Rad Tech’s Guide to
Radiation protection is a core element of radiologic technology programmes
and daily practice alike. Rad Tech’s Guide to Radiation Protection is a comprehensive yet compact guide designed to illuminate the extensive field
of radiation protection for technologists, trainees, and radiology students.
Organised into ten digestible chapters, the second edition of this popular
book provides new discussions of dose factors in computed tomography, the
debate concerning the use of the LNT model, Diagnostic Reference Levels
(DRLs), dose optimization, and more.
Written by a recognised expert in medical radiation sciences, this valuable guide:
• Reflects the most current standards for radiation protection, with
references to relevant organisations and resources
• Covers basic radiobiology, sources of radiation exposure, dose
management regulations and optimization, and more
• Presents essential information in a bulleted, easy-to-reference format
Rad Tech’s Guide to Radiation Protection is a must-have resource for student
radiographers and radiology technologists, particularly those preparing for
the American Registry of Radiation Technologist (ARRT) exams.
www.wiley.com
Seeram
Cover Design: Wiley
Cover Image: © Hero Images/Getty Images
Second Edition
Euclid Seeram, PhD, FCAMRT, is a Full Member of the Health Physics
Society and has academic appointments as Honorary Senior Lecturer in
Medical Radiation Sciences at the University of Sydney, Australia; Adjunct
Associate Professor of Medical Imaging and Radiation Sciences at Monash
University, Australia; Adjunct Professor in the Faculty of Science at Charles
Sturt University, Australia; and Adjunct Professor of Medical Radiation Sciences at the University of Canberra, Australia.
RADIATION PROTECTION
• Helps students and technologists acquire the skills required to protect
patients, personnel, and members of the public in the radiology
department
Rad Tech’s Guide to
RADIATION
PROTECTION
Second Edition
Euclid Seeram
SERIES EDITOR
Rad Tech’s
Guide to
Radiation
Protection
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Rad Tech’s
Guide to
Radiation
Protection
Second Edition
Euclid Seeram, PhD, FCAMRT
Full Member – Health Physics Society
Academic Appointments
Honorary Senior Lecturer; Medical Radiation
Sciences; Faculty of Health Sciences; University of
Sydney, Australia | Adjunct Associate Professor;
Medical Imaging and Radiation Sciences; Monash
University, Australia | Adjunct Professor; Faculty
of Science; Charles Sturt University, Australia |
Adjunct Professor; Medical Radiation Sciences;
Faculty of Health; University of Canberra, Australia
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This edition first published 2020
© 2020 John Wiley & Sons Ltd
Edition History
Blackwell Science. Inc. (1e, 2001)
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Set in 11.5/13.5pt STIX Two Text by SPi Global, Pondicherry, India
10
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This book is dedicated with love and affection to my lovely and
charming wife Trish, a brilliant, hard‐working, caring, and
loving individual, who taught me the very essence of life.
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Contents
Preface to the Second Edition.. . . . . . . . . . . . . . . . . . .xi
Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . .xiii
1 Nature and Scope of Radiation Protection. . . . . . . . . 1
What is Radiation Protection?. . . . . . . . . . . . . . . . . . . . . . . . . .2
Scope of Radiation Protection . . . . . . . . . . . . . . . . . . . . . . . . .2
Diagnostic Radiology Modalities. . . . . . . . . . . . . . . . . . . . . . .3
Why Protect Patients and Personnel in Diagnostic
Radiology? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Framework for Radiation Protection. . . . . . . . . . . . . . . . . . . .4
Basic Schemes for Patient Exposure in Digital
Radiography, Fluoroscopy, and Computed
Tomography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Factors Affecting Dose in Diagnostic Radiology. . . . . . . . . .8
Dose Management Techniques . . . . . . . . . . . . . . . . . . . . . . .10
Pregnancy: Radiation Protection Considerations. . . . . . . .11
2 Diagnostic X-Rays: Essential Physical Factors . . . . . 13
X-Ray Production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Mechanisms for Creating X-Rays. . . . . . . . . . . . . . . . . . . . . .14
X-Ray Spectrum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
X-Ray Attenuation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
X-Ray Interactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Increasing kV and Scatter Production.. . . . . . . . . . . . . . . . .26
3 Radiation Quantities and Units . . . . . . . . . . . . . . . . . 27
Sources of Radiation Exposure. . . . . . . . . . . . . . . . . . . . . . . 29
Types of Exposure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
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viii Contents
Quantities and Units for Quantifying
Ionizing Radiation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
Quantities and Units for Quantifying Biologic Risks . . . . 33
Radiation Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Wearing a Personnel Dosimeter . . . . . . . . . . . . . . . . . . . . . 38
4 Basic Radiobiology. . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
What Is Radiobiology?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41
Essential Physics and Chemistry . . . . . . . . . . . . . . . . . . . . . .41
Fundamental Concepts of Radiobiology. . . . . . . . . . . . . . . 44
Deterministic Effects (Early Effects of Radiation). . . . . . . .49
Stochastic Effects (Late Effects of Radiation). . . . . . . . . . . .51
Radiation Exposure During Pregnancy. . . . . . . . . . . . . . . . 54
5 Current Standards for Radiation Protection.. . . . . . 55
Radiation Protection Organizations . . . . . . . . . . . . . . . . . . 56
Objectives of Radiation Protection . . . . . . . . . . . . . . . . . . . .57
Radiation Protection Criteria and Standards. . . . . . . . . . . 58
Recommended Dose Limits . . . . . . . . . . . . . . . . . . . . . . . . . 60
Diagnostic Reference Levels: A Useful Tool
for Optimization of Protection. . . . . . . . . . . . . . . . . . . . . . . 62
6 Dose Factors in Digital Radiography. . . . . . . . . . . . . 65
Digital Radiography: Essential Considerations. . . . . . . . . 66
The Standardized Exposure Indicator: Basics. . . . . . . . . . 68
Factors Affecting Dose in Digital Radiography. . . . . . . . . .70
7 Dose Factors in Fluoroscopy. . . . . . . . . . . . . . . . . . . . 77
Major Components of Fluoroscopic
Imaging Systems.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .78
Factors Affecting Dose in Fluoroscopy. . . . . . . . . . . . . . . . 82
Scattered Radiation in Fluoroscopy. . . . . . . . . . . . . . . . . . . 87
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Contents
ix
8 Factors Affecting Dose in Computed
Tomography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Computed Tomography: A Definition. . . . . . . . . . . . . . . . . 90
Nobel Prize for CT Pioneers . . . . . . . . . . . . . . . . . . . . . . . . . .91
CT Principles: the Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . .91
Multislice CT Technology: The Pitch . . . . . . . . . . . . . . . . . . .93
Dose Distribution in CT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .93
CT Dose Metrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
Factors Affecting the Dose in CT. . . . . . . . . . . . . . . . . . . . . 96
Dose Optimization in CT . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
9 Dose Management Regulations
and Optimization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101
Federal Regulations for Dose Management . . . . . . . . . . 103
Equipment Specifications for Radiography. . . . . . . . . . . 104
Equipment Specifications for Fluoroscopy. . . . . . . . . . . . 106
Procedures for Minimizing Dose
to Patients and Personnel. . . . . . . . . . . . . . . . . . . . . . . . . . 109
Shielding: Design of Protective Barriers. . . . . . . . . . . . . . .112
Quality Assurance: Dose Management
and Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .114
10 Pregnancy: Essential Radiation Protection
Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
Rationale for Radiation Protection
in Pregnancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .118
Factors Affecting Dose to the Conceptus. . . . . . . . . . . . . .119
Estimating the Dose to the Conceptus. . . . . . . . . . . . . . . .120
Continuing/Terminating a Pregnancy
After Exposure.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .120
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x
Contents
Dose Reduction Techniques for Pregnant
Patients. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .121
The Pregnant Worker. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .121
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .129
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Preface to the
Second Edition
R
adiation protection in medicine has experience an evolution
through the years and has focused mainly on developing
new procedures for optimizing the dose to the patient, based
on objective evidence and knowledge of the biological effects
of radiation exposure to not only humans but animals as well.
These data have suggested that the doses from medical radiation
are high and that every effort must be made to manage the dose
to both patients and personnel. Another concept developed from
these data is that of a dose‐risk model or simply a dose‐response
relationship. Several models have been proposed but the one
that has been supported for use in medical radiation exposure
to patients is the Linear No Threshold (LNT) model. Experts
Hendee and O’Connor (2012) provide a guiding argument for
the use of the LNT model in diagnostic radiology as follows:
This model is not chosen because there is solid biologic or epidemiologic data supporting its use. Rather,
it is used because of its simplicity and because it is a
conservative approach … For the purpose of establishing radiation protection standards for occupationally exposed individuals and members of the public,
a conservative model that overestimates the risk is
preferred over a model that underestimates risk
Radiation protection is an essential core subject of radiologic
technology programs. To meet the needs of these programs,
a handful of books on radiation protection is currently available to enable students and technologists alike to acquire the
skills required to protect patients, personnel, and members of
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xii Preface to the Second Edition
the public in the radiology department. This book, Rad Tech’s
Guide to Radiation Protection, provides a comprehensive practical guide for technologists and students engaged in the art
and science of radiation protection. It main goal is to provide a
resource that is brief, clear, and a concise coverage of the subject
in preparation for their professional certification examination.
Rad Tech’s Guide to Radiation Protection is not a textbook
and it is not intended to replace the vast resources on radiation
protection. Rather, it provides a précis of the extensive coverage
of radiation protection topics for technologists.
Rad Tech’s Guide to Radiation Protection contains 10 short
chapters that cover a wide scope of topics on radiation protection. For this second edition, a new chapter on Dose Factors in
Computed Tomography has been added, and a few new concepts have been introduced into the appropriate chapters. For
example, the debate concerning the use of the LNT model has
been included in Chapter 4, While Diagnostic Reference Levels
(DRLs) are briefly outlined in Chapter 5, and Dose Optimization
is reviewed briefly in Chapter 9.
Chapter 1 discusses the nature and scope of radiation protection and sets the framework for the remaining chapters. While
Chapter 2 presents a description of the essential physics for radiation protection, Chapter 3 describes radiation quantities and
their units. Chapter 4 outlines the basic concepts of radiobiology and Chapter 5 provides a rationale for radiation protection.
Chapters 6 and 7 address the factors that affect dose levels in
digital radiography and fluoroscopy, respectively. Additionally,
Chapter 8 is a new Chapter on Factors Affecting the Dose in
Computed Tomography. Chapter 9 provides a discussion of Dose
Management Regulations and Optimization. Finally, Chapter 10
reviews radiation protection considerations in pregnancy.
Enjoy the pages that follow and remember – your patients
will benefit from your wisdom.
Euclid Seeram, PhD, FCAMRT
British Columbia, Canada
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Acknowledgments
I
t is a pleasure to acknowledge the contributions of experts in
the fields of radiobiology and radiation protection, from whom
I have learned a great deal that allows me to write this book.
First, I must acknowledge James Watson, Commissioning
Editor, Wiley, Oxford, UK, who understood and evaluated the
need for a second edition of this book. Additionally, I am grateful to Anupama Sreekanth, the project editor for this title at
Wiley, for all the advice and support provided to me during the
writing of this book.
Second, I am indeed grateful to all those who have dedicated
their energies in providing several comprehensive volumes
on medical radiation protection for the radiologic community.
I would like to acknowledge the notable medical physicist,
Dr. Stewart Bushong, ScD, FAAPM, FACR, and experimental
radiobiologist, Dr. Elizabeth Travis, PhD. I have learned a great
deal on radiologic science from the works of Dr. Bushong, a
professor of radiologic science in the Department of Radiology,
Baylor College of Medicine, Houston, Texas. In addition, I have
gained further insight into the nature, scope, and depth of
radiobiology, and particularly its significance in radiology, from
Dr. Travis, a researcher in the Department of Experimental
Radiotherapy, University of Texas, MD, Anderson Cancer Center,
Houston, Texas. Furthermore, I would like to thank Dr. Hans
Swan, PhD, and Dr. Rob Davidson, PhD, who served as my primary supervisors for my PhD dissertation entitled Optimization
of the Exposure Indicator of a Computed Radiography Imaging
System as a Radiation Dose Management Strategy.
I must acknowledge, too, all others, such as the authors
whose papers I have cited and referenced in this book – thank
you for your significant contributions to the radiation protection
knowledge base. Additionally, I would like to express my sincere thanks to Dr. Perry Sprawls, PhD, FACR, FAAPM, FIOMP,
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xiv Acknowledgments
Distinguished Emeritus Professor, Emory University, Director,
Sprawls Educational Foundation, http://www.sprawls.org, Co‐
Director, College on Medical Physics, ICTP, Trieste, Italy, and
Co‐Editor, Medical Physics International, http://www.mpijournal.
org. Dr. Sprawls has always supported my writing and I appreciate his gracious permission to use his materials (illustrations
in particular) in my textbooks on radiation protection. Thank
you, Perry.
Finally, I must acknowledge the warm and wonderful support
of my family; my lovely wife, Trish, a very wise and caring
person, and my very smart son, David, a very special young
man, and the best Dad in the universe. Thank you both for your
unending love, support, and encouragement.
Last, but not least, I want to express my gratitude to all the
students in my radiation protection classes – your questions
have provided me with a further insight into teaching this
important subject.
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1
Nature
and Scope
of Radiation
Protection
Chapter at a Glance
What is Radiation Protection?
Scope of Radiation Protection
Diagnostic Radiology Modalities
Why Protect Patients and Personnel in
Diagnostic Radiology?
Framework for Radiation Protection
Radiation Protection Principles
Radiation Protection Actions
Basic Schemes for Patient Exposure in Digital
Radiography, Fluoroscopy, and Computed
Tomography
Digital Radiography
Fluoroscopy
Computed Tomography
Factors Affecting Dose in Diagnostic Radiology
Radiographic Factors
Fluoroscopic Factors
Computed Tomography Factors
Rad Tech’s Guide to Radiation Protection, Second Edition. Euclid Seeram.
© 2020 John Wiley & Sons Ltd. Published 2020 by John Wiley & Sons Ltd.
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2
Rad Tech’s Guide to Radiation Protection
Dose Management Techniques
Pregnancy: Radiation Protection Considerations
R
adiation protection is one of the most important subjects
in the education and practice of medical imaging technologies using ionizing radiation, because it deals with the safe use
of ionizing radiation when imaging patients for the purpose of
restoring their health.
The purpose of this chapter is to orient both students and
practicing technologists to the nature and scope of radiation
protection specifically in diagnostic x‐ray imaging (radiography/
radiology) and to provide a brief overview of topics essential to
the theory and practice of radiography.
What is Radiation Protection?
Radiation protection is concerned with the protection of persons
from the harmful effects of exposure to radiation. In radiography/radiology radiation protection deals with the protection of
patients and operators, as well as members of the public. Why?
◼◼ Patients are exposed to varying amounts of radiation from
the x‐ray tube, depending on the type of examination.
◼◼ Personnel may be exposed to radiation scattered from the
patient and the equipment.
◼◼ Members of the public, in this instance, are individuals
who are working or waiting in close proximity to an x‐ray
room. These include, for example, radiology support staff
and family members of patients.
Scope of Radiation Protection
The scope of radiation protection includes knowledge and
­understanding of the following:
◼◼ Physical factors. These are related to the physics
of ­r adiation, including energy dissipation in matter,
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Nature and Scope of Radiation Protection
3
c­ haracteristics of radiation, interaction of radiation with
matter, and the physical and chemical factors of radiation.
◼◼ Technical factors. These include a range of topics such
as technical components of imaging systems that affect
dose, standards of radiation protection, dose management
techniques, and shielding considerations.
◼◼ Procedural factors. These relate to personnel practices
during the examination and include such tasks as equipment set‐up, patient communication and positioning,
selection of technical factors, gonadal shielding, image
processing, and image assessment.
◼◼ Biologic factors. The biologic effects of radiation result
from the physical and chemical interaction of radiation
with matter (tissue). Bioeffects can be somatic (effects that
appear in the individual exposed to radiation) or they
can be genetic (effects that appear in the offspring of the
individual exposed). Bioeffects are now classified as stochastic and deterministic (nonstochastic) effects. These
are described further in Chapter 3.
Diagnostic Radiology Modalities
The diagnostic radiology modalities that are described in this
book are digital radiography, fluoroscopy, and computed tomography (CT). The basic approaches to exposing a patient to the
radiation beam during imaging, depend on the beam geometry.
These approaches are described in brief later in the chapter.
Why Protect Patients and Personnel
in Diagnostic Radiology?
There are several major reasons for protecting patients undergoing radiographic examination as well as for protecting the
personnel conducting the examination. These reasons include:
◼◼ Biologic effects data demonstrate beyond question that
radiation exposure is harmful to humans.
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4
Rad Tech’s Guide to Radiation Protection
◼◼ Patients receive more radiation exposure from diagnostic
radiology compared with any other man‐made radiation sources.
◼◼ No dose of radiation is considered safe (there is no risk‐
free dose of radiation).
◼◼ Various research studies reveal that some diagnostic x‐ray
examinations, particularly involving fluoroscopy and CT,
deliver high doses of radiation to patients.
◼◼ Technologists assume full responsibility for all aspects
of the radiographic examination, including radiation
exposure of the patient. Protection of patients and operators (technologists and radiologists) depends on the
technical expertise of both the technologist and the
radiologist.
◼◼ Radiation safety is guided by several principles and actions
set forth by the International Commission on Radiological
Protection (ICRP); these are meant to keep all exposures
as low as reasonably achievable (ALARA).
Framework for Radiation Protection
A framework refers to a supporting structure or a basic system.
A radiation protection framework includes a number of concepts
intended to prevent and minimize the harmful effects of radiation exposure.
The ICRP offers one comprehensive framework that is accepted
by various national radiation protection organizations, including
the National Council of Radiation Protection and Measurements
(NCRP) in the United States, the Radiation Protection Bureau
(part of Health Canada), the National Radiological Protection
Board (NRPB) in the United Kingdom, as well other radiation
protection organizations around the world.
The ICRP framework encompasses several notable concepts,
including the types of exposure from which individuals can receive
a dose of radiation and a concept of significance to technologists
and radiologists called the two triads of radiation protection.
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Nature and Scope of Radiation Protection
5
The types of exposure include:
◼◼ Occupational exposure. Exposure from work activities.
◼◼ Medical exposure. Exposure from diagnostic and
therapy procedures, which does not include occupational
exposure.
◼◼ Public exposure. All other exposures from natural
sources of radiation, such as radon gas.
There are several other concepts that constitute the ICRP
framework. However, the two triads of radiation protection are
discussed here. These two triads define current radiation protection standards and include:
◼◼ Radiation protection principles.
◼◼ Radiation protection actions.
Radiation Protection Principles
Radiation protection principles include the following three
fundamental guiding concepts:
◼◼ Justification. This concept focuses on net benefit. That is,
there must be a benefit associated with any new modality,
procedure, or exposure.
◼◼ Optimization. This concept dictates that all exposures
be kept as low as reasonably achievable (ALARA), taking
into consideration social and economic factors.
◼◼ Dose limitation. By establishing dose limits to persons
exposed to radiation, certain harmful effects can be prevented
and others can be minimized. These limits are numerical
values representing an upper limit of exposure annually. For
example, the ICRP recommends an annual dose limit to the
lens of the eye of 150 millisieverts (mSv) or 15 000 millirem
(mrem) for occupationally exposed individuals.
Dose limits are discussed in Chapter 5.
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6
Rad Tech’s Guide to Radiation Protection
Radiation Protection Actions
Radiation protections actions are based on the following
three concepts:
◼◼ Time. Because dose is directionally proportional to the
length of time of exposure, it is important to decrease the
time of the exposure to decrease the dose. If the time is
decreased by a factor of two, then the dose will be reduced
by a factor of two.
◼◼ Shielding. To reduce the dose to patients and others, it is
essential to place a shield between the source of radiation
(x‐ray tube) and the individual exposed (patient). Gonadal
shielding is a prime example of this concept.
◼◼ Distance. The dose an individual receives is inversely
proportional to the square of the distance. This factor is
known as the inverse square law and it is expressed as
I 1/d 2
where I is equal to the intensity of the radiation and d
equals the distance from the source of the radiation to the
individual exposed. As the distance is increased, the dose
is reduced proportionally to the square of the distance.
Basic Schemes for Patient Exposure
in Digital Radiography, Fluoroscopy,
and Computed Tomography
The basic scheme for patient exposure refers to the beam geometry used to expose the anatomical area of interest, as illustrated in Figure 1.1, for radiography, fluoroscopy, and CT. The
beam geometry refers to the size and shape of the x‐ray beam
emanating from the x‐ray tube. Beam geometry also refers to
whether the beam is fixed or is moving during the exposure.
Whereas in radiography, the beam is fixed during the exposure
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Nature and Scope of Radiation Protection
X-ray tube
X-ray tube
Open beam geometry
7
X-ray tube
Beam geometry
Patient
Patient
Patient
Digital detector
Digital detector
Two-dimensional detector
(a) Radiography
(b) Fluoroscopy
(c) Computed Tomography
Figure 1.1 The basic scheme for patient exposure refers to the
beam geometry used to expose the anatomical area of interest, as
illustrated for (a) digital radiography, (b) fluoroscopy, and (c) computed
tomography (CT).
(with the exception of digital radiographic tomosynthesis), it is
moved about the patient during a fluoroscopic examination. In
CT, the beam rotates around the patient during the exposure.
The details of each of are described in later chapters.
Digital Radiography
In radiography, specifically digital radiography:
◼◼ The beam geometry describes an open beam (Figure 1.1a)
shaped by the collimator to fall on the area of interest on
the patient.
◼◼ The beam is fixed on this area of interest during
the exposure.
◼◼ The beam is collimated to the size of the digital image
receptor used for the examination.
◼◼ Radiographic exposure technique factors, such as mA,
kV, and exposure time in seconds, are used to produce
the x‐rays needed to create images of the patient.
◼◼ Images are static and each image requires a separate x‐
ray exposure.
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8
Rad Tech’s Guide to Radiation Protection
Fluoroscopy
In fluoroscopy (conventional or digital):
◼◼ The beam geometry describes an open beam (Figure 1.1b)
shaped by the collimator to fall on the regions of interest
being imaged.
◼◼ The beam is moving during fluoroscopic exposures. This
movement is necessary to track the flow of contrast media
through the anatomy, such as the gastrointestinal tract.
◼◼ The beam is collimated to the size of the image receptor.
◼◼ Fluoroscopic exposure technique factors, generally low
mA (approximately 1–3 mA) and high kV, are used to show
fluoroscopic images displayed on a television ­monitor.
The x‐ray tube is energized for longer periods compared
with the short exposure times used in radiography.
◼◼ Radiographic exposure technique factors are used by the
technologist who records “overhead” images after the
fluoroscopic portion of the examination. This is the radiographic component of a routine fluoroscopic examination.
Computed Tomography
In CT, the x‐ray beam is a cone (Figure 1.1c) that rotates 360°
around the patient while the patient moves through the CT scanner
gantry opening during the collection of transmission x‐ray readings, to build up a sectional image of the anatomy being imaged.
Factors Affecting Dose
in Diagnostic Radiology
To protect patients, personnel, and members of the public from
radiation in diagnostic radiology, it is mandatory that technologists have a firm understanding of the various factors affecting
dose. In this text, only essential dose factors in radiography,
fluoroscopy, and CT are considered.
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Nature and Scope of Radiation Protection
9
Radiographic Factors
The technical factors affecting patient dose in radiography are:
◼◼ Type of x‐ray generator.
◼◼ X‐ray exposure technique factors (mA, kV, and time).
◼◼ Beam energy and filtration.
◼◼ Collimation and field size.
◼◼ Distance from the x‐ray tube to the patient (source‐
to‐skin distance) and the distance from the x‐ray
tube to the image receptor (source‐to‐image receptor
­distance, SID).
◼◼ Patient thickness and density.
◼◼ Antiscatter grids.
◼◼ Image receptor sensitivity.
◼◼ Shielding.
◼◼ Repeat examinations.
◼◼ Patient orientation.
Fluoroscopic Factors
The technical factors affecting dose in fluoroscopy are numerous
and include:
◼◼ Beam energy and tube current.
◼◼ Collimation.
◼◼ Source‐to‐skin distance.
◼◼ Patient‐to‐image intensifier distance.
◼◼ Beam‐on time.
◼◼ Antiscatter grids.
◼◼ Image magnification.
◼◼ Conduct of the fluoroscopic portion of the examination.
◼◼ Conduct of the radiographic portion of the fluoroscopic
examination.
The factors for radiography and fluoroscopy are elaborated
further in Chapters 6 and 7 respectively.
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10 Rad Tech’s Guide to Radiation Protection
Computed Tomography Factors
There are several technical factors affecting dose in CT. However,
only the ones that are under the control of the technologist are
listed here. These include:
◼◼ Exposure technique factors (mAs and kV).
◼◼ Collimation and slices.
◼◼ Pitch.
◼◼ Noise index.
◼◼ Automatic exposure control.
◼◼ Overranging and overbeaming.
◼◼ Iterative reconstruction algorithms.
These factors are described further in Chapter 8.
Dose Management Techniques
The goal of radiation protection is to reduce the dose to patients
and personnel. Dose management techniques are intended to
reduce and optimize the imaging process to produce diagnostic
image quality using the ALARA philosophy. These techniques are
governed objectively by the guidelines and recommendations of
not only the ICRP but also of various national regulatory authorities and by radiation protection organizations issuing Radiation
Protection Reports. The guidelines and recommendations for
dose management in radiology address the four major areas.
◼◼ Equipment design and performance. Equipment must
be designed or upgraded to meet certain specifications
that will allow the operator to optimize image quality
while protecting patients and personnel from unnecessary radiation. These guidelines and recommendations
focus on specific technical parameters such as filtration,
collimation, and source‐to‐skin distance for radiographic
and fluoroscopic (fixed and mobile) equipment.
◼◼ Personnel practices. These recommendations focus
on the conduct of the examination using the ALARA
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Nature and Scope of Radiation Protection
11
philosophy. For example, the primary beam must always
be collimated to the size of the image receptor or smaller;
technologists should not hold patients to achieve immobilization during an examination.
◼◼ Shielding. Shielding is one of three radiation protection
actions intended to protect individuals from radiation
exposure. Recommendations for shielding address the use
of lead shields to protect radiosensitive organs such as the
gonads. Additionally, walls of x‐ray rooms are lined with
lead to prevent radiation from penetrating and exposing
individuals outside the room.
◼◼ Education and training. Guidelines and recommendations for the safe use of radiation also address the need for
operators to be educated and trained in a wide range of
subjects. These subjects include radiation physics, instrumentation, radiation risks, and radiation protection, and
are intended to minimize the radiation dose to patients,
personnel, and members of the public. These techniques
are described further in Chapter 7.
The operations needed to observe ALARA include dose
reduction and dose optimization. A good dictionary defines the
term reduction as “reduce or diminish in size, amount, extent
or number;” the term optimization refers to “an act, process,
or methodology of marking something (as a design, system, or
decision) as fully perfect, functional, or effective as possible.”
Optimization is viewed as a much more involved process and
requires a careful examination of reducing the dose so as not to
compromise the diagnostic quality of the image. Optimization
addresses both image quality and dose.
Pregnancy: Radiation Protection
Considerations
This is an important topic in radiation protection for everyone
working in radiology because of the sensitivity of the conceptus
(any product of conception, embryo, or fetus) to radiation.
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12 Rad Tech’s Guide to Radiation Protection
There are several major considerations with respect to
in-utero exposure in radiology. It is not within the scope of this
chapter to outline all of the factors relating to in-utero exposure.
However, a few significant points to be noted are:
◼◼ Data suggest that, depending on the dose and gestational
stage of exposure, there can be malformations and radiation‐induced childhood malignancy.
◼◼ If a patient must have an x‐ray examination (because the
benefits outweigh the risks), then several precautions
should be taken:
◼◼ Shielding must be used only if it will not interfere
with diagnostic information.
◼◼ High‐kVp techniques and increased filtration are
advocated.
◼◼ Fluoroscopy must be kept to an “absolute minimum.”
◼◼ The number of images must be kept to a minimum.
◼◼ Several factors affect dose to the fetus, including the
technical factors previously mentioned. Additionally, direct
exposure (fetus in the field‐of‐view) and indirect exposure
(fetus outside the field‐of‐view) are significant factors.
◼◼ Fetal doses can be estimated to provide information on
the risks to the fetus and on any actions to be taken.
◼◼ Following an exposure, the NCRP (Report 54) recommendations should be observed. These points are examined
in Chapter 9.
◼◼ Ultrasound (rather than radiology) is now used to evaluate
fetal maturation and placental localization.
Several issues and concerns surround the pregnant technologist. The pregnant technologist should notify the department
of her pregnancy and use a second dosimeter for the remainder
of the pregnancy (under the apron at the waist level). The work
schedule of a pregnant technologist may be altered.
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2
Diagnostic
X-Rays
Essential Physical
Factors
Chapter at a Glance
X-Ray Production
Mechanisms for Creating X-Rays
X-Ray Spectrum
Form of an X-Ray Emission Spectrum
X-Ray Beam Quality and Quantity
Controlling Beam Quality and Quantity
X-Ray Attenuation
Definition
Factors Affecting Attenuation
X-Ray Interactions
Photoelectric Absorption
Compton Scattering
Increasing kV and Scatter Production
X
-rays are produced when high-speed electrons strike
a target. In diagnostic radiology, an X-ray tube is used
to produce X‐rays to image patients. The X‐ray tube consists
basically of an anode and a cathode. The cathode consists of a
filament that when heated emits electrons. In turn, the electrons
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14 Rad Tech’s Guide to Radiation Protection
are accelerated at high speeds to strike a small spot on the anode,
called the target. The electron–target interaction results in the
production of heat and X‐rays.
The purpose of this chapter is to outline several physical
factors relating to the basic physics of X‐ray production, X‐ray
interactions with matter, and X‐ray attenuation, all of which are
important to radiation protection. An understanding of how
the X‐ray beam is produced, how it interacts with the patient,
and what happens when the beam is transmitted through the
patient is key to optimizing image quality and minimizing radiation dose. The technologist has control over several parameters
that affect X‐ray production, attenuation, and interaction with
the patient.
This chapter is pivotal in ensuring that the technologist/
radiographer has the fundamental cognitive skills that are
required for good radiation protection practices.
X-Ray Production
To produce X‐rays for a particular examination, the technologist
sets up the appropriate voltage (kV), current (mA), and time
(in seconds [s]) on the control panel. These exposure technique
factors determine the type of radiation beam that will be produced by the X‐ray tube. In this section, the following points
are summarized: the mechanisms by which electrons from the
filament of the X‐ray tube create X‐rays; the X‐ray emission spectrum, including X‐ray beam quantity, X‐ray beam quality, and
the factors affecting both; and, finally, X‐ray attenuation.
Mechanisms for Creating X-Rays
There are two mechanisms by which high‐speed electrons
from the filament of an X‐ray tube create X‐rays. These are the
characteristic and the bremsstrahlung processes, which give rise
to characteristic and bremsstrahlung radiation, respectively.
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Diagnostic X-Rays
M
L
K
–
–
15
Incoming
high-speed
electron
–
Nucleus
K-characteristic
X-rays
Ejected K-shell electron
Figure 2.1 Characteristic X-rays are emitted when an inner-shell
electron is ejected from the atom (ionization) and an outer-shell
electron moves in to fill the vacancy in the inner shell.
◼◼ Characteristic radiation. When a high‐speed electron
from the filament interacts with an inner‐shell electron of
the target atom, and the electron has enough kinetic energy,
then the electron will be ejected from its orbit leaving a
vacancy in the orbit. An electron from an outer shell will
move into the inner‐shell vacancy. This movement results in
an emission of characteristic X‐rays, as shown in Figure 2.1.
◼◼ Bremsstrahlung radiation. This is the result of electrons
interacting with the target nucleus rather than its electrons. The production of bremsstrahlung (brems) radiation is illustrated in Figure 2.2. When incident electrons
approach the charged nucleus, they decelerate, change
direction, and exit with reduced energy. This loss of energy
appears in the form of radiation called brems radiation.
The following points are important with respect to
characteristic and brems radiation in diagnostic radiology:
◼◼ In the range of kV values used in diagnostic radiology for
general radiographic and fluoroscopic procedures, most
of the X‐rays emitted from the tube are brems radiation.
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16 Rad Tech’s Guide to Radiation Protection
K-shell
Incoming
high-speed
electron
–
KEx
Brems
radiation: E
Nucleus
KEY –
Outgoing
electron
Figure 2.2 Brems radiation is emitted from the X-ray tube when
high-speed electrons interact with the nucleus of the target atoms.
Electrons slow down, change direction, and leave with reduced
kinetic energy. This loss of energy results in brems radiation.
◼◼ In mammography, however, characteristic radiation is
used, as the narrow band of energy is most useful when
imaging the soft tissues of the breast.
◼◼ K‐characteristic radiation is used in radiography.
◼◼ Brems radiation has a wide range of energies suitable for
all radiographic and fluoroscopic examinations.
◼◼ The output radiation from the X‐ray tube consists of
both brems and characteristic radiation. If the intensity
of the radiation is plotted as a function of its energy, then
an X‐ray emission spectrum is the result.
X-Ray Spectrum
When a technologist makes an exposure for an examination,
both characteristic and brems radiation are emitted from the
tube. This emission is known as the X‐ray emission spectrum,
which can be represented as a graph illustrating the intensity
of X‐rays (number of X‐rays per unit energy) plotted as a function
of the X‐ray energy.
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Diagnostic X-Rays
17
Form of an X-Ray Emission Spectrum
◼◼ The general form of an X‐ray emission spectrum is shown
in Figure 2.3.
◼◼ Two spectra are shown:
1. The brems or continuous spectrum.
2. The characteristic or discrete spectrum.
An understanding of both of these spectra helps the technologist realize how exposure technique factors and filtration can
affect image quality and radiation dose.
◼◼ The area under the curve represents the number of pho-
Number of X-ray photons
per unit energy
tons in the X‐ray beam. A greater area indicates more
photons, thus a higher dose. The area is also referred to
as the X‐ray quantity.
◼◼ The energy distribution of the beam is shown on the
horizontal axis and is expressed in keV. The term X‐ray
quality is used to describe the energy distribution, thus
the shape of the curve.
A
Bremsstrahlung spectrum
(continuous spectrum)
Characteristic
spectrum
(discrete
spectrum)
B
Energy (keV)
Figure 2.3 The form and shape of the brems and characteristic
X-ray spectra.
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18 Rad Tech’s Guide to Radiation Protection
X-Ray Beam Quality and Quantity
The intensity of the beam from the tube can be described in
terms of both the quantity and quality. The quantity and quality
of the beam emanating from the X‐ray tube affect the dose to
the patient.
◼◼ X‐ray quantity, also referred to as radiation exposure,
refers to the number of photons in the beam. More photons increase the dose to the patient. Numerous factors
determine X‐ray quantity.
◼◼ X‐ray quality refers to the energy or penetrating power
of the photons in the beam. The beam consists of both
high‐energy and low‐energy photons. Numerous factors
affect beam quality.
Controlling Beam Quality and Quantity
There are several factors that affect the quality and quantity of
the X‐ray beam. However, few are under the direct control of
the technologist.
The factors affecting quality and quantity are:
◼◼ kV. which is under the direct control of the technolo-
gist. The penetration of the beam and the beam quality
is greater as the kV increases. High‐kV techniques result
in a low dose. The beam intensity is directly proportional
to the square of the ratio of the kV change, that is (kVnew/
kVold)2. If the kV is doubled, then the intensity increases
by a factor of four. This factor means that by increasing
the kV, the quantity can be increased. Increasing the kV
by 15% is comparable to doubling the mAs.
◼◼ mA/mAs, which are under the direct control of the
technologist. mA/mAs affects the quantity of radiation
in that the quantity is proportional to the mA. If the
mA is doubled, then the quantity is doubled, and the
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Diagnostic X-Rays
19
dose is increased by a factor of two. The same applies
to the mAs.
◼◼ Filtration. Filtration affects both the quality and quantity
of the beam. A filter is always inserted in the X‐ray beam
to remove low‐energy photons. This reduces the quantity
and, as a result, the mean energy of the beam increases.
The beam becomes more penetrating or harder. Thicker
filters reduce the quantity of the beam, but increase
beam quality. A filter is intended to protect the patient
by removing these low‐energy photons.
◼◼ Target material. Target materials with higher atomic
numbers increase both the quantity of photons slightly
and the quality (energy) of the beam. Tungsten produces a significantly more efficient spectrum than
molybdenum. The technologist has no control over this
parameter.
◼◼ Type of generator. The X‐ray generator provides power
to energize the X‐ray tube to produce X‐rays. This generator determines the voltage waveform to the tube. Three
types of generators are available: single‐phase, three‐
phase, and high‐frequency generators. State‐of‐the‐art
X‐ray equipment uses high‐frequency generators, which
are more efficient since they produce a greater quantity
of photons (area under the curve) with higher effective
energies (greater quality) than single‐ and three‐phase
generators. This means that the dose can be reduced when
using high‐frequency generators by appropriately adjusting exposure technique factors (e.g. high‐kV techniques
can be used).
◼◼ Source‐to‐image receptor distance (SID). This is under
the direct control of the technologist. The SID affects the
quantity of photons but has no effect on the quality. The
quantity is affected by the inverse square law, which states
that the intensity (quantity) is inversely proportional to the
square of the distance. If the distance is increased, then
the quantity decreases by l/d2.
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20 Rad Tech’s Guide to Radiation Protection
X-Ray Attenuation
Definition
When a beam of X‐rays passes through matter, it is attenuated
(reduced in intensity) before it reaches the image receptor.
◼◼ Attenuation is the reduction in the intensity of the beam
as it passes through any material.
◼◼ The materials that are of importance in radiology are, of
course, the X‐ray tube filter and the patient.
◼◼ The reduction in intensity is a result of absorption and
deflection of photons from the radiation beam.
◼◼ Absorption of photons produces a greater dose to
the patient.
◼◼ Increased attenuation produces more absorption, thus
increasing the dose to the patient.
◼◼ A measure of the radiation quantity attenuated by a
given thickness of material is defined as the attenuation
coefficient.
◼◼ The linear attenuation coefficient (μ) is the fractional
reduction of the radiation per unit thickness of the
material traversed.
◼◼ Although μ is a measure of per unit length attenuation,
it can also be expressed as per unit mass of the absorbing
material. This is the mass attenuation coefficient, which
is equal to μ divided by the density (ρ) of the material. The
unit of μ/ρ is centimeter2 per gram (cm2/g).
◼◼ The attenuation of a homogeneous beam (all photons
have the same energy or are monochromatic) of X‐rays or
gamma rays is shown in Figure 2.4. The following points
are important:
◼◼ Equal thicknesses of absorber remove equal
amounts of radiation (Figure 2.4a)
◼◼ The attenuation is exponential (Figure 2.4b) and
can be described by the equation:
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Diagnostic X-Rays
(a)
21
Attenuation of a HOMOGENEOUS beam of radiation
Beam
quantity
1000
Photons
IN
32
Photons
OUT
Beam
quality
(40 kVp)
(40 kVp)
Beam
quantity
Beam
quality
1 cm
(b)
I0
I
I = I0e–μx
Figure 2.4 Exponential attenuation of a homogeneous
(monochromatic) beam of radiation in which all the photons have the
same energy.
I Io e
x
where Io = intensity of the incident photons
I = intensity of the transmitted photons
e = base of the natural logarithm
x = thickness of the material
◼◼ The X‐ray beam from the X‐ray tube is a heterogeneous or
polychromatic beam in which the photons have different
energies, and the attenuation is somewhat different from
a monochromatic or homogeneous beam.
◼◼ The attenuation of a heterogeneous beam is illustrated in
Figure 2.5. The following points are important:
◼◼ Equal thicknesses of material remove different
amounts of radiation.
◼◼ Low‐energy photons are attenuated more rapidly
compared with high‐energy photons. This event
changes both the quantity and quality of the beam.
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22 Rad Tech’s Guide to Radiation Protection
Beam
quantity
Attenuation of a HETROGENEOUS beam of radiation
Beam
quantity
1000
Photons
IN
32
Photons
OUT
(40 kVp)
(57 kVp)
Beam
quality
1 cm
Beam
quality
Figure 2.5 Attenuation of a heterogeneous beam of radiation. See
text for further explanation.
◼◼ The beam quantity decreases and the beam quality
increases. That is, the mean energy of the photons
increases. This increase is referred to as beam hardening, a term used to indicate that the beam is more
penetrating.
Factors Affecting Attenuation
Since attenuation affects the dose to the patient through the
absorption process, it is necessary to examine the factors
that increase attenuation (more patient dose) and factors that
decrease attenuation (less patient dose). These factors include:
◼◼ Thickness of the absorbing material. In general, the
thicker the material, the greater the attenuation.
◼◼ Density (mass per unit volume). As the density of the
material increases, attenuation increases.
◼◼ Atomic number (Z) or number of protons in the
nucleus. As atomic number increases, attenuation
increases. Contrast agents – barium and iodine compounds – increase attenuation and are used to visualize
blood vessels and the gastrointestinal track because their
atomic numbers are different from the surrounding
soft tissue. These compounds attenuate more radiation
rendering them visible on the image.
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Diagnostic X-Rays
23
◼◼ Beam energy. This factor is determined by the kV used
to image the material (patient).
◼◼ Low kV results in more absorption of the beam
(increased attenuation), thus a higher dose to
the patient.
◼◼ High kV results in more transmission of the beam
through the patient (less absorption), thus a lesser
dose to the patient.
To understand how beam energy and attenuation are related,
it is necessary to explain what occurs when X‐rays interact
with matter.
X-Ray Interactions
One of the goals of radiology is to optimize the dose to the patient
without compromising image quality. It is the nature of the interactions of radiation with matter that determines not only image
contrast but also the dose to the patient.
There are several interactions of radiation with matter, including
Rayleigh (coherent) scattering, photoelectric absorption, Compton
scattering, and pair production. Among these, the photoelectric
absorption and Compton scattering are the most important attenuation mechanisms in diagnostic ­radiology. Therefore, only these
two are presented in this chapter. The others do not occur with
any noticeable significance in diagnostic radiology. The physics of
the interactions will not be described but, rather, a basic overview
is presented to demonstrate how these two interactions relate to
dose to the patient, as well as beam energy.
Photoelectric Absorption
Photoelectric absorption (also referred to as the photoelectric
effect) is shown in Figure 2.6. The following points are noteworthy:
◼◼ The incident photon interacts with an inner‐shell (K or L)
electron (tightly bound).
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24 Rad Tech’s Guide to Radiation Protection
M
L
Photoelectron
–
K
Incident
photon
–
Nucleus
Figure 2.6 Photoelectric absorption is an attenuation mechanism
whereby an incident photon interacts with inner-shell electrons,
ejecting them from the atom. See text for further explanation.
◼◼ The photon is completely absorbed and the elec-
tron is ejected from the atom. This electron is called a
photoelectron.
◼◼ The absorption of the incident photons in this interaction increases the dose to the patient as they do not pass
through the patient and reach the image receptor.
◼◼ The probability that the photoelectric effect will occur
depends on the photon energy (E) and the atomic number
(Z) of the absorbing material.
◼◼ The probability is directly proportional to Z3 and inversely
proportional to E3.
◼◼ At low beam energies (low‐kV techniques) the
photoelectric effect predominates and the patient dose
increases. However, image contrast is greater in materials with high atomic numbers.
Compton Scattering
Another X‐ray interaction that is significant in diagnostic radiology is Compton scattering, as shown in Figure 2.7. In Compton
scattering:
◼◼ Incident photons interact with free electrons, which are
loosely bound electrons in the outer shell of the atom.
◼◼ The incident photon is scattered in a new direction with
energy less than that of the incident photon.
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Diagnostic X-Rays
25
Compton
– electron
Nucleus
λ1
Incident
X-ray
photon
–
θ
Angle of deflection
λ2
Scattered
X-ray photon
Figure 2.7 Compton interaction or scattering is an attenuation
mechanism whereby an incident photon interacts with an outer-shell
electron and ejects it from the atom. The incident photon loses its energy
and changes direction of travel. This deflected photon is the scattered
photon that may reach the image detector and destroy image contrast.
◼◼ The incident photon also ejects the electron from its orbit.
This electron is called a recoil or scattered electron.
◼◼ The scattered photons (scattered radiation) reach the film
and degrade image contrast. Compton scattering produces
most of the scattered radiation in diagnostic radiology.
◼◼ The probability that a Compton interaction will occur
depends on the number of outer‐shell electrons and the
photon energy (E).
◼◼ Compton interaction is directly proportional to the
number of outer‐shell electrons and inversely proportional
to E. Characteristics of a Compton interaction are:
◼◼ As the number of outer‐shell electrons (loosely bound
electrons) increases, Compton scattering increases.
◼◼ As the X‐ray energy increases, the probability of
Compton interaction decreases.
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26 Rad Tech’s Guide to Radiation Protection
◼◼ The probability of Compton interaction relative to
photoelectric absorption increases.
◼◼ The probability of penetration of the beam through
the absorber increases.
◼◼ In radiography and fluoroscopy, the patient is the major
source of scattered radiation. Technologists and radiologists must protect themselves from exposure to scatter by
remaining in the control booth or by wearing protective
aprons during the exposure.
Increasing kV and Scatter
Production
High‐kV techniques are used in diagnostic radiology to penetrate the anatomy (as well as contrast agents such as barium) and
to reduce the dose to the patient. High‐kV techniques produce
images with poor image contrast resulting from the increasing
scatter that reaches the image detector.
◼◼ Scatter production in the patient is decreased because
the probability of Compton interaction decreases with
increasing kV.
◼◼ The forward scatter leaving the patient and reaching the
image detector increases because:
◼◼ The fraction of the total scatter produced, traveling
in a forward direction, increases as the kV rises.
◼◼ The mean energy of the scattered radiation
increases, thus less of it is absorbed by the patient.
Therefore, the major reason for an increase in
scatter at the image detector is the increase in the
mean energy of the scatter.
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3
Radiation
Quantities
and Units
Chapter at a Glance
Sources of Radiation Exposure
Types of Exposure
Occupational Exposure
Medical Exposure
Public Exposure
Quantities and Units for Quantifying
Ionizing Radiation
Exposure
KERMA
Absorbed Dose
f-Factor
Linear Energy Transfer
Quantities and Units for Quantifying
Biologic Risks
Dose Equivalent
Radiation Weighting Factor
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28 Rad Tech’s Guide to Radiation Protection
Equivalent Dose
Effective Dose
Tissue Weighting Factor
Radiation Measurement
Ionization Chamber
Film Dosimetry
Thermoluminescent Dosimetry
Optically Stimulated Luminescence (OSL)
Dosimetry
Wearing a Personnel Dosimeter
H
uman beings are exposed to several sources of radiation and subject to different types of radiation exposure.
Quantifying ionizing radiation and biologic risks demands an
understanding of radiation quantities and the units associated
with each of them.
The purpose of this chapter is to identify the two major
sources of radiation exposure and define the situations in which
individuals are exposed to radiation. In addition, several radiation quantities and their units are highlighted.
These topics are important to the technologist for several reasons:
◼◼ Annual dose limits for individuals are based on the types
of exposure.
◼◼ Dose magnitudes from various radiological procedures
are often compared with those from natural background
radiation exposure.
◼◼ Dose limits demand a clear understanding of radiation
quantities and their associated units.
◼◼ Radiation quantities and their units are essential ingredients when quantifying ionizing radiation and biologic risks.
◼◼ Measuring and reporting patient dose in radiology require
the intelligent use of these quantities.
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Radiation Quantities and Units
29
Sources of Radiation Exposure
There are two main sources of radiation exposure:
◼◼ Natural radiation sources
◼◼ Radiation sources of human origin
Natural radiation sources comprise three major categories:
cosmic radiation, earth sources (terrestrial radiation), and
internal sources.
◼◼ Among the natural sources, radon contributes the high-
est exposure to the population. Specifically, radon‐222
(222Rn) is a naturally occurring radioactive gas that arises
from the radioactive decay of radium‐226 (226Ra). Radon
exposure is particularly important to the public since its
intensity depends on where an individual lives. Radon gas
can enter a building. Breathing radon gas can expose the
lungs to alpha particles, which the gas emits. When the
level of radon gas exceeds 4 picocuries per liter (pCi/L) in
a building, the United States Environmental Protection
Agency (EPA) recommends that efforts be made to eliminate or reduce this exposure.
◼◼ Cosmic radiation includes solar and galactic radiation (e.g.
gamma rays, high‐energy protons, neutrons). Cosmic radiation intensity depends on latitude and altitude and is greatest
at the poles and least at the equator. Additionally, cosmic
radiation increases with altitude.
◼◼ Earth sources include radiation from the air, terrestrial
radiation, radiation from buildings, and endogenous
radiation. The last source refers to radiation arising
from several internal body sources, such as carbon‐14,
potassium‐40, rubidium‐87, and strontium‐90. Sleeping
with a partner will increase an individual’s exposure.
Radiation sources of human origin, conversely, consist of
several sources, including medical X‐rays, nuclear medicine,
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30 Rad Tech’s Guide to Radiation Protection
consumer products (e.g. smoke alarms), and other sources such
as fall‐out from nuclear testing.
◼◼ Medical X‐ray exposure represents the greatest source of
exposure to the population.
◼◼ Nuclear medicine examinations result in the second largest
radiation source of human origin compared with all others.
Exposures from medical X‐rays and nuclear medicine
procedures constitute medical exposure, compared with
other types of exposure such as occupational exposure and
public exposure.
Types of Exposure
The International Commission on Radiological Protection
(ICRP) identifies three situations in which individuals are
exposed to radiation.
Occupational Exposure
This situation refers to all exposures received in the workplace by
radiologic technologists. All exposures received when working
in radiography, fluoroscopy, mobile, and operating room radiography, computed tomography, and angiography constitute
occupational exposure. Occupational exposure excludes medical exposure.
Medical Exposure
This situation includes exposures from medical and therapy
­procedures for diagnosis and treatment, respectively. It also
includes exposures resulting from individuals assisting patients
having diagnostic or therapeutic examinations. It does not
include radiation scattered from patients having examinations
or occupational exposure of staff members.
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Public Exposure
The ICRP (2007) states that:
Public exposure encompasses all exposures other than
occupational and medical exposures. The component
of public exposure due to natural sources is by far the
largest, but this provides no justification for reducing
the attention paid to smaller but more readily controlled exposures to artificial sources (p. 34).
Quantities and Units for Quantifying
Ionizing Radiation
There are three radiation quantities: exposure, absorbed dose,
and effective dose. The first two refer to the radiation, and the
third (effective dose) relates to the biologic risk of the radiation
absorption. In this section, we focus only on quantities and units
for quantifying ionizing radiation.
Recently, the ICRP began using the International System of
Units (SI units) for radiation quantities. SI units are now used in
several countries, including the United Kingdom (UK), Canada,
and Australia. In the United States, the National Council of
Radiation Protection and Measurements (NCRP) made the following statement with respect to the use of SI units in radiation protection and measurements: “After 1989, it is recommended that SI
units be used exclusively.” SI units are used throughout this book.
Exposure
Exposure is a radiation quantity referring to the intensity of radiation. Exposure:
◼◼ Can be measured using an ionization chamber, which
contains a volume of air.
◼◼ Ionizes the air in the chamber.
◼◼ Is the total change liberated in a cubic centimeter of air.
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32 Rad Tech’s Guide to Radiation Protection
◼◼ Is measured in coulombs per kilogram (C/kg) in the SI
system and in roentgens (R) in the old system of units.
◼◼ 1R = 2.58 × 10 4 C/kg; 1C = 1.6 × 1019 electrons.
◼◼ Follows the inverse square law, which states that the
intensity of radiation decreases inversely as the square
of the distance. If distance from the source of exposure
is increased by a factor of three, then the exposure is
decreased by a factor of nine:
◼◼ Is equal to the exposure rate multiplied by time. The
exposure rate is the exposure per unit time.
KERMA
KERMA is a quantity that characterizes the radiation field.
◼◼ KERMA is an acronym for kinetic energy released per
unit mass.
◼◼ KERMA quantifies the energy transferred from the radi-
ation beam to charged particles (protons and electrons)
in matter.
◼◼ The unit of KERMA is joules per kilogram (J/kg).
◼◼ KERMA may replace the quantity exposure in the SI system.
Absorbed Dose
Absorbed dose is another quantity to quantify ionizing radiation.
◼◼ Absorbed dose (D) is the energy deposited in an absorbing
medium from ionizing radiation. (Absorbed dose is a measure of the amount of energy absorbed.)
◼◼ The SI unit for absorbed dose is the gray (Gy); the old unit
is the rad (radiation absorbed dose).
◼◼ 1 Gy = 100 rad.
◼◼ 1 rad = 10 mGy.
◼◼ 1 Gy of absorbed dose is equal to 1 J of energy deposited
per kilogram of absorbing medium.
◼◼ Biologic effects are associated with the amount of
absorbed dose.
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Radiation Quantities and Units
33
f-Factor
When it is necessary to calculate the absorbed dose (D) given
only the exposure, the f‐factor is used.
◼◼ D = f × Exposure.
◼◼ f converts roentgens to rads.
◼◼ For radiology, the f‐factor for air and soft tissues is approx-
imately one, although it ranges from four (low energy) to
one (high energy) for bone.
Linear Energy Transfer
The linear energy transfer (LET) is a physical factor quantifying
the radiation beam.
◼◼ LET is the rate at which the radiation transfers energy to
surrounding tissues.
◼◼ The unit of LET is kilo electron volt per micrometer
(keV/μm).
◼◼ The LET for X‐rays is 3.0 keV/μm and 100 keV/μm for 5‐
MeV alpha particles.
◼◼ As LET increases, bioeffectiveness increases.
Quantities and Units for Quantifying
Biologic Risks
Radiation absorption in biologic systems can lead to ­excitation
and ionization. Both excitation and ionization lead to the production of free radicals, which subsequently produce ­biologic
damage.
Several quantities are used to quantify biologic risks of
radiation exposure; these include dose equivalent, equivalent
dose, and effective dose. The current quantity is the
Effective Dose (E).
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34 Rad Tech’s Guide to Radiation Protection
Dose Equivalent
The dose equivalent (H) is a quantity defined for radiation protection purposes. Since different sources of radiation have different efficiencies in producing biologic damage, a quantity is
required to address the differences in bioeffectiveness. The dose
equivalent represents this quantity.
◼◼ Dose equivalent is equal to the absorbed dose (D) multiplied by a quality factor (Q), which depends on the LET
of the radiation.
H DQ
◼◼ The SI unit of H is the sievert (Sv); the old unit is the rem
(rad equivalent man).
◼◼ The badges worn by technologists record occupational
exposure in millisieverts (mSv).
◼◼ The sievert is related to absorbed dose as follows:
Sievert Gray Radiation Weighting Factor WR
◼◼ 1 Sv = 100 rem
◼◼ 1 mSv = 100 mrem
◼◼ 10 mSv = 1 rem
◼◼ For the sake of simplicity (in radiology),
1Roentgen 1rad 1rem
◼◼ In the SI system,
2.58 10 4 C / kg 0.01Gy 0.01Sv
Radiation Weighting Factor
Bioeffects of radiation depend not only on the absorbed dose (D)
but also on the type and energy of the radiation.
◼◼ Radiation weighting factor (W ) is 1 for X‐rays and gamma
R
rays, 5 for high‐energy protons, and 20 for alpha particles
and fission fragments.
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Equivalent Dose
In 1990, the ICRP revised its radiation protection recommendations and the term dose equivalent (H) was replaced by the term
equivalent dose (HT). While H is the weighted absorbed dose at
a point, HT is the weighted absorbed dose in tissues or organs.
◼◼ H = ΣW ·D
T
R
TR
◼◼ The above is read as the equivalent dose is equal to the
sum of the weighted absorbed doses. DTR is the absorbed
dose averaged over the tissue or organ, T, for the type of
radiation, R.
Effective Dose
The effective dose (E) was previously referred to as the effective
dose equivalent (HE).
◼◼ E is the equivalent dose weighted for the type of
tissue (organ).
◼◼ E is used to quantify the different risks from partial body
exposure compared with risks from an equivalent whole‐
body dose.
◼◼ E = ΣW ·H , where W is the tissue weighting factor.
T
T
T
◼◼ The SI unity E is sievert (Sv); the old unit is the rem.
◼◼ The dose limits recommended for occupational, public, students in training, and the embryo or fetus are expressed as E.
◼◼ If the effective dose for an upper gastrointestinal tract
examination is 2.45 mSv, then this value means that the
risk from an upper gastrointestinal tract examination is
equivalent to the risk of an exposure dose of 2.45 mSv to
the whole body.
Tissue Weighting Factor
Bioeffects depend not only on the absorbed dose and type and
energy of the radiation, but also on the type of tissue.
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36 Rad Tech’s Guide to Radiation Protection
◼◼ The tissue weighting factor, W , provides data on the
T
relative contribution of the tissue or organ to the total
biologic response from whole‐body irradiation.
◼◼ The W for the gonads, active bone marrow, breast, and thyT
roid, for example, are 0.20, 0.12, 0.05, and 0.05, respectively.
Radiation Measurement
There are a number of tools currently available for measuring
and monitoring the amount of radiation to an individual. Of
­relevance to diagnostic radiology are the ionization chamber,
film dosimetry, thermoluminescent dosimetry (TLD), and
optically stimulated luminescence (OSL) dosimetry.
Ionization Chamber
The ionization chamber comprises a gas‐filled chamber and is
used to measure radiation exposure.
◼◼ When radiation falls on the chamber, the gas ionizes to
produce ions, which are collected and counted.
◼◼ The total charge (Q coulombs) determines the exposure
expressed in Roentgens or C/kg.
◼◼ Ionization chambers are used as dosimetry devices to
measure the output from an X‐ray tube. These chambers
are also used in automatic exposure timing.
◼◼ A pocket ionization chamber can be used for personnel
dosimetry (i.e. to measure occupational exposures).
Film Dosimetry
Film dosimetry is an older approach used to monitor occupational
exposures in the form of a film badge. Some places may still be
using film dosimetry! In any case:
◼◼ The film badge consists of small X‐ray films placed bet-
ween special filters to detect beta, gamma, and X‐rays.
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37
◼◼ These badges can detect occupational exposure at or
above 0.1 mSv (10 mrem) and are not sensitive enough to
detect lower levels of exposure.
◼◼ Film dosimetry is being replaced by TLD.
Thermoluminescent Dosimetry
Thermoluminescent dosimetry can be used to measure patient
exposures and to monitor personnel occupational exposures.
◼◼ TLD is based on thermoluminescence.
◼◼ Lithium fluoride (LiF) chips are used in diagnostic
radiology.
◼◼ When exposed to X‐rays, electrons in the LiF are raised to
another energy level and are trapped there until the TLD
chip is heated. This heating causes the electrons to return
to their original orbits causing the emission of light. The
amount of light emitted is directly proportional to the
radiation exposure.
◼◼ TLD can measure doses from 0.1 mGy (10 mrad) to
approximately 10 Gy (1000 mrad).
Optically Stimulated Luminescence
(OSL) Dosimetry
The OSL dosimeter was developed in the 1990s by Landauer
for the purpose of monitoring staff in an environment in which
they are exposed occupationally. The dosimeter contains an
aluminum oxide (Al2O3) detector. There are three important
steps to using this dosimeter. First, when the Al2O3 detector is
exposed to radiation, electrons are raised into an excited state.
Secondly, during processing, a laser light stimulates the electrons trapped in the excited state and causes them to return to
their ground state. This process results in light being emitted.
Finally, this light is measured and is found to be proportional to
the dose falling upon the OSL. The OSL as a personnel monitor
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38 Rad Tech’s Guide to Radiation Protection
is more sensitive that the TLD dosimeter, since it can detect very
small doses, such as 10 μGy, for example.
Wearing a Personnel Dosimeter
In radiography and fluoroscopy, technologists must wear personnel dosimeters to record their occupational exposures in
millisieverts.
◼◼ In radiography, the dosimeter can be worn at the level of
the waist, at the level of the collar in the upper chest area,
or on the anterior surface of the individual.
◼◼ In fluoroscopy, when a protective apron must be worn,
the NCRP (1989) states:
When the apron is worn, a decision must be made as
to whether to wear one or more than one dosimeter.
If only one is worn and it is worn under the apron, it
can represent the dose to most internal organs, but
it may underestimate the dose to the head and neck
(including the thyroid gland). If only one is worn and
it is worn at the collar, it may represent the dose to the
organs contained in the head and neck, but it may
overestimate the dose to the organs in the trunk of the
body (p. 49).
◼◼ Health Canada Radiation Safety Code SC 35 (2008) states
that for diagnostic radiology, the dosimeter must be worn
under the apron; and when the radiation levels are considered to be high, additional dosimeters should be worn
on the extremities.
As noted by Bushong (2017), “we assume the occupational
­effective dose to be 10% of the monitor dose. Assuming the effective dose of 10% of the occupational monitor dose is conservative.
In actual fact, it is something less than 10%.”
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4
Basic
Radiobiology
Chapter at a Glance
What Is Radiobiology?
Essential Physics and Chemistry
Basic Physics
Basic Chemistry
Fundamental Concepts of Radiobiology
Types of Bioeffects
Radiosensitivity
Dose-Response Relationships
The LNT Model Debate Concerns
Radiation Effects on DNA and Chromosomes
Target Molecule
Target Theory
Direct and Indirect Effects
DNA Damage
Chromosome Damage
Deterministic Effects (Early Effects of Radiation)
LD50/60
Acute Radiation Syndromes
Rad Tech’s Guide to Radiation Protection, Second Edition. Euclid Seeram.
© 2020 John Wiley & Sons Ltd. Published 2020 by John Wiley & Sons Ltd.
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40 Rad Tech’s Guide to Radiation Protection
Stochastic Effects (Late Effects of Radiation)
Tissue Effects
Life-Span Shortening
Radiation-Induced Cancers
Hereditary Effects
Radiation Exposure During Pregnancy
I
t is well known that radiation can cause biologic damage,
as derived from both animal and human studies. In diagnostic radiology, the benefits associated with radiation
exposure far outweigh any risks to the patient. These radiation risks (bioeffects) have received increasing attention since
patients received more radiation from diagnostic X-ray examinations than from any other source of radiation exposure
(Chapter 3).
The purpose of this chapter is to review the fundamental
concepts relating to the manifestation of biologic damage and
to describe the early and late effects of radiation exposure.
Specifically, radiobiology will be defined and sources of data
on the bioeffects of radiation will be identified. This section will
be followed by a description of radio sensitivity, dose response
models, effects of radiation on cells and water, direct and indirect
action of radiation, and the target theory. Finally, the chapter
will conclude with a summary of the early and late effects of
radiation.
It is essential that technologists have a firm understanding
of these topics because:
◼◼ Current radiation protection standards and recommen-
dations are based on biologic effects.
◼◼ The dose delivered to the patient in radiology can vary
from low, to moderate-to-high, depending on the type of
examination.
◼◼ Dose-response models used in diagnostic radiology suggest that there is no risk-free dose of radiation.
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Basic Radiobiology
41
What Is Radiobiology?
Radiation transfers energy to biologic systems. The energy
is absorbed and subsequently produces a sequence of events
leading to biologic expression of harm.
◼◼ Radiobiology is the study of effects of radiation on bio-
logic systems.
The effects occur at the:
◼◼ Molecular level and include physical processes such as
ionization and excitation of atoms, and chemical interactions such as the production and reaction of free radicals.
◼◼ Cellular level, where damage to DNA and chromosomes
occurs, leading to damage at the tissue and organ levels
(e.g. atrophy).
◼◼ Whole-body levels, where the responses are categorized
as early and late effects.
This chapter reviews the mechanisms leading to these effects.
Essential Physics and Chemistry
Biologic systems are made up of atoms. The interaction of radiation with a living system requires an understanding of a number
of topics in physics and chemistry.
Basic Physics
The physics topics that are of relevance to radiobiology and are
of importance to technologists in diagnostic radiology include
atomic structure, the nature and properties of X-rays, ionization,
excitation, linear energy transfer (LET), and relative biologic
effectiveness (RBE).
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42 Rad Tech’s Guide to Radiation Protection
It is not within the scope of this text to describe any of these
in detail; the student must refer to radiological physics textbooks
for elaboration. The following points, however, are noteworthy:
◼◼ The atom consists of a nucleus positioned at the center of
electrons, which occupy specific orbits around the nucleus.
Electrons close to the nucleus are tightly bound while electrons far away from the nucleus are loosely bound and can
easily be removed from the atom (ionization).
◼◼ The nature of X-rays in terms of production and interaction with matter was the subject of Chapter 2. Essentially,
X-rays can produce ionization and excitation of atoms in
biologic systems.
◼◼ Ionization is the removal of an electron from an atom.
Ionization results in ion pairs, the electron that has been
removed and the positively charged atom remaining.
Outer-shell electrons are easily removed from the atom,
as they are loosely bound to the nucleus.
◼◼ Excitation is a process by which electrons are transferred
into orbital levels farther away from the nucleus as a result of
energy absorption. Electrons are not removed from the atom.
◼◼ Both ionization and excitation can lead to bioeffects.
However, the mechanism of excitation is not fully
understood.
◼◼ Linear Energy Transfer (LET) is the efficiency of radiation
to produce ionization and excitation. Specifically, LET
measures the rate at which energy is transferred from
the radiation to the living system. The units of LET are
kilo-electron volt per micrometer of length in soft tissue.
As LET increases, biologic damage increases. The LET for
X-rays used in radiology is approximately 3.0 keV/μm; for
alpha particles, the value is 300 keV/μm.
◼◼ Relative Biologic Effectiveness (RBE) is the efficiency with
which different types of radiation can cause damage
to biologic systems. Specifically, the RBE is a ratio of a
standard (200–250 kV X-rays) radiation dose required to
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43
produce a given biologic effect to the test radiation dose
required to produce the same effect. The RBE for diagnostic X-rays is 1.0. As LET increases, RBE increases since
high LET produces more ionization compared with low
LET radiation.
Basic Chemistry
In diagnostic radiology, most of the interactions of radiation and
the patient occur with water, since the body contains 70–85%
water. This interaction results in ionization of water, forming
ion pairs and free radicals.
◼◼ Radiolysis of water refers to the breakdown of water by
radiation leading to the followingchemical reactions:
◼◼ H O + radiation → H O+ + e−
2
2
◼◼ e− + H O → H O−
2
2
◼◼ Two ions (H O+ and H O −) are the by-products of the
2
2
initial interaction.
Each of these two ions is unstable and exists for only a
short time. Each will dissociate as follows:
◼◼ H O+ → H+ + OH
2
◼◼ H O− → H + OH−
2
◼◼ Now, there are two ions: a hydrogen ion (H+) and a
hydroxyl ion (OH−); and two free radicals: a hydroxyl free
radical (OH ) and a hydrogen free radical (H ).
◼◼ A free radical, symbolized by a dot (●), as shown in
the above reactions, is an atom or a molecule with an
unpaired electron in the outermost orbit.
◼◼ Free radicals are highly unstable chemical species and
can react to form other chemical species that are harmful to the cell.
◼◼ Two ions, H+ and OH−, can recombine as follows:
◼◼ H+ + OH+ → H O
2
◼◼ Free radicals can also recombine as follows:
◼◼ H + OH → H O
2
●
●
●
●
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44 Rad Tech’s Guide to Radiation Protection
◼◼ In the last two reactions, water is formed and there is no
damage to the cell. However, free radicals can react as
follows to form other molecules that are toxic to the cell:
◼◼ OH + OH → H O
2 2
◼◼ H + O → HO
2
2
◼◼ H O is hydrogen peroxide, which is toxic to the cell.
2 2
◼◼ HO is hydroperoxyl free radical, which is highly reac2
tive and can combine with biologic macromolecules to
produce more and more free radicals.
●
●
●
●
●
Fundamental Concepts
of Radiobiology
There are several concepts of radiobiology that are important
to the technologist and that provide a foundation for a good
understanding of the bioeffects of radiation.
Types of Bioeffects
Bioeffects can be stochastic or deterministic effects:
◼◼ Stochastic effects are those for which the probability of
occurrence increases with dose and for which there is
no threshold dose. Any dose of radiation, however small,
has the potential to cause biologic harm. There is no riskfree dose.
◼◼ Deterministic effects are those for which the severity of the
effect increases with increasing dose and for which there
is a threshold dose.
Bioeffects can also be somatic and genetic effects:
◼◼ Somatic effects are those that occur in the individual
exposed to the radiation.
◼◼ Genetic effects are hereditary effects and occur in the off-
spring of the individual exposed to radiation.
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45
Additionally, bioeffects can be discussed as early and late effects.
◼◼ Early effects appear minutes, hours, days, weeks, or
months after exposure to high doses of radiation. Early
effects are deterministic effects.
◼◼ Late effects occur years after exposure to low doses of radiation. Late effects are stochastic effects.
Early and late effects of radiation are reviewed further in a later
section of this chapter.
Radiosensitivity
The radiation quantity (effective dose) takes into consideration
the sensitivity of tissues to radiation exposure. This sensitivity is
referred to as radiosensitivity. In 1906, Bergonie and Tribondeau,
two French scientists, performed experiments to demonstrate
this radiosensitivity of various tissues. Their results generated the
law of Bergonie and Tribondeau, which indicates the following:
◼◼ Immature cells (stem cells) are more radiosensitive than
mature cells (end cells).
◼◼ Radiosensitivity is directly proportional to the prolifera-
tion rate for cells and the growth rate for tissues. As both
rates increase, radiosensitivity increases.
◼◼ Young tissues and organs are more radiosensitive than
older tissues and organs.
As noted by Bushong, this law “serves to remind us that the
fetus is considerably more sensitive to radiation exposure than
the child or the mature adult” (2017).
Radiosensitivity also varies with the phases of the cell cycle
as follows:
◼◼ The most radiosensitive phase is mitosis (M), which
includes prophase, metaphase, anaphase, and telophase.
◼◼ G -phase (post-DNA synthesis) is also extremely sensitive.
2
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46 Rad Tech’s Guide to Radiation Protection
◼◼ The most radioresistant phase is the S-phase (DNA-
synthesis phase),
Certain cell types are more radiosensitive than others. For example,
◼◼ Lymphocytes, spermatogonia erythroblast, and the
crypt cells of the gastrointestinal tract are extremely
radiosensitive.
◼◼ Endothelial cells, osteoblasts, and fibroblasts, for example,
have moderate radiosensitivity.
◼◼ Muscle cells and nerve cells have low radiosensitivity.
Radiosensitivity for tissues and organs is as follows:
◼◼ Lymphoid tissue, bone marrow, and the gonads have a
high level of radiosensitivity.
◼◼ The skin, gastrointestinal tract, cornea, growing bone,
kidney, liver, and thyroid are moderately radiosensitive.
◼◼ Muscle, brain tissue, and the spinal cord have a low level
of radiosensitivity.
Dose-Response Relationships
A dose-response relationship shows the relationship between a
biologic response (bioeffect) as a function of radiation dose. Two
popular linear relationships include:
◼◼ Linear dose-response relationship without a threshold.
This relationship shows that as the dose increases, the
­biologic response increases. This relationship also indicates that there is no risk-free dose (i.e. no dose of radiation is considered safe).
◼◼ Linear dose-response relationship with a threshold.
This relationship shows that there is a level of dose – the
threshold dose (D T ) – below which no response is
observed. At the threshold dose, a response is observed
and it (the response) increases as the dose is increased.
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47
Radiation protection standards and guidelines in diagnostic
radiology are based on the linear dose–response relationship
without a threshold.
The Linear Nonthreshold (LNT) Model
Debate Concerns
A recent literature review by Tran and Seeram (2017) examining
the debate over the LNT model shows the following:
However, increasing biological and epidemiological
studies have raised doubts on the validity of
the model especially at low levels of radiation
(<100 mSv), whereby no definitive effects have been
demonstrated in humans. A review of literature
was conducted using Ovid Medline and Scopus
databases to evaluate the controversy surrounding
the use of the LNT model and the current perspective
of radiation protection organisations on its use.
The literature debate consists of arguments against
the data obtained from epidemiological studies as
well as the consequence effects of the LNT model on
the public. In response, alternative dose response
models that contradict the accepted LNT model,
especially at low doses, have been suggested. These
include hormesis, hypersensitivity and threshold
models. However, there remains a need for continued
research on the effects of low doses radiation on
specific organs and tissues to further quantify risk
estimates. Further knowledge and understanding
of these effects will allow for improved radiation
protection for patients.
Radiation Effects on DNA and Chromosomes
The human cell consists of two major components: the central
nucleus surrounded by the cytoplasm.
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48 Rad Tech’s Guide to Radiation Protection
Target Molecule
Compared with the cytoplasm, the nucleus is more radiosensitive because:
◼◼ It contains target molecules that are essential for
cell survival.
◼◼ The target molecule is DNA, which is present in all ge-
netic and somatic cells in relatively small quantities.
◼◼ Experiments have shown that lower doses of radiation
can cause the cell to die when the nucleus (rather than
the cytoplasm) is irradiated.
Target Theory
The target theory states that inactivation of the critical target
molecule (DNA) after irradiation will cause the cell to die.
Direct and Indirect Effect
Radiation interaction with a cell can be by either direct or
indirect action.
◼◼ Direct action occurs when the radiation interacts
directly with the critical target to cause a series of events (e.g.
ionization) leading to changes that are damaging to the cell.
◼◼ Indirect action occurs when radiation interacts with other
molecules leading to free radicals, which subsequently
interact with the critical target molecule to deactivate it.
DNA Damage
DNA is made up of two strands forming a double helix composed
of sugar base pairs of adenine, guanine, cytosine, and thiamine.
Irradiation of DNA can result in the following events:
◼◼ Single-strand breaks or double-strand breaks.
◼◼ Both breaks can be repaired.
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49
◼◼ Single-strand breaks are less damaging than double-
strand breaks, which can lead to cell death.
◼◼ Loss or change of base that leads to genetic mutations.
◼◼ Interstrand crosslink resulting in a separation of bases.
The previously mentioned lesions produced in DNA by irradiation result in cell death, malignant disease, and genetic effects.
Chromosome Damage
Chromosomes contain DNA, and damage to DNA leads to
chromosome damage. Chromosome damage is also referred to as
chromosome aberrations (or breaks). These aberrations can be:
◼◼ Single-hit aberrations, which can occur via the direct or
indirect effect. The hit, which is an interaction of radiation
with the chromosome, will cause a noticeable derangement of the chromosome. Examples of single-hit aberrations include chromatid breaks and chromosome deletions.
◼◼ Multi-hit aberrations, which result in dicentrics, ring
chromosomes, multicentrics, and reciprocal translocations.
Experiments have demonstrated that these chromosome aberrations lead to malignancies such as Burkitt’s lymphoma, acute
promyelocytic leukemia, and ovarian cancer. Chromosome deletions, specifically, lead to small cell lung cancer, neuroblastoma,
retinoblastoma, and Wilms’ tumor.
Deterministic Effects (Early Effects
of Radiation)
When the whole body is exposed to high doses of radiation
(greater than 1 Gy), several bioeffects are observed, depending
on the dose. These effects are called early effects as they can
occur within minutes, hours, days, weeks, and months after
the exposure.
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◼◼ Early effects of radiation are considered deterministic
effects because the severity of the effect depends on the
dose. These effects have a threshold dose and increase
with increasing dose.
◼◼ Death is the major effect of whole-body exposure to high
doses of radiation.
LD50/60
The dose required to kill 50% of the human population in 60 days
is termed the lethal dose 50/60 (LD50/60). For humans the LD50/60
is 3.5 Gy (350 rad).
Acute Radiation Syndromes
Exposure to large doses of radiation to the whole body will result
in the following syndromes:
◼◼ Bone marrow syndrome, which occurs after an acute
whole-body exposure of 2–10 Gy. The dose affects the
stem cells in the bone marrow and other blood-forming organs.
◼◼ Gastrointestinal syndrome, which occurs after an acute
whole-body exposure of doses between 10 and 100 Gy.
Damage to the crypt cells of the small intestines will cause
death, which occurs much faster than that caused by the
bone marrow syndrome.
◼◼ Central nervous system (CNS) syndrome, which is also
referred to as the cerebrovascular syndrome and requires
doses in excess of 100 Gy. Death results because blood
vessels in the brain are damaged, leading to edema that
causes an increase in skull pressure.
Early or deterministic effects can occur in local tissues,
such as the skin and gonads, the hemopoietic system, and cell
chromosomes.
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51
◼◼ Damages to the skin include skin erythema (reddening)
and epilation (loss of hair), which is a result of damage to
the basal cells of the epidermis.
◼◼ The principal effect of high doses of radiation on the
gonads (ovaries and testes) is atrophy (reduction in size).
In women, the oocyte is highly radiosensitive. Radiation
exposure of the ovaries can delay or suppress menstruation, as well as cause temporary or permanent sterility.
A dose of 2 Gy will cause temporary sterility; 5 Gy will
cause permanent sterility.
◼◼ In men, the spermatogonia are most radiosensitive
compared with spermatocytes and spermatids. A dose
of 100 mGy decreases the spermatozoa count. A dose
of 2 Gy produces temporary sterility; 5 Gy will produce
permanent sterility. Spermatogonia are also considered
to be among the most radiosensitive of body cells (as well
as lymphocytes).
◼◼ The blood system or hemopoietic system is made up of
the bone marrow, lymphoid tissue, and circulating blood.
The main effect of radiation on this system is to reduce the
number of blood cells in the peripheral circulation. Among
the blood cells, lymphocytes are the most radiosensitive.
◼◼ Chromosomal damage includes single- and multiplechromatid breaks, ring chromosomes, and dicentrics, as
well as reciprocal translocations.
Stochastic Effects (Late Effects
of Radiation)
Bioeffects that occur years after exposure to low doses of radiation (less than 0.25 Gy) are considered stochastic effects or
late effects.
Stochastic effects are those for which the probability of
the effect increases as the dose increases and for which there
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is no threshold dose. For stochastic effects, there is no riskfree dose.
Stochastic effects can occur at the local tissue level and can
cause life-span shortening, radiation-induced malignancy, and
hereditary effects.
Tissue Effects
These effects are nonmalignancies that can appear in the skin,
eyes, and chromosomes.
◼◼ “The skin develops a weathered, callused, and discolored
appearance” (Bushong 2017).
◼◼ Damage has been observed in the chromosomes of
circulatory lymphocytes.
◼◼ Cataracts have also been observed in the eye as a late
effect of radiation.
It is important to note that these effects are not significant
in diagnostic radiology, except for skin damage from patients
who may have lengthy interventional angiographic procedures.
Life-Span Shortening
There is no need for radiologists and technologists working in
diagnostic radiology to be concerned about life-span effects.
Radiation-Induced Cancers
Cancer induction is the most significant stochastic effect of radiation. The evidence stems from radium dial painters, uranium
miners, atomic bomb survivors, patients exposed to radiation
in fluoroscopy, and for ankylosing spondylitis.
Additionally, cancer induction is associated with tissues and
organs that are highly radiosensitive, such as the lymphoid tissue,
bone marrow, the breast and gonads, and the gastrointestinal tract.
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Radiation-induced cancers include:
◼◼ Leukemia. The incidence of leukemia is dependent on the
dose (incidences increase with increased dose). Leukemia
may appear 2–3 years after exposure.
◼◼ Lung cancer
◼◼ Breast cancer
◼◼ Thyroid cancer
◼◼ Bone cancer
◼◼ Liver cancer.
Hereditary Effects
Hereditary effects are stochastic or late effects that occur in the
offspring of the irradiated individual.
Irradiation of germ cells (spermatozoa or ova) can result in:
◼◼ Chromosomal mutations
◼◼ Gene mutations.
Genetic effects of radiation have been studied extensively
in animals, the results of which have been extrapolated to
humans because the data for humans are limited. As Travis
(1997) points out:
◼◼ Most mutations are harmful.
◼◼ Any dose of radiation, no matter how small, can cause
genetic changes.
◼◼ There is a linear relationship between dose and the
number of mutations, thus information for low doses can
be extrapolated from high-dose data.
◼◼ Humans are not more sensitive than mouse and may, in
fact, be less sensitive.
◼◼ Mouse data can provide reasonable estimates of the risk
of radiation-induced genetic effects in humans.
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Radiation Exposure
During Pregnancy
The human embryo and fetus are highly sensitive to radiation,
thus every effort must be made to protect them from unnecessary radiation exposure. A recent review on “Ionizing Radiation
Exposure During Pregnancy: Effects on Postnatal Development
and Life” by Sreetharan et al. (2017) and published in Radiation
Research Journal provides the following summary:
There is a major consensus that the atomic bomb
survivors’ data shows increased incidence of microcephaly and reductions in IQ of A-bomb survivors,
whereas with diagnostic radiography in utero there
is no conclusive evidence of increased cancer risk.
Due to the relatively limited data (particularly for
low-dose exposures) in humans, animal models have
emerged as an important tool to study prenatal effects
of radiation. These animal models enable researchers
to manipulate various experimental parameters and
make it possible to analyze a wider variety of end
points. In this review, we discuss the major findings
from studies using mouse and rat models to examine
prenatal ionizing radiation effects in postnatal
development of the offspring. In addition, we broadly
categorize trends across studies within three major
stages of development: pre-implantation, organogenesis and fetal development. Overall, long-term effects
of prenatal radiation exposure (including the possible
role on the developmental programing of disease) are
important factors to consider when assessing radiation risk, since these effects are of relevance even in
the low-dose range.
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5
Current
Standards
for Radiation
Protection
Chapter at a Glance
Radiation Protection Organizations
Objectives of Radiation Protection
Radiation Protection Criteria and Standards
Radiation Protection Principles
Radiation Protection Actions
Recommended Dose Limits
Dose Limits for Occupational Exposure
Dose Limits for Pregnant Workers
Dose Limits for Members of the Public
Diagnostic Reference Levels: A Useful Tool for
Optimization of Protection
I
n Chapter 4, the bioeffects of radiation were described. In
summary, there are two types of bioeffects:
◼◼ Deterministic effects. These are effects (nonstochastic)
for which the severity of the effect increases as the dose
increases, and for which there is a threshold dose; below
this threshold, no effect occurs. Examples include skin
reddening (erythema), epilation (loss of hair), cataracts,
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56 Rad Tech’s Guide to Radiation Protection
and impairment of fertility. These effects require high
doses of radiation, greater than 0.5 Gy (50 rad), and are
also referred to as early effects of radiation.
◼◼ Stochastic effects. These are effects for which the probability of occurrence increases with increasing dose and for
which there is no threshold dose, meaning that there is no
risk‐free dose. With doses of less than 0.5 Gy (50 rad), cancer and genetic damage are concerns. Stochastic effects
are also referred to as late effects, among which radiation‐
induced malignant disease (which may appear years after
the exposure) is of major concern in diagnostic radiology.
The purpose of this chapter is to outline the essential elements
of the steps required to prevent deterministic effects and minimize stochastic effects. In other words, this chapter examines
the objectives of radiation protection and the two notable triads
that serve as a general framework for radiation protection.
Radiation Protection Organizations
There are several radiation protection organizations responsible
for providing guidelines and recommendations on radiation
protection. While some of these address radiation risks, others
are devoted to radiation protection based on radiation risk data.
Important organizations include:
◼◼ The International Commission on Radiologic Protection
(ICRP), founded in 1928, issued Publication 26 in 1977.
Subsequently in 1990, the commission issued Publication
60, which provides updated recommendations to
Publication 26. These two reports provide the basis for
regulations and recommendations, not only in the United
States and Canada but also in other countries as well.
◼◼ The International Commission on Radiological Units and
Measurements (ICRU), established in 1925, deals primarily
with radiation quantities and units and measurement
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57
t­ echniques. This commission ensures uniform reporting
of data and information on radiation risks and protection.
◼◼ The United Nations Scientific Committee on the Effects of
Atomic Radiation (UNSCEAR) is a committee that issues
reports concerning the risks associated with radiation.
◼◼ The Biological Effects of Ionizing Radiation Committee
(BEIR) also deals with radiation risks. In 1990, the BEIR
issued Report V, which provides updated information on
radiation risks. This report states that the risks of radiation
are 3–4 times greater than BEIR had previously estimated.
It was this finding that led the ICRP to revise its radiation
protection guidelines and recommendations in 1990.
◼◼ The National Council on Radiation Protection and
Measurements (NCRP) is a United States organization
that advises federal and state regulators on matters of
radiation protection.
◼◼ The Food and Drug Administration (FDA) in the United
States regulates the design and manufacture of X‐ray
equipment; the regulations are in Title 21 of the Code of
Federal Regulations (CFR).
◼◼ In Canada, the organization responsible for radiation protection in diagnostic radiology is the Radiation Protection
Bureau, Health Canada (RPB‐HC). For diagnostic radiology,
recommendations are stated in Safety Code 35: Radiation
Protection in Radiology‐Large Facilities. Safety Procedures
for the Installation, Use, and Control of X‐Ray Equipment in
Large Medical Radiological Facilities
Objectives of Radiation Protection
The objectives of a radiation protection are twofold:
◼◼ To prevent the occurrence of deterministic effects by
ensuring that doses are kept below the threshold levels.
◼◼ To minimize the induction of stochastic effects by using a
radiation‐protection philosophy that ensures that ­radiologic
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examinations are optimized to produce the best image
quality with the minimum radiation dose to the patient.
A radiation protection philosophy that forms the basis of
current radiation protection criteria and standards can achieve
these objectives.
Radiation Protection Criteria
and Standards
Radiation protection criteria and standards are based on
two triads:
◼◼ Radiation Protection Principles
◼◼ Radiation Protection Actions
Radiation Protection Principles
The ICRP advocates the following three principles of radiation
protection that are currently used by the NCRP and the RPB‐
HC (and other radiation protection organizations around the
world) to provide guidelines for the safe use of radiation when
imaging patients:
◼◼ Justification. There should be a net benefit associ-
ated with exposure to ionizing radiation in light of the
known risks of radiation. As pointed out by Wolbarst et al.
“Justification provides an essential moral stance for the
intelligent use of radiation” (2013).
◼◼ Optimization – ALARA. The principle of optimization ensures that doses are kept As Low As Reasonably
Achievable (ALARA), with economic and social factors
taken into account. The ICRP also uses the term optimization of radiation protection (ORP). ALARA and ORP
are synonymous. ALARA dictates that technologists
­optimize radiologic examinations by practicing the guidelines and recommendations for the safe and prudent use
of radiation when imaging patients. Essentially, ALARA
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deals with the technical elements of radiation protection,
which are intended to ensure optimal image quality using
minimal radiation doses. These technical elements are
described later in Chapter 9.
◼◼ Dose limitation. This principle addresses the legal limits
on the radiation dose received per year or accumulated
over a working lifetime for persons who are occupationally exposed and for others as well. There are no limits
for patients. The ICRP and national radiation protection organizations, such as the NCRP and RPB‐HC,
have established these limits. Additionally, these limits
are intended to minimize stochastic effects to acceptable
levels and certainly to prevent deterministic effects since
they are well below threshold doses.
Radiation Protection Actions
These actions are the second triad of radiation protection and
include time, shielding, and distance.
◼◼ Time. Because the exposure dose is directly proportional
to the time of exposure, keeping the time as short as possible will reduce the dose to the individual exposed.
◼◼ Shielding. This aspect involves the use of materials such
as concrete or lead between the source of the radiation
exposure and the individual exposed. Lead is used in
aprons and in the walls of X‐ray rooms. The lead will
attenuate the radiation, thus reducing the intensity of
the beam. Lead aprons are worn by radiation workers in
fluoroscopy, for example, and are also used for gonadal
shielding.
◼◼ Distance. Distance is governed by the inverse square law
stated as:
1
I
d2
where I represents the radiation intensity and d is the distance between the individual exposed and the source of
radiation. The law states that the radiation intensity (dose)
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60 Rad Tech’s Guide to Radiation Protection
is inversely proportional to the square of the distance.
Therefore, the dose decreases as the distance increases. If
the distance from the radiation source is doubled, then the
dose decreases by a factor of four. This law has applications
in mobile and operating room radiography and in fluoroscopy. Because the patient is the main source of scatter, the
technologists performing mobile radiography and fluoroscopy should stand as far away as possible from the patient
when exposing the patient to the radiation beam.
For mobile radiography units, the NCRP recommends that the
length of the exposure cord be at least 2 m in length. In Canada,
the RPB‐HC recommends a length of 3 m. These lengths allow
the technologist to stand between 2 and 3 m from the patient
during mobile work.
Recommended Dose Limits
As stated earlier in this chapter, dose limitation is an important
element in radiation protection. Dose limitation is based on the
fact that radiation can cause biologic harm. That is, radiation can
produce stochastic and deterministic effects. The following aspects
regarding dose limitation are significant. Dose limits have been set:
◼◼ To minimize the risks of stochastic effects.
◼◼ Using the linear dose‐response model without a threshold.
◼◼ For occupational and nonoccupational exposures.
◼◼ By the ICRP and the NCRP (United States) and the RPB
(Health Canada) and are expressed as effective dose (E).
Dose Limits for Occupational Exposure
Occupational exposures are incurred by virtue of work. For
example, technologists and radiologists are occupationally
exposed. The dose limits for occupational exposure have been set
for uniform whole‐body exposure and individual organs, excluding
exposures from medical procedures and from natural background.
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Technologists wear dosimeters to measure their occupational
exposures. Although the dosimeters do not measure the effective dose (E), “for regulatory compliance it is considered to be
E” (Bushong 2017).
◼◼ The ICRP recommended dose limit for occupational
exposure is 20 mSv/year averaged over defined periods
of five years.
◼◼ The ICRP recommended dose limits for the lens of the
eye, skin, and hands and feet, are 150, 500, and 500 mSv/
year, respectively.
◼◼ The dose limits recommended by the NCRP (1993) for
occupational exposures are 50 mSv/year; while it is
10 mSv × age of the individual for a cumulative dose limit.
◼◼ The NCRP (1993) recommended annual dose limits for
the lens of the eye and for the skin, hands, and feet are
150 and 500 mSv, respectively.
◼◼ The NCRP recommended annual dose limits for students
(under the age of 18) in training is 1 mSv; while it is 15
and 50 mSv/year for the lens of the eye and for the skin,
hands, and feet, respectively. Students above the age of 18
are subject to the same limits for occupational exposure.
◼◼ The RPB (Health Canada) recommended annual dose
limits for occupational exposures are the same as the
ICRP limits.
Dose Limits for Pregnant Workers
With regard to pregnant workers who are in the occupational
exposure category, the fetus is “normally considered to be a
member of the public” (Huda and Slone 1995). Pregnant workers
wear dosimeters under the apron to monitor dose limits to the
embryo‐fetus. Bushong notes that:
◼◼ “Once pregnancy is known, the recommended dose limit
for the embryo‐fetus takes precedence over the dose limit
for the pregnant radiation worker” (2017).
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62 Rad Tech’s Guide to Radiation Protection
◼◼ The NCRPs recommended dose limits for the fetus is:
◼◼ 0.5 mSv/month
◼◼ 5.0 mSv for the 9‐month period
◼◼ The ICRPs recommended dose limit for the fetus
(9 months) is 1 mSv.
Dose Limits for Members of the Public
Members of the public are individuals who accompany patients
having X‐ray examinations and are waiting in a room in close
proximity to an X‐ray room. In general, recommended dose
limits for these individuals are less than that of occupationally
exposed individuals.
◼◼ The ICRP annual dose limits for members of the public
is 1 mSv; while it is 15 and 50 mSv for the lens of the eye
and for the skin, hands, and all other organs, respectively
◼◼ For persons who receive continuous or frequent exposures, the NCRP annual limit is 1 mSv; for persons who
receive infrequent exposures, the limit is 5 mSv.
◼◼ The NCRP annual dose limits for the lens of the eye and
for the skin, hands, and feet (for members of the public)
are 15 and 50 mSv, respectively.
In this respect, radiology facilities must be shielded (i.e. the
walls of X‐ray rooms should be lined with lead to ensure that
the dose limits to the public are not exceeded).
Diagnostic Reference Levels: A Useful
Tool for Optimization of Protection
As described previously, dose limits have been established for
occupationally exposed individuals such as radiologists and
technologists, pregnant workers, and members of the public.
What about exposure limits for the patient? In this regard the
ICRP, as early as 1990, introduced the notion of Diagnostic
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Reference Levels (DRLs), and recommends the use of DRLs for
patients. Dose limits and DRLs are not the same thing.
The DRL has been defined by several organizations; however, only two, the ICRP and the American College of Radiology
(ACR) definitions, are presented here for the sake of brevity. The
ICRP states that DRLs “are a form of investigation level, applied
to an easily measured quantity, usually the absorbed dose in air,
or tissue‐equivalent material at the surface of a simple standard phantom or a representative patient.” The ACR, on the
other hand, defines a DRL as “an investigation level to identify
unusually high radiation dose or exposure levels for common
diagnostic medical X‐ray procedures.”
All the definitions reflect that the DRL is a means to optimize
the radiation dose to the patient, and it “is to provide a benchmark for comparison, not to define a maximum or minimum
exposure limit.” Additionally, the ICRP states that DRLs “apply
to medical exposure, not to occupational and public exposure,
thus they have no link to dose limits or dose constraints … The
values should be selected by professional medical bodies, and
renewed at intervals that represent a compromise between the
necessary stability and long‐term changes in the observed dose
distributions. The selected value will be specific to a country
or region.” DRLs are tools radiology departments can use to
measure and assess radiation doses to patients for a defined set
of procedures. If the doses delivered are consistently greater
than established DRLs for that facility’s country or region, then
the department should be concerned about its radiation protection procedures, investigate why exposures are beyond the
established DRLs, and take corrective action.
The details of how DRLs are established is not be described
in this text. However, the reader is encouraged to consult any
good radiologic physics textbook for a comprehensive explanation. The following points are noteworthy:
◼◼ The general purpose of DRLs is to address patient dose
using the principle of ORP.
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64 Rad Tech’s Guide to Radiation Protection
◼◼ The following points summarize existing ICRP guide-
lines for DRLs:
◼◼ The DRL is an advisory, not a regulatory, measure.
It is not related to dose limits established for radiation workers and members of the public.
◼◼ The DRL is intended to identify high levels of radiation doses to patients.
◼◼ The DRL applies to common examinations and
specific equipment.
◼◼ Dose quantities and techniques should be easy to
measure (e.g. the entrance skin exposure).
◼◼ The DRL selection is established by professional
organizations, using a percentile point on the
observed distribution for patients, and specific
to a country or region. One pragmatic method is
to use what has been referred to as a diagnostic
reference range. While the upper level of the range
is established at the 75th percentile, the lower
level is set at the 25th percentile of the computed
patient dose.
◼◼ Below the 25th percentile level, image quality may
be compromised; above the 75th percentile may
indicate excessive dose.
◼◼ For actual DRL values for various imaging examinations, the reader should consult their respective
national radiation protection organizations
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6
Dose Factors
in Digital
Radiography
Chapter at a Glance
Digital Radiography: Essential Considerations
Definition
Major Differences Between FSR and DR
The Standardized Exposure Indicator: Basics
Factors Affecting Dose in Digital Radiography
Beam Energy and Filtration
Exposure Technique Factors
Beam Collimation
Patient Size
Source-to-Image Receptor Distance
Radiographic Antiscatter Grids
Image Detector Sensitivity or Speed
Detective Quantum Efficiency
F
ilm‐screen radiography (FSR) has been used in radiology
ever since the discovery of X‐rays by W.C. Roentgen in 1895.
Today FSR is now obsolete and has been replaced by digital
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radiographic imaging or, simply, digital radiography. Digital
radiography (DR) technologies include not only digital image
acquisition modalities but also digital image processing, display,
storage, and image communication. Specifically, digital image
acquisition modalities include computed radiography, flat‐panel
digital radiography, digital mammography, digital tomosynthesis,
and digital fluoroscopy for routine gastrointestinal fluoroscopy
and vascular imaging.
To optimize image quality using exposures as low as reasonably achievable (ALARA), it is mandatory that technologists
have a firm understanding of how these factors influence patient
dose. The purpose of this chapter is to review the major factors
contributing to patient dose in digital radiography.
Digital Radiography: Essential
Considerations
Definition
The American Association of Physicists in Medicine (AAPM)
has offered a definition of digital radiography as “radiographic
imaging technology producing digital projection images such
as those using photostimulable storage phosphor (computed
radiography, or CR), amorphous selenium, amorphous silicon,
charge‐coupled device (CCD), or metal oxide semiconductor–
field effect transistor (MOSFET) technology.” These technologies can produce acceptable image quality over a wider range
of exposure techniques compared to FSR.
Major Differences Between FSR and DR
One of the major differences between FSR and DR is the image
receptor or image detector. While DR uses a digital detector, the
image receptor in FSR is a film cassette (a film sandwiched between two intensifying screens). Digital detectors capture and
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67
convert X‐ray attenuation data from the patient into electronic
signals (analog signals) that are subsequently converted into
digital data for processing by a digital computer. The result of
processing is a digital image that must be converted into one
that can be displayed on a computer monitor for viewing by
an observer.
The other major difference between FSR and DR is the
response of the digital detector to radiation exposure. While
film‐screen image receptors have a narrow exposure latitude
described by the film characteristic curve (Bushong 2017), DR
detectors have a wide exposure latitude. The narrow exposure
latitude of FSR means that exposure technique selection must
be accurate to achieve an image with acceptable density and
contrast, and has been considered a problem for technologists.
DR solves this problem, since the wide exposure latitude (linear
response to exposure) is about 100 times that of FSR (Don et al.
2012). Additionally, DR images taken with low and high exposures appear visually the same on the viewing monitor (due
to the image processing of DR systems). While low exposures
produce images with more noise (grainy image), high exposures produce images with less noise but at the expense of
increased dose to the patient. The result is that technologists
face a difficult task of recognizing underexposed and overexposed images. If overexposed images cannot be determined,
the patient receives an unnecessary dose. Overexposures
5–10 times a normal exposure will appear acceptable to the
­technologist. Subsequently, this will lead to what has been
popularly referred to as exposure creep or dose creep (Seibert
and Morin 2011).
Whereas the indicator that the technologist used to determine
acceptable image in FSR is the degree of film blackening, in DR
the technologist uses an exposure indicator (EI) (also referred
to as an exposure index), seen on the image after processing,
and which is used to give the technologist some indication of
the exposure level to the digital detector. It is important to note
that the EI is not patient dose.
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68 Rad Tech’s Guide to Radiation Protection
With respect to the EI, different DR manufacturers (Fuji,
Carestream, Agfa, Konica, Siemens, Philips, and so on) have
developed their own proprietary methods to calculate the EI,
which has therefore led to different EI names. For example,
while Fuji refers to its indicator as a “sensitivity” (S) number,
Carestream uses the term “exposure index,” and Agfa uses the
term “log of the median of the histogram (lgM).” These differences have created “widespread confusion and frustration.”
As a result, the International Electrotechnical Commission
(IEC 2008) and the AAPM (2009) have developed a standardized EI.
The Standardized Exposure
Indicator: Basics
The standardized EI uses a linear proportional scale and
is related to the detector exposure, that is, if the detector
exposure is doubled, the standardized EI doubles. Further
details of the standardized EI are described by the IEC (2008),
AAPM (2009) and, recently, in a textbook by Seeram (2019).
There are four parameters of the IEC standardized EI that are
of importance to the technologist, and the radiologist as well.
These include the EI, EI T, DI, and VOI, which are defined by
the IEC as:
◼◼ EI is a “measure of the detector response to radiation in
the relevant image region of an image acquired with a
digital X‐ray imaging system.”
◼◼ EI is the “expected value of the exposure index when
T
exposing the X‐ray image receptor properly.”
◼◼ DI is a “number quantifying the deviation of the actual
exposure index from a target exposure index.”
◼◼ VOI is the “central tendency of the original data in the
relevant image region. The central tendency is a statistical
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Dose Factors in Digital Radiography
69
term depicting generally the center of a distribution. It
may refer to a variety of measures such as the mean,
median, or the mode.”
Once the above parameters have been established and images
obtained, the DI is seen on every image produced by the technologist for the examination and is used as follows:
◼◼ A DI = 0 means that the intended exposure to the detector
is correct (i.e. EI = EIT).
◼◼ A positive DI indicates overexposure, a negative DI indi-
cates underexposure.
◼◼ A DI of +1 = an overexposure of 26% more than the
desired exposure.
◼◼ A DI of −1 = an underexposure of 20% less than the
desired exposure.
◼◼ A DI of +3 = 100% more than the desired exposure.
◼◼ A DI of −3 = 50% less than the desired exposure.
◼◼ The acceptable range of DI numbers is approximately
+1 to −1, and the DR system is able to deliver the EI T
established by the department.
◼◼ Numbers greater than +1 and less than −1 indicate gross
overexposure and underexposure, respectively.
Having a firm understanding of the above parameters,
especially the DI, technologists can now embrace dose‐image
quality optimization with the objective of reducing patient dose
without compromising image quality, and hence remove the
phenomenon of dose creep.
In summary, DR systems now use a standardized EI to provide immediate feedback to the technologist as to whether the
correct exposure was used for the examination, through the
visual use of the DI value seen on the image. These DI values
are affected by a number of technical factors that affect the dose
to the patient. These factors are outlined below.
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70 Rad Tech’s Guide to Radiation Protection
Factors Affecting Dose in Digital
Radiography
There are numerous factors affecting the dose to the patient in
both FSR and digital radiography. The same technical factors
used in FSR to create a latent image on the image receptor are
also used in DR. The major factors are illustrated in Figure 6.1.
The major factors affecting dose to the patient in digital radiography include beam energy and filtration, exposure technique
X-ray
generator
X-ray
tube
X-ray beam filter
X-ray beam collimator
Source-to-image
receptor
distance
(SID)
X-ray beam
Patient
(thickness
and
density)
X-ray
control console
mA
kV
Time
X-ray table top
Grid
Digital detector
Operator
(Technologist)
Figure 6.1 Major technical factors affect dose to the patient in digital
radiography. The radiation passing through the patient is used to
create the image, and it is the exposure to the image receptor (digital
detector) which ultimately determines the Deviation Index (DI), a
parameter of the standardized exposure indicator (EI).
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Dose Factors in Digital Radiography
71
factors, beam collimation, the size of the X‐ray field, the size
of the patient, source‐to‐image receptor distance (SID), grids,
image receptor sensitivity or speed, and detective quantum
efficiency (DQE). Each of these is now examined briefly.
Beam Energy and Filtration
The energy of the X‐ray beam is often expressed as the beam
quality or the penetrating power of the beam. Beam energy is
dependent on at least three factors.
◼◼ Kilovoltage or kV is the voltage applied between the
cathode and anode; it affects the energy of the photons
striking the patient. High kV results in higher energy photons and a more penetrating beam; low‐kV techniques
produce low‐energy photons and a less penetrating beam.
The dose to the patient is less in high‐kV techniques compared with low‐kV techniques since more photons are
transmitted through the patient. In addition, at low kV,
photoelectric absorption predominates and the patient
absorbs more photons, thus the dose increases. These
were described further in Chapter 2.
◼◼ X‐ray generator. The generator determines the
voltage waveform across the X‐ray tube. An important
characteristic that describes the way in which the voltage
is supplied to the tube is the ripple. When the value of
the ripple is low, the efficiency of the generator is greater.
Among the three types of generator mentioned earlier –
single phase, three phase, and high frequency – the high‐
frequency generator has the lowest ripple, which is less
than 3%, compared with 100%, 13%, and 4% for single‐
phase full wave, three‐phase six‐pulse, and three‐phase
twelve‐pulse generators, respectively.
This factor means that a high‐frequency generator can
deliver greater X‐ray quality and quantity compared to
single‐ and three‐phase generators. Additionally, higher
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72 Rad Tech’s Guide to Radiation Protection
kV values (lower mAs) can be used to decrease the dose
to the patient.
◼◼ Filtration is intended to protect the patient by removing
the low‐energy photons in the X‐ray beam. Filters are
added to the X‐ray tube (added filtration) to reduce
the quantity of photons and increase the mean energy
(quality) of the beam. The beam becomes harder (loss
of low‐energy photons) or more penetrating. Filtration
reduces patient skin dose.
In radiography, aluminum is used as an added filter, the thickness of which depends on the kilovoltage used. The National
Council on Radiation Protection and Measurements (NCRP)
recommends that for most general radiographic examinations,
the total filtration in the beam should be at least a 2.5 aluminum
equivalent.
Exposure Technique Factors
Exposure technique factors include kV, mA, and time of the
exposure. These factors are under the direct control of the technologist, who determines the appropriate set of factors depending on the type of examination. These factors control the quality
and quantity of radiation reaching the patient; thus they affect
the dose in several ways.
◼◼ As noted earlier, kV determines the penetration of the
beam and controls the beam quality. High‐kV techniques
decrease the dose to the patient, since more radiation penetrates the patient with less absorption.
◼◼ The mA determines the quantity of photons falling on
the patient. The radiation dose is directly proportional to
the mA. If the mA is doubled, then the dose is increased
by a factor of two.
◼◼ The exposure time is directly proportional to the dose.
If the time is doubled, then the dose is increased by a
factor of two.
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Dose Factors in Digital Radiography
73
◼◼ Automatic exposure control (AEC) is designed to
reduce exposures to patient, since repeat exposures are
minimized.
Beam Collimation
Collimation is a radiation protection mechanism included in
the ALARA philosophy.
◼◼ Collimation reduces the dose to the patient by restricting
the size of the radiation beam to the area of clinical interest.
This protects the patient from unnecessary exposure.
◼◼ Collimation improves radiographic quality because the
amount of scattered radiation reaching the film and the
total mass of the patient irradiated are decreased.
Patient Size
The technologist has no control over the size of the
patient. However:
◼◼ As the thickness and the density (mass per unit volume)
increase, the dose to the patient increases because more
radiation is required to produce the image.
◼◼ Technologists should always use technique charts that
provide proper exposure factors for various examinations
and patient sizes.
Source-to-Image Receptor Distance
The SID or the source‐to‐skin distance are major factors that
affect patient dose. Most radiology examinations are given at
101 cm (40 in.) because:
◼◼ A long SID produces a less divergent beam at the patient’s
surface and reduces patient dose, since the concentration
of photons (or surface exposure) decreases (Figure 6.2a).
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74 Rad Tech’s Guide to Radiation Protection
(a)
(b)
x
x
x
Figure 6.2 The dose to the patient decreases with a long SID (a), and
increases with a short SID (b), since the concentration of photons
per unit area increases. (Source: courtesy of Dr. Perry Sprawls, PhD;
Distinguished Emeritus Professor, Emory University; Director, Sprawls
Educational Foundation, www.sprawls.org; Co-Director, College
on Medical Physics, ICTP, Trieste, Italy; Co-Editor, Medical Physics
International, http://www.mpijournal.org.)
◼◼ At a short SID, the dose to the patient increases, the radi-
ation beam is more divergent at the patient’s surface,
and the concentration of photons per unit area (surface
exposure) increases (Figure 6.2b).
Radiographic Antiscatter Grids
A grid improves radiographic contrast by absorbing scattered
radiation from the patient, thus preventing it from reaching the
image receptor. Whenever a grid is used in an examination, the
dose to the patient increases compared with nongrid examinations. Among the performance characteristics of a grid, the grid
ratio affects the dose to the patient.
◼◼ As the grid ratio increases, the dose to the patient
increases.
◼◼ High‐ratio grids should be used with high‐kV techniques.
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Dose Factors in Digital Radiography
75
◼◼ When high‐ratio grids are used with high‐kV techniques,
the dose to the patient decreases, because more radiation
is transmitted through the patient and not absorbed by
the patient. The entrance doses for a 12:1 grid at 70, 90,
and 110 kV are 2.1, 2.0, and 1.5 mGy, respectively (using
a 400 speed image detector) (Bushong 2017).
◼◼ Moving grids that have the same characteristics as
stationary grids require approximately 15% more
exposure. Therefore, moving grids increase the dose to
the patient (Bushong 2017).
Image Detector Sensitivity or Speed
The image detector is the image receptor used to produce images of
the patient. The speed refers to the amount of radiation used to produce the image (i.e. sensitivity refers to the overall efficiency of the
image receptor). Image receptor speeds are expressed as numbers
and may range from 200 to 400 to 800 and may extend beyond 800.
◼◼ The dose to the patient (including gonadal dose) is inversely
proportional to the sensitivity of the image receptor.
◼◼ This factor means that increasing the speed by a factor of
two (i.e. going from a 200‐speed system to a 400‐speed
system) will reduce the dose by one half.
◼◼ Depending on the phosphors being used and their
conversion efficiencies, rare‐earth image receptors can
reduce exposures by 50% or more, compared with older
calcium tungstate image receptors.
◼◼ Image receptors with carbon fiber fronts will reduce
the dose by 6–12% compared with cassettes having
aluminum fronts.
Detective Quantum Efficiency
A characteristic related to the image receptor sensitivity or
speed is the DQE, which is intended to measure the efficiency
with which the detector converts an input exposure into a
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76 Rad Tech’s Guide to Radiation Protection
­ iagnostic output image. The performance of a perfect detector
d
is 1 or 100% which reflects that there is no loss of information in
the conversion of the input exposure into a useful image.
It is not within the scope of this chapter to describe the details
of the DQE The interested reader may, however, refer to a textbook by Seeram (2019) for a further description of the DQE at
the technologist level.
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7
Dose
Factors in
Fluoroscopy
Chapter at a Glance
Major Components of Fluoroscopic
Imaging Systems
Image Intensifier Fluoroscopy: Major System
Components
Digital Fluoroscopy: Major System Components
Factors Affecting Dose in Fluoroscopy
Beam Energy
Tube Current
Beam-on Time
Automatic Exposure-Rate Control
Collimation
Source-to-Skin Distance
Patient Size
Antiscatter Grids
Image Magnification
Last Image Hold
Pulsed Fluoroscopy
Scattered Radiation in Fluoroscopy
Rad Tech’s Guide to Radiation Protection, Second Edition. Euclid Seeram.
© 2020 John Wiley & Sons Ltd. Published 2020 by John Wiley & Sons Ltd.
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78 Rad Tech’s Guide to Radiation Protection
F
luoroscopy is an X‐ray imaging technique that allows
the radiologist to observe real‐time images on a televi‑
sion screen to study the dynamics or motion of organ sys‑
tems, particularly circulation and hollow internal anatomy.
Fluoroscopy can provide diagnosis and guidance in interven‑
tional X‐ray studies.
There are several reasons why the dose in fluoroscopy is
important to radiology workers:
◼◼ Several recent reports indicate that patient doses in fluo‑
roscopy are extremely high compared with most radiology
procedures, and that some patients have experienced
radiation‐induced injuries such as skin burns (Bushberg
et al. 2012).
◼◼ Occupational exposures are highest in fluoroscopy
(Bushong 2017).
◼◼ The technologist is an integral part of the fluoroscopic
examination and is present in the room to deal with the
patient and to assist the radiologist during the fluoroscopic
portion of the examination.
The purpose of this chapter is to provide a description of the
major components of a fluoroscopic system and to highlight the
significant factors affecting patient dose in fluoroscopy.
Major Components of Fluoroscopic
Imaging Systems
Two major f luoroscopic imaging systems are shown in
Figure 7.1. The older fluoroscopic imaging system is shown in
Figure 7.1a, while a newer digital fluoroscopic imaging system
is illustrated in Figure 7.1b. The former systems, however, are
rapidly being replaced by digital fluoroscopic imaging systems.
Therefore, there are two types of fluoroscopy system used in
medical imaging departments:
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Dose Factors in Fluoroscopy
(a)
79
(b)
X-ray
tube
Patient
Grid
X-ray
image
intensifier
tube
Flat-panel
digital detector
Digital data
Output
analog
signal
Image
Computer
Circular
image
on TV
monitor
Rectangular
image
on TV
monitor
Figure 7.1 Two major fluoroscopic imaging systems in use today. An
older fluoroscopic imaging system is shown in (a), a newer digital
fluoroscopic imaging system is illustrated in (b).
1. Image Intensifier‐Based Fluoroscopy Systems (Image
Intensifier Fluoroscopy)
2. Flat‐Panel Digital Detector‐Based Fluoroscopy Systems
(Digital Fluoroscopy)
Image Intensifier Fluoroscopy: Major System
Components
The major components as shown in Figure 7.1a are:
◼◼ X‐ray tube and generator. The X‐ray generator provides
the power to the X‐ray tube for the production of X‐rays.
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80 Rad Tech’s Guide to Radiation Protection
In fluoroscopy, the kV is high and the mA is low, since
the X‐ray tube is energized continuously. Naturally, other
factors affect the choice of exposure aspects in fluoros‑
copy. For undertable fluoroscopy, the X‐ray tube is located
below the table; for overhead or overtable fluoroscopy,
the X‐ray tube is positioned over the table and is used for
both fluoroscopy and radiography (images taken after the
fluoroscopic portion of the examination). These images
are generally referred to as “overhead images.”
◼◼ Image intensifier tube. The purpose of the image inten‑
sifier tube in the fluoroscopic imaging chain is to increase
the brightness of the fluoroscopic image using extremely
low mA values applied to the X‐ray tube. Radiation trans‑
mitted through the patient falls on an input screen (cesium
iodide phosphor) that converts X‐ray photons to light. The
light photons, in turn, are converted to electrons by a pho‑
tocathode in the tube. These electrons are subsequently
accelerated across the tube to strike an output screen that
is approximately one‐tenth the diameter of the input screen
(zinc cadmium sulfide phosphor). The image at the output
screen is increased in brightness from the acceleration of
the electrons and the minification at the output screen.
This image cannot be observed directly, thus it must be
sent to a monitor for display and viewing.
To aid in visualization and diagnosis, image intensifiers
can also magnify the image viewed on the television monitor.
Magnification plays a significant role in the dose to the patient.
◼◼ Image distributor. This component of the imaging chain
distributes the light from the output screen of the image
intensifier tube. Approximately 90% of the light reaches
the film recording camera (cine or spot‐film camera); 10%
is directed to the television camera.
◼◼ Television camera or charge‐coupled device (CCD).
The image at the output screen is picked up using a
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Dose Factors in Fluoroscopy
81
t­ elevision camera tube or a solid‐state device called a
CCD and is transmitted via a coaxial cable to the televi‑
sion monitor.
◼◼ Television monitor. The television monitor receives a
video signal from the television camera tube or CCD and
creates the image on the output screen of the image inten‑
sifier tube. The television monitor is the picture tube or
cathode ray tube that displays the image for viewing by
the radiologist.
◼◼ Spot‐film recording devices. These devices include cas‑
sette‐loaded spot films and films recorded by photo‐spot
cameras, and are now obsolete.
Digital Fluoroscopy: Major System
Components
The major components, as shown in Figure 7.1b, are:
◼◼ X‐ray tube and generator. The X‐ray generator provides
the power to the X‐ray tube for the production of X‐rays,
as reviewed above.
◼◼ Scattered radiation grid. This device is positioned
between the patient and the detector for the purpose
of reducing the scattered radiation from the patient
and preventing these scattered rays from reaching
the detector.
◼◼ Flat‐panel digital detector. The digital detector for
fluoroscopy is a dynamic detector. This means that the
detector must be capable of producing moving images
that can be displayed in real time. There are two types of
digital fluoroscopy detector: (i) the cesium iodide, amor‑
phous‐silicon thin film transistor (TFT) (CsI a‐Si TFT)
indirect digital detector, and (ii) the amorphous‑selenium
TFT direct detector (a‐Se TFT). These detectors will not
be described further in this text; however, the reader is
encouraged to refer to Seeram (2019), Bushong (2017),
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82 Rad Tech’s Guide to Radiation Protection
and Bushberg et al. (2012) for detailed functional expla‑
nations of how these detectors work.
◼◼ Computer to process all data from the patient and subse‑
quently display images on a television monitor.
◼◼ Image display monitor. Current displays are flat‐screen
liquid crystal display monitors.
Factors Affecting Dose
in Fluoroscopy
The major factors affecting the dose to the patient in fluoros‑
copy include:
◼◼ Beam energy or filtration
◼◼ Tube current
◼◼ Beam‐on time
◼◼ Automatic dose‐rate control
◼◼ Collimation
◼◼ Source‐to‐skin distance (SSD)
◼◼ Patient‐to‐image intensifier distance
◼◼ Patient size
◼◼ Antiscatter grids
◼◼ Image magnification
◼◼ Last image hold
◼◼ Image recording method
◼◼ Pulsed fluoroscopy.
Beam Energy
Beam energy refers to the penetrating power of the X‐ray photons
in the X‐ray beam emanating from the X‐ray tube. Kilovoltage
(kV), filtration, and the type of X‐ray generator used to provide
power to the X‐ray tube affect the beam energy.
◼◼ When kV is high, penetration of the X‐ray beam is greater
and more photons are transmitted through the patient
(less absorption), lowering the dose to the patient.
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Dose Factors in Fluoroscopy
83
◼◼ Higher kV beams, however, result in a loss of
image contrast.
◼◼ Fluoroscopic imaging systems are provided with filters
for the purpose of protecting the patient.
◼◼ Filters reduce the dose to the patient by absorbing the
low‐energy photons from the X‐ray beam that would oth‑
erwise be absorbed by the patient’s skin surface and fail
to reach the film.
◼◼ The removal of these photons increases the mean energy
of the beam, thus increasing the penetrating power of the
beam. The result is a smaller dose to the patient, since
more photons are transmitted to the image receptor.
◼◼ High‐frequency generators produce more photons with
higher energies than single‐phase and three‐phase X‐ray
generators. Higher beam energies allow patients to receive
a smaller dose.
Tube Current
The tube current is the mA. In fluoroscopy, the mA is low
because the fluoroscopic exposure time can range from min‑
utes to hours. Additionally, since high kV values are generally
used, the mA is reduced substantially.
◼◼ Fluoroscopic mA used in image intensifier fluoroscopy
systems ranges from 1 to 3 mA.
◼◼ For digital fluoroscopy systems the mA is measured in
hundreds of mA instead of less than 5 mA (Bushong
2017). In order to deal with dose increases from the use
of hundreds of mA, a technique known as pulse‐progressive fluoroscopy is used.
◼◼ Lower kV, however, will increase the mAs, thus increasing
the dose to the patient.
Beam-on Time
The beam‐on time refers to the length of time that the X‐ray
tube is energized to produce X‐rays for real‐time image display.
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84 Rad Tech’s Guide to Radiation Protection
◼◼ Continuous exposures increase beam‐on time, thus
increasing patient dose. The dose is directly proportional
to the exposure time. If the beam‐on time is doubled, the
dose is increased by a factor of two.
◼◼ Short, intermittent exposures reduce patient dose, which
is referred to as intermittent fluoroscopy (short bursts of
beam‐on time).
Automatic Exposure-Rate Control
Automatic exposure‐rate control (AERC) is also referred to as
automatic brightness control (ABC) and is a technique that
maintains the image brightness on the television monitor when
the anatomic part thickness changes.
◼◼ The AERC plays an important role in maintaining constant
signal‐to‐noise ratio (SNR) in the image, by altering the
mA and kV automatically as the thickness of the patient
varies in the beam path, thus resulting in dose control.
◼◼ When a nonuniform object such as a wedge (or from the
patient’s neck to abdomen) is being examined by fluoros‑
copy, the image brightness will change as the X‐ray tube
moves from the thick part to the thin part and from bright
to extremely bright, respectively.
◼◼ The X‐ray beam intensity (therefore the dose) increases
as the tube moves from the thick part to the thin part.
Without AERC the dose to the thin part would increase.
◼◼ With AERC the exposure technique factors change automat‑
ically to maintain the same brightness of the image as the
tube moves from the thicker to the thinner anatomic part.
Collimation
As stated in Chapter 6, collimation is intended to protect the
patient from unnecessary radiation by limiting or restricting
the beam to the clinical area of interest.
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Dose Factors in Fluoroscopy
85
◼◼ Fluoroscopy collimators ensure that the clinical area of
interest receives minimal radiation by shaping the beam
to cover only the desired region under study.
◼◼ The smallest possible field size is recommended to ensure
low doses to the patient.
◼◼ Collimation also ensures good quality images by reducing
the amount of scattered radiation reaching the image
receptor (image intensifier input screen).
Source-to-Skin Distance
The SSD in fluoroscopy can affect the dose to the patient, since
the concentration of photons per unit area is greatest for shorter
SSD. The SSD can be described as short (X‐ray tube close to the
patient) or long (X‐ray tube farther away from the patient).
◼◼ The patient dose is reduced when the image intensifier tube
is close to the patient and the X‐ray tube is farther away
from the patient. This arrangement represents the best
geometry of three geometries commonly used in practice.
◼◼ A poor beam geometry is a situation where the X‐ray
tube is too close to the patient, thus resulting in a greater
dose compared to a situation where the detector (digital
detector or image intensifier) is close to the patient using
the same SSD.
◼◼ Furthermore, if the image intensifier tube is too far
away from the patient, the dose increases compared to
a situation where the detector is close to the patient dur‑
ing imaging.
Patient Size
This factor is not under the control of the fluoroscopist.
◼◼ Thicker patients will require more radiation to produce
optimal image brightness, detail, and contrast compared
with thinner patients.
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86 Rad Tech’s Guide to Radiation Protection
◼◼ The kV and mA must be increased to image thicker
patients, resulting in a higher patient dose.
Antiscatter Grids
A grid is used in fluoroscopy to improve image contrast by pre‑
venting scattered radiation from reaching the input screen of
the image intensifier tube.
◼◼ Grids introduced into the radiation beam during the
examination will increase the patient dose by a factor of
two or more. Therefore, not using a grid can reduce the
dose by a factor of two or more.
◼◼ Increasing the distance between the patient and the image
intensifier tube by a reasonable quantity, can reduce the
scattered radiation by using the air‐gap technique (radi‑
ation scattered at extremely oblique angles will fall out
of the gap and will not reach the image intensifier tube).
Image Magnification
The image in fluoroscopy can be magnified to facilitate diag‑
nosis. Image magnification can be achieved by geometric and
electronic means. The former involves increasing the distance
between the patient and the image intensifier tube; the latter
involves collimating the beam automatically to fall only on a
small central portion of the image intensifier input screen.
◼◼ Geometric magnification increases the patient dose in
fluoroscopy, as described earlier.
◼◼ Electronic magnification results in an increase in mA to
maintain the brightness level on the television monitor,
thus increasing the dose to the patient.
◼◼ The dose increase is approximately equal to the ratio of
the sizes of the input screen used for the normal mode
and for the magnification mode.
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Dose Factors in Fluoroscopy
87
◼◼ A dual‐mode (25/17 cm) image intensifier tube will deliver
a 2.2 times (252 ÷ 172) greater dose to the patient when
operating in the 17 cm mode or magnification mode
(Bushong 2017).
Last Image Hold
Last image hold is a technique used to describe images that can
be stored digitally and displayed on the television continuously
without the need for continuous fluoroscopy. Usually the last
image is displayed on the monitor electronically for a period of
time. Last‐image‐hold techniques can reduce patient dose by
50–80% (Bushberg et al. 2012).
Pulsed Fluoroscopy
Pulsed fluoroscopy is a technique during which the X‐ray beam
is pulsed (produced in short bursts) compared with being pro‑
duced continuously.
◼◼ Pulsed fluoroscopy reduces the dose to the patient by as
much as 90% (depending on the number of pulses per
second) compared with nonpulsed units.
◼◼ Pulse fluoroscopy can reduce the scattered radiation expo‑
sures to personnel.
◼◼ Pulsed fluoroscopy can also be used as an effective means
to reduce doses to children undergoing fluoroscopic
examination.
Scattered Radiation in Fluoroscopy
Because occupational exposures are highest in fluoroscopy, it is
essential to consider the nature of this exposure.
Technologists and radiologists are exposed to scattered radia‑
tion in fluoroscopy, as they are in the room assisting the patient
during the examination.
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88 Rad Tech’s Guide to Radiation Protection
◼◼ The main source of scattered radiation is the patient,
although the equipment can produce scattered radia‑
tion as well.
◼◼ When higher levels of primary beam exposures (also
referred to as the high dose‐rate mode or high‐level
control fluoroscopy) are used, more scattered radiation
is produced.
◼◼ A higher level of scattered radiation reaches the fluorosco‑
pist when the primary beam is off the patient’s mid line,
because there is less attenuation of the primary beam by
the patient, as it is closer to the operator.
◼◼ The distribution of scattered radiation when no protective
curtain is hanging on the spot‐film device is higher com‑
pared to when a protective curtain is used (Bushong 2017).
◼◼ To reduce occupational exposures in fluoroscopy, always
wear lead aprons and try to stand back from the table
during beam‐on times.
◼◼ The radiation monitor must be worn at the level of the
collar, outside the protective apron (in the United States)
and under the apron at the waist level (in Canada) to
record the dose from scattered radiation.
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8
Factors
Affecting
Dose in
Computed
Tomography
Chapter at a Glance
Computed Tomography: A Definition
Nobel Prize for CT Pioneers
CT Principles: The Basics
Physics and Technology
Multislice CT Technology: The Pitch
Dose Distribution in CT
CT Dose Metrics
CT Dose Index
Dose Length Product
Size-Specific Dose Estimate
Effective Dose
Factors Affecting the Dose in CT
Exposure Technique Factors
Pitch
Effective mAs
Rad Tech’s Guide to Radiation Protection, Second Edition. Euclid Seeram.
© 2020 John Wiley & Sons Ltd. Published 2020 by John Wiley & Sons Ltd.
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Collimation and Slices
Overranging and Overbeaming
ATCM and Image Quality Index
Automatic Voltage Selection
Iterative Reconstruction Algorithms
Dose Optimization in CT
Computed Tomography: A Definition
Computed tomography (CT) is a sectional imaging technique
that produces direct cross‐sectional digital images referred to
as transverse axial images (transaxial images). These images
have been defined as planar sections that are perpendicular to
the long axis of the patient. The word “computed” implies that a
computer is used to process and reconstruct X‐ray transmission
data collected from the patient.
Ever since its invention in the early 1970s, there have been
several notable developments in CT scanning. These have
resulted in improved scanning technologies, and improved
image quality, both of which play a vital role in the care and
management of the patient during CT examinations. The CT
scanner has evolved from single slice CT scanners to ­multislice
CT scanners (MSCT). State‐of‐the‐art CT scanners are now
MSCT scanners capable of a wide range of applications. These
applications range from scanners dedicated to imaging the
beating heart with excellent image quality to scanners that are
used not only in nuclear medicine imaging but also in radiation
treatment planning. These technological advances have contributed to the increasing use of CT to image not only adults but
children as well.
One important consequence of such increasing use is that
CT delivered the highest collective dose in the United States
compared to other medical imaging modality (Fleischmann and
Boas 2011).
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Nobel Prize for CT Pioneers
Two individuals shared the Nobel Prize in Medicine and
Physiology in 1979 for their development of the CT scanner.
These were Godfrey Newbold Hounsfield in the United Kingdom
(UK), who invented the first clinically useful scanner, and Allan
Cormack, a physicist at Tufts University in Massachusetts. For a
detailed account of each of their contributions, the reader should
refer to a CT textbook by Seeram (2016).
CT Principles: The Basics
CT is a multidisciplinary technology and has its roots in physics,
mathematics, engineering, and computer science. It is not within
the scope of this chapter to describe the elements of each of
these subjects, however it is essential for the user to have a
broad understanding of the fundamental concepts related to
the physics and technology. These concepts are important to
understanding the factors affecting the dose to the patient having a CT examination.
Physics and Technology
The CT process comprises at least three major system components that are used to produce the CT image. These system components include: (i) the data acquisition system, (ii) the computer
system, and (iii) the image display, storage and communication systems.
Data acquisition means that radiation attenuation data are
collected from the patient during the scanning. In this respect,
an X‐ray tube coupled to special electronic detectors rotate
around the patient to collect and measure attenuation readings
as the X‐ray beam passes through the patient.
The attenuation is according to Beer–Lambert’s law
I
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I oe
x
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92 Rad Tech’s Guide to Radiation Protection
where I is the transmitted X‐ray beam intensity, Io is the original
X‐ray beam intensity, e represents Euler’s constant, μ is the
linear attenuation coefficient, and Δx is the finite thickness of
the section. In CT, the system calculates all linear attenuation
coefficients for all structures seen on the image. Special detectors and detector electronics are used to calculate the attenuation
data and convert them into integers (0, a positive number, or a
negative number), referred to as CT numbers, using an image
reconstruction algorithm to build up the image in numerical
format. The CT numbers (numerical image format) are converted into a gray scale image for display on a monitor for the
observer to interpret.
The CT numbers are calculated using the following
relationship:
CT number
tissue
water
water
K
where K represents a scaling factor. In general, K is equal to 1000.
When Hounsfield invented the scanner, K was equal to 500.
The main message in the above paragraphs is that the attenuation depends on several physical factors (physics) that all play
a role in the dose to the patient. These are described briefly later
in the chapter.
The technology aspects of CT are complex but they enable
the attenuation values collected around the patient for 360° to be
used to build up an image of the internal anatomy of the patient,
and for such an image to be displayed for interpretation by radiologists. The technology addressing the collection of these values
includes the X‐ray tube, which is coupled to special electronic
detectors and detector electronics. Another major technology
component in CT is the computer system, which captures the
raw data from the detectors and uses sophisticated image reconstruction algorithms to create the image from the raw data.
Display, storage, and communication of processed images are
accomplished using digital technologies. A display monitor also
allows the interpreter to manipulate the image using special
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93
image processing operations. Images are subsequently stored
on magnetic or optical data carriers and can be communicated
by electronic means to other locations using Picture Archiving
and Communication Systems (PACS).
Multislice CT Technology: The Pitch
As noted in the introduction, all present day CT scanners are
MSCT scanners. One characteristic feature of MSCT scanners is
the two‐dimensional detector array, compared to the one‐dimensional detector array of single slice CT scanners (SSCT). The
principles of MSCT are not within the scope of this chapter and
the reader is encouraged to refer to major physics textbooks, such
as the one by Bushberg et al. (2012), or to consult CT‐specific
textbooks, such as one by Seeram (2016) or a review article
(Seeram 2018). This means that there will be additional specific
technical factors that affect the dose in CT. These factors are
described below. One such notable factor is the pitch (P), which
is defined by the International Electrotechnical Commission
(IEC) as the distance the table travels per rotation (D) divided by
the total collimation (W). This can be expressed algebraically as:
P
D/W
Furthermore, the total collimation (W) is equal to the number
of slices (M) times the collimated slice thickness (S). The pitch
can now be expressed algebraically as:
P
D/ M S
Dose Distribution in CT
The increasing use of CT has led to widespread concerns about
high patient radiation doses from CT examinations relative to
other radiography examinations. The distribution of the dose to
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94 Rad Tech’s Guide to Radiation Protection
the patient in CT is significantly different than the distribution of
the dose in radiography. Whereas radiography uses a stationary
X‐ray beam and a stationary detector, CT uses a rotating X‐ray
beam and detectors around the patient for 360°. Because of this,
the dose distribution in radiography is considered nonuniform
and results in a distribution of the dose that is 100% at the surface
of the patient and about 3% at the detector. In CT, the dose distribution is more uniform at the surface of the patient and decreases
toward the center. Additionally, in CT there is a concern about
the relative contribution of scatter to the absorbed dose. These
differences require additional CT‐specific dose metrics.
CT Dose Metrics
There are four essential CT‐specific dose metrics: the Computed
Tomography Dose Index (CTDI), the Dose Length Product
(DLP), the Size‐Specific Dose Estimate (SSDE), and the Effective
Dose (ED). The basics of these four metrics is described briefly
here. For detailed descriptions, the reader should refer to the
American Association of Physicists in Medicine (AAPM) report
by Task Group 204 (2011).
CT Dose Index
The CTDI is a standardized measure of the radiation output
from a CT scanner and is used to compare the radiation output
different CT scanners (Bushberg et al. 2012). Specifically, the
dose in the z‐axis of the patient (line drawn from head to toe)
in the CT scanner is referred to as the CTDIvol for MSCT; it is
expressed in milligray (mGy). The following algebraic expression is used to calculate the CTDIvol.
CTDI vol
CTDI weighted / PITCH
where the weighted CTDI (CTDIw) is used to account for the
average dose in the x–y axis of the patient instead of the z‐axis.
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For a pitch of 1, the CTDIvol is equal to the CTDTw. The CTDIvol
is not used to estimate the dose to the patient.
Dose Length Product
The CTDIvol is used to provide a measurement of the dose per
slice of tissue and is the same whether the user scans a 10 mm
or a 100 mm scan length. Thus another metric is required. This
is the DLP, which is a much more accurate reflection of the dose
for a defined length of tissue. The DLP is directly proportional
to the scan length (L) in centimeters (cm) along the z‐axis and
is expressed in mGy‐cm. The DLP can be calculated as follows:
DLP
CTDI vol L
The CTDIvol is the same regardless of the size of the patient,
that is, the CTDIvol is the same for a large patient as it is for a
small patient (keeping all scan parameters the same). Therefore,
another metric is used to take into consideration the size of
the patient.
Size-Specific Dose Estimate
The SSDE is given by the formula:
SSDE16
CTDI vol for a16 cm CTDI phantom
Conversion factor f for the16 cm CTDI phantom
or
SSSD32
CTDI vol for a 32 cm CTDI phantom
Conversion factor f for the32 cm CTDI phantom
The conversion factors (ƒ 16 and ƒ 32) are provided in the AAPM
report (2011) for different patient sizes. The reader is encouraged
to consult this report for further details. These details are not
within the scope of this book.
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96 Rad Tech’s Guide to Radiation Protection
Effective Dose
In order to consider the risk of a CT examination, the use of the
ED is the metric used. ED is used in radiation protection to relate
exposure to risk, and it takes into account that different tissues
have different radiosensitivities. Wolbarst et al. (2013) noted that
the DLP can provide only a rough estimate of ED In practice,
however, the DLP is approximately proportional to the ED and
can be calculated using the following relationship:
ED
k DLP
where k is a constant. While the k value (mSv/mGy‐cm) for a CT
scan of the head is 0.0021, it is 0.015 for a CT scan of the pelvis
(McNitt‐Gray 2002).
The ED can be used to compare the CT dose with the dose
received from natural background radiation. The annual ED
from natural background radiation is reported to be 3 mSv,
while it is 1.5 mSv for CT and 3 mSv for medical imaging
(Bushong 2017).
Factors Affecting the Dose in CT
Several factors affect the dose in CT. Major factors include
exposure technique factors (mAs and kV), pitch, effective
mAs, collimation and slices, overbeaming and overranging,
automatic tube current modulation (ATCM), automatic voltage
selection (AVS), and iterative reconstruction (IR) algorithms
(McCollough 2019).
Exposure Technique Factors
The dose is directly proportional to the mAs and directly proportional to the square of the kV change.
CTDI vol mAs
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Factors Affecting Dose in Computed Tomography
CTDI vol
kVnew / kVold
97
2
Pitch
The pitch is defined above by the IEC. The CT dose is related
to the pitch (when all parameters such as for example, mA and
rotation time) are held constant, as follows:
CTDI vol 1/ Pitch
Effective mAs
The effective mAs is a ratio of the true mAs to the pitch.
CTDI vol mAseffective
Collimation and Slices
The X‐ray beam width is defined by the collimation and for
MSCT, the slice or section thickness (width) is defined by the
number of detector elements grouped or binned together in
each detector channel (Bushberg et al. 2012; Seeram 2016).
As the collimation width increases and the slice thickness
decreases, the dose decreases and increases respectively.
Decreasing the section thickness means that the exposure
technique be increased to maintain the same signal‐to‐noise
ratio (SNR).
Overranging and Overbeaming
As described by Goo (2012), while overranging refers to the use
of additional rotations before and after the planned length of
tissue so the first and last images can be reconstructed, overbeaming is the excess dose beyond the edge of the detector rows
per rotation of a multisection. Both of these parameters increase
the patient dose. To address this problem, adaptive collimation is
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98 Rad Tech’s Guide to Radiation Protection
used in MSCT scanners to reduce patient dose at the beginning
and end of scanning, and dose reduction of about 40% is possible
(Christner et al. 2010).
ATCM and Image Quality Index
The overall goal of ATCM, categorized as Automatic Exposure
Control (AEC) is to adjust the mA in either the z‐axis
(longitudinal), the x‐y axis (angular), or both, in an effort to
reduce patient dose. An important feature of AEC systems is a
preselected image quality index, also referred to as a reference or
target image quality index. This index is an operator‐selectable
parameter. Different CT vendors have different names for this
index. For example, General Electric (GE) Healthcare refers to
this index as the Noise Index (NI).
Coakley et al. (2011), for example, showed that ATCM can
reduce the dose by 20–40% in adults. Furthermore, as the NI
increases, the patient dose decreases but at the expense of a noise,
since the NI is inversely proportional to the square root of the dose.
Automatic Voltage Selection
The purpose of AVS is to optimize kV for the diagnostic task and
patient size in an effort to obtain lower CTDIvol while maintaining acceptable image quality. The kV is not modulated as is the
case with ATCM technique. It is important to note, however,
that as the kV is changed the mA changes as well to maintain
a constant Contrast‐to‐Noise Ratio (CNR).
A study conducted by Oda et al. (2011) showed that low tube
voltage in cardiac CT imaging reduced the dose by about 55%
in slim patients while maintaining image quality.
Iterative Reconstruction Algorithms
As mentioned earlier, CT uses an image reconstruction
algorithm to create the image knowing the values of all attenuation data collected from the patient. In the past, the filtered
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back projection (FBP) algorithm was commonly used. However,
the FBP algorithm, produces image noise and streak artifacts,
and therefore other algorithms are now being used to solve these
problems. These algorithms are referred to as IR algorithms
and all major CT manufacturers employ IR algorithms on their
scanners (Beister et al. 2012; Kaza et al. 2014).
The major goals of IR algorithms are to reduce image noise
and minimize the higher dose inherent in the FBP algorithm.
IR algorithms use the FBP algorithm image data (measured
projrction data) and create simulated data. The simulated data
are then compared with the initial measured projection data to
determine the differences in image noise. Once the difference
is determined, it is applied to the simulated data to correct for
inconsistencies.
A systematic review of the IR literature entitled “Does IR
improve image quality and reduce dose in CT” (Qiu and Seeram
2016) concluded that there is general consensus that IR algorithms can faithfully reduce radiation dose and improve image
quality in CT in comparison with the FBP algorithm. A number
of studies for example have shown that IR can reduce the dose
from 30 to 50% (Maldjian and Goldman 2013; Kaza et al. 2014).
Dose Optimization in CT
The International Commission on Radiological Protection
(ICRP) optimization framework refers to optimization as keeping the dose as low as reasonably achievable (ALARA) while
not compromising the diagnostic quality of the image.
In an early paper, Goo (2012) provides a checklist for dose
optimization in CT in which several “checkup items” and recommendations are provided in clear and simple terms, These
items include body size‐adapted, CT protocols, tube current
modulation, optimal tube voltage at equivalent radiation dose,
longitudinal scan range, repeated scans, scan modes, and noise‐
reducing image reconstruction algorithms.
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An important consideration in dose optimization in CT
means that image quality must not be compromised at lower
doses. Furthermore, the diagnostic performance of radiologists
must also be taken into consideration as the dose is reduced
(McCollough 2019). In this regard, there are several resources for
the user to consult. Two such resources are the AAPM Alliance
for Quality CT, and the Image Gently® and Image Wisely®
campaigns.
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9
Dose
Management
Regulations
and
Optimization
Chapter at a Glance
Federal Regulations for Dose Management
Safety Reports for Guidelines and
Responsibilities
Definition of Terms in Safety Reports
Equipment Specifications for Radiography
X-Ray Control Panel
Leakage Radiation from the X-Ray Tube
Filtration
Collimation and Beam Alignment
Source-to-Image Receptor Distance
Source-to-Skin Distance
Exposure Switch for Radiography
Equipment Specifications for Fluoroscopy
Filtration
Collimation
Source-to-Skin Distance
Exposure Switch
Cumulative Timer
Rad Tech’s Guide to Radiation Protection, Second Edition. Euclid Seeram.
© 2020 John Wiley & Sons Ltd. Published 2020 by John Wiley & Sons Ltd.
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Protective Curtain
Table and Bucky-Slot Shielding
Accessory Protective Clothing
Procedures for Minimizing Dose to Patients
and Personnel
Procedural Factors for Minimizing
Dose to Patients in Radiography
and Fluoroscopy
Diagnostic Reference Levels (DRL)
Procedural Factors for Minimizing
Dose to Personnel
Shielding: Design of Protective Barriers
Quality Assurance: Dose Management and
Optimization
Dose Management
Dose Optimization
T
he factors affecting dose in radiography and fluoroscopy
have been described in Chapters 6 and 7, respectively.
This chapter outlines the techniques and methods for dose
management. Dose management examines various methods
to reduce the dose to patients, personnel, and members of the
public. It is also guided by the two triads of radiation protection:
time, shielding, and distance (radiation protection actions) and
justification, optimization, and dose limitation (radiation protection principles) described in Chapter 5.
The purpose of this chapter is to summarize the major federal
regulations for dose management in radiology, including the
guidelines and recommendations for equipment specifications,
procedures for minimizing the dose to patients and personnel,
and shielding.
These topics are important to the technologist as they provide
the technologist with the techniques that are mandatory to optimize the examination in an effort to reduce the dose to all individuals who may be exposed to radiation in the radiology department.
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Federal Regulations for Dose
Management
Safety Reports for Guidelines
and Responsibilities
The guidelines and recommendations for dose management are
outlined in various reports. It is essential for users of diagnostic
X‐ray equipment to consult the radiation safety codes for their
respective countries. In this book, the regulations from the following are cited:
◼◼ National
Council on R adiation Protection and
Measurements (NCRP) Report No. 102 (NCRP 2015).
◼◼ Code of Federal Regulations (CFR) Title 21: Performance
Standards for Ionizing Radiation Emitting Products
(DHHS 2018).
It is not within the scope of this text to address details of these
reports. The student and instructor must refer to them for additional topics not covered here. Specifically, this chapter focuses
on major recommendations for equipment specifications, procedures for minimizing dose to patients and personnel, and,
finally, shielding requirements for radiology.
Definition of Terms in Safety Reports
In the NCRP Report No. 102, the terms shall and should are used
in the recommendations. The exact meanings are as follows:
◼◼ Shall and shall not are used to indicate that adherence
to the recommendation is considered necessary to meet
accepted standards of protection.
◼◼ Should and should not are used to indicate a prudent
practice to which exceptions may be occasionally made
in appropriate circumstances (NCRP 1989, p. 2).
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Equipment Specifications
for Radiography
The regulations for equipment specifications are obviously
intended for manufacturers of radiation emitting devices such
as the X‐ray machine. There are specifications particularly
intended to protect both patients and operators when X‐ray
machines are producing radiation.
The specifications for radiographic equipment to be summarized here are those relating to the X‐ray control panel,
leakage radiation from the X‐ray tube, filtration, collimation,
source‐to‐image receptor distance (SID), source‐to‐skin distance (SSD), and the exposure switch for fixed and mobile
radiographic systems.
X-Ray Control Panel
The X‐ray control panel features warning signs, markings,
indicator lights, and various icons to indicate when the machine
is being used and to ensure that exposures occur when the operator conducts the examination. The specifications are such that
the control panel must:
◼◼ Bear visible signs indicating that only authorized and qual-
ified personnel are permitted to operate the equipment.
◼◼ Show controls, meters, and lights indicating the opera-
tion of the various machine parameters, such as a visible
light or audible tone when the exposure is made (when
the beam is on).
Leakage Radiation from the X-Ray Tube
The X‐ray tube housing must be lined with lead to reduce leakage radiation from the tube such that, at a distance of 1 m from
the X‐ray tube, the leakage radiation must be less than 1 mGy/h
(100 mR/h)
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Filtration
Remember that the purpose of filtration is to protect the patient
by removing low‐energy photons from the X‐ray beam. The recommendations from the minimum total filtration (inherent and
added) in the useful beam are:
◼◼ 2.5 mm aluminum equivalent when the tube is operating
above 70 kV.
◼◼ 1.5 mm aluminum equivalent when the tube is operating
between 50 and 70 kV.
◼◼ 0.5 mm aluminum equivalent when the tube is operating
below 50 kV.
Note that the thickness of the filter depends on the kV and
increases when the kV increases.
Collimation and Beam Alignment
Collimation is intended to protect the patient by restricting the
beam to the clinical area of interest. The recommendation for
collimation and beam alignment is that the X‐ray field and the
light beam must be aligned to within 2% of the SID at the center
of the image receptor.
Source-to-Image Receptor Distance
The SID is important in terms both of image quality and radiation dose. Examinations must be carried out at the correct SID
(a chest X‐ray should be performed at 180 cm [72 in.]). The recommendation is as follows:
◼◼ An SID indicator (tape measure or laser measurement
system) must be provided with all X‐ray systems and it
must be accurate to within 2% of the SID used.
◼◼ The SID for tabletop work in radiography should not be
less than 100 cm (40 in.).
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Source-to-Skin Distance
The SSD affects dose to the patient given that it determines
the concentration of photons per unit area of the surface of the
patient. The NCRP recommendation is that the SSD shall not be
less than 30 cm (12 in.) and should not be less than 38 cm (15 in.).
Exposure Switch for Radiography
The purpose of the exposure switch is to allow the technologist
to make an exposure for image formation. Recommendations
for the exposure switch are:
◼◼ For fixed radiographic equipment, the exposure switch
must be on the control panel to ensure that the operator
remains in the control booth during the exposure.
◼◼ The switch must be a “dead man” switch. That is, pressure
must be applied to the switch for the exposure to occur.
◼◼ For mobile radiography units, the exposure switch shall
be so arranged on a long cord to allow the operator to
stand at least 2 m (6 ft) from the patient and the unit.
Equipment Specifications
for Fluoroscopy
The specifications for the fluoroscopic equipment are numerous
and, therefore, only the major categories are reviewed here.
These specifications include filtration, collimation, SSD,
exposure switch, cumulative timer, protective curtain, table and
Bucky‐slot shielding, and accessory protective clothing.
Filtration
Filtration of the beam used in fluoroscopy is mandatory,
and since high kV beams are generally used in fluoroscopic
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examinations, the total filtration must be at least a 2.5 aluminum
equivalent to protect the patient from low‐energy photons.
Collimation
The NCRP recommendations for the collimator in f luoroscopy are:
◼◼ The collimator shall be coupled and centered to the
image receptor and should be confined to the receptor
at any SID.
◼◼ “For spot‐film radiography, the shutters shall automatically change to the required field size before each
exposure” (NCRP 1989, p. 15).
Source-to-Skin Distance
In fluoroscopy, the SSD shall not be less than 38 cm (15 in.) for
stationary fluoroscopic units and shall not be less than 30 cm
(12 in.) for mobile fluoroscopic units.
Exposure Switch
The exposure switch for fluoroscopy must be a dead man type
of switch. Both the foot switch and the switch on the spot‐film
device are the dead man type of switch.
Cumulative Timer
This timer records the beam‐on time during the examination.
◼◼ The timer shall provide the operator with an audible
or visual signal when five minutes of beam‐on time
have elapsed.
◼◼ The signal should last at least 15 seconds, at which time
it must be reset to continue fluoroscopy.
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Protective Curtain
A protective lead curtain is part of the fluoroscopic equipment that hangs from the spot‐film device to reduce and prevent the scattered radiation from the patient from reaching the
radiologist.
◼◼ The dimensions of the curtain should be not less than
45.7 cm × 45.7 cm (18 in. × 18 in.).
◼◼ The curtain should have at least 0.25 mm lead‐equivalent
thickness.
Table and Bucky-Slot Shielding
The Bucky slot is an opening on the table about 5 cm wide that
allows the operator to move the Bucky tray to one end of the table
when fluoroscopy is being conducted. The Bucky slot is shielded
by a length of metal that covers the opening to prevent scattered
radiation from reaching the operator at the gonadal level.
◼◼ Bucky‐slot shielding should have a lead‐equivalent thick-
ness of at least 0.25 mm.
Similarly, the fluoroscopic table must also provide shielding
to prevent scattered radiation from reaching personnel.
◼◼ The table should have a lead‐equivalent thickness of
0.25 mm to provide protection from scattered radiation.
Accessory Protective Clothing
These items include lead aprons, thyroid shields, and protective
gloves worn by operators. Recommendations are as follows:
◼◼ Protective aprons shall have at least a 0.5 mm lead‐
equivalent thickness.
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◼◼ Protective thyroid shields shall have at least a 0.5 mm
lead‐equivalent thickness.
◼◼ Protective gloves shall have a lead‐equivalent thickness
of at least 0.25 mm.
Procedures for Minimizing Dose
to Patients and Personnel
There are a number of practices and procedures to reduce
the dose to the patients and personnel and to keep exposures
according to the as low as reasonable achievable (ALARA) philosophy. Only the more commonplace procedures are reviewed
here. Technologists must refer to the safety codes for more
detailed information.
Procedural Factors for Minimizing Dose
to Patients in Radiography and Fluoroscopy
There are several factors under the direct control of the technologist that are intended to reduce the dose to the patient during
the conduct of the examination. It is not within the scope of
this book to discuss all of these factors. However, the following
actions are significant and illustrative:
◼◼ The radiologist should be consulted to ensure that the
examination is warranted, especially in situations that
are confusing.
◼◼ Selection of correct image receptor should be appropriate
to the examination, since the dose is inversely proportional to the sensitivity of the image receptor. Faster image
receptor systems are now used to control the dose to the
patient without compromising image quality.
◼◼ Selection and use of the best possible exposure
technique factors should keep the dose as low as
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reasonably ­achievable without compromising image
quality. Generally, high‐kV techniques reduce the dose
to the patient, as described in Chapter 6.
◼◼ The correct use of automatic exposure control can play a
significant role in dose reduction, specifically with regard
to repeat examinations.
◼◼ Repeat examinations should be kept to a minimum.
Errors in positioning, exposure technique factors, collimation, and patient motion are major factors that affect
repeat rates.
◼◼ Collimation to the size of the image receptor or smaller is
the preferred action, and evidence of collimation on the
image should be shown. Collimation alone can reduce
the genetically significant dose (population gonadal
dose) by 65%.
◼◼ The smallest image receptor size should be chosen to
match the diagnostic goal of the examination.
◼◼ The optimal SID appropriate to the anatomy being imaged
should be used.
◼◼ Shielding should always be used to protect radiosensitive
organs. Careful placement of the shield is mandatory to
protect organs, while not compromising the goals of the
examination.
During fluoroscopy, paying careful attention to the following
factors can minimize the dose to the patient:
◼◼ The patient must understand the nature of the examina-
tion to avoid repeat exposures. Understanding breathing
instructions and being asked to assume various positions
are examples.
◼◼ Fluoroscopy must be carried out by an appropriately
trained individual, such as a radiologist.
◼◼ Although not under the direct control of the technologist, the fluoroscopy time should be kept as short
as possible.
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Dose Management Regulations and Optimization
111
◼◼ The exposure rate in fluoroscopy must not exceed the
recommended limits.
◼◼ Because the radiologist performs the examination, he or
she should be certain that the beam is filtered and collimated, and that the fluoroscopic exposure technique is
consistent with the clinical objectives of the examination.
Diagnostic Reference Levels (DRL)
DRLs were briefly outlined in Chapter 5. The NCRP has prepared a report that specifically deals with DRLs for different
modalities and examinations – NCRP Report No. 172, Reference
Levels and Achievable Doses in Medical and Dental Imaging:
Recommendations for the United States (NCRP 2015).
Procedural Factors for Minimizing Dose
to Personnel
To achieve the goal of reducing the dose to personnel, technologists must work within the ALARA philosophy. Examples of
recommendations to ensure personnel dose reduction are:
◼◼ Only essential personnel must be present in an X‐ray room
during the exposure.
◼◼ Technologists must remain in the control booth during
radiographic exposures and must wear protective aprons
when the situation makes this impossible.
◼◼ Technologists must always pay attention to patients by
observing them through the shielded transparent window
in the control booth during the exposure.
◼◼ All room doors must be closed during the exposure to
prevent scattered radiation from reaching personnel in
close proximity to the room.
◼◼ Radiology personnel (including technologists) must refrain
from holding patients during an examination. Immobilizing
devices should be used or other nonradiology personnel.
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In the latter case, these individuals must be provided with
protective clothing.
◼◼ Personnel dosimeters must always be worn during scheduled periods of work in the radiology department or when
performing operating room and mobile radiographic
examinations.
During fluoroscopic procedures:
◼◼ Aprons must always be worn during the fluoroscopic por-
tion of the examination.
◼◼ Technologists should stand a few steps away from the
patient during fluoroscopic exposures unless the situation
warrants that someone assists the patient directly.
During mobile radiography:
◼◼ Technologists must stand the maximum distance allowed
by the exposure cord (2 m) from the X‐ray machine and
the patient.
◼◼ Protective aprons must always be worn, or the technologist can stand behind a portable shield if one is available.
◼◼ Direct the primary beam to the patient only and be certain
that no others will be exposed to the useful beam.
Shielding: Design
of Protective Barriers
Shielding is a radiation protection action and a design criterion
that is intended to protect patients, personnel, and members of
the public. Shielding includes specific area shielding (protecting
radiosensitive organs) and protective barriers (walls) positioned
between the source of radiation and the individual. This type
of shielding is specifically intended to protect personnel and
members of the public from unnecessary radiation.
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113
◼◼ Because X‐ray rooms are in close proximity to other rooms
that are occupied by personnel (lounges and offices) and
by patients (waiting rooms and corridors), the walls of
the X‐ray room must be shielded.
◼◼ X‐ray rooms are shielded with lead in most cases (concrete
can also be used).
◼◼ The thickness of the lead depends on whether the wall is
exposed to the primary beam (primary protective barriers)
or to scattered radiation (secondary protective barrier).
◼◼ Lead may not be used for shielding in secondary barriers since its four layers of thickness may be too thin
(<0.4 mm). Therefore, a lead acrylic gypsum may be used
(⅝ inch gypsum board with ½ inch plate glass will offer
adequate protection).
◼◼ The control booth is considered a secondary protective
barrier and is subject to the same lead thickness criterion
for secondary barriers (the primary beam must never be
directed toward the control booth).
There are several factors that affect the thickness of protective
barriers in radiology. These factors include the exposure rates
for controlled and uncontrolled areas occupied by individuals,
the distance between the radiation source and the barrier, workload, occupancy factor, use factor, and the kV for examinations.
◼◼ A controlled area is an area in which an individual is
occupationally exposed and includes radiology personnel
and as well as patients. Barriers for controlled areas
must minimize the exposure rate to less than 1 mSv/wk
(100 mrem/wk).
◼◼ An uncontrolled area is one occupied by any individual.
Barriers for uncontrolled areas must reduce the exposure
rate to that of members of the public, that is 1 mSv/yr
(100 mrem/yr).
◼◼ Barriers for uncontrolled areas contain more lead than
do barriers for controlled areas.
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◼◼ The workload refers to the number of examinations per-
formed per week, expressed as milliampere‐minutes per
week (mA‐min/wk).
◼◼ The occupancy factor refers to the time that the area is
occupied, expressed as a fraction of the work week. Levels
of occupancy can be full (1), partial (¼), and occasional
( 116) occupancy.
◼◼ The use factor is the fraction of time during which the
primary beam is on and aimed at the barrier. Use factors
are provided for levels of full use (1), partial use (¼), and
occasional use ( 116).
◼◼ The maximum and average kV must also be known to
calculate barrier thickness.
The barrier thickness can be determined by referring to
precalculated shielding requirement tables or by performing
a calculation using the data for the various factors previously
identified.
Quality Assurance: Dose
Management and Optimization
Quality assurance (QA) includes quality administration and
quality control (QC). Quality administration deals with the
people and management of the entire QA program; QC deals
with the technical aspects of machine performance.
Dose Management
The goals of QA and QC are to:
◼◼ Reduce the dose to the patient.
◼◼ Ensure optimal image quality to facilitate diagnostic
interpretation.
◼◼ Reduce costs to the institution and the consumer.
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115
QC ensures dose management and optimization through its
three major steps of:
◼◼ Acceptance testing, during which new equipment is tested
to ensure that manufacturer’s specifications are met.
◼◼ Routine performance or carrying out QC tests on var-
ious parameters that affect patient dose and image quality.
Specifically, equipment parameters are evaluated using a
defined set of tools and procedures.
◼◼ Error correction, whereby the problems detected during routine performance testing are remedied to ensure
proper functioning of the equipment.
Additionally, QC uses an important concept known as the
tolerance limit or acceptance criterion.
◼◼ Tolerance limits can be assessed qualitatively (as a pass–
fail) or quantitatively (as a ± value).
◼◼ If the QC test results fall within the tolerance limit, then
the parameter under investigation is deemed acceptable
and the equipment can be used to image the patient.
◼◼ If the test results fall outside the tolerance limits, the
performance criterion has not been met and the equipment
must be repaired to produce acceptable image quality.
◼◼ The NCRP tolerance limits for kV accuracy, for example,
are ±5% and less over a limited range (±2 kV for
60–100 kV).
◼◼ The NCRP tolerance limit for the light field and X‐ray field
alignment is ±3% of the SID.
◼◼ QC tests should be conducted on all the variables that
affect image quality and dose, including collimation, filtration, focal spot size, kVp accuracy, mA linearity, and
exposure timer accuracy, among others.
A QA–QC program is a management operation that ensures
optimal image quality with reduced radiation dose to patients
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116 Rad Tech’s Guide to Radiation Protection
and personnel through a performance evaluation of the factors
affecting both image quality and radiation dose.
Dose Optimization
In addressing the ALARA principle and the need to ensure there
is a balance between acquiring high‐quality diagnostic images
with the least amount of dose, the literature most often uses two
terms: reduction and optimization. Reduction means to “reduce
or diminish in size, amount, extent, or number.” Optimization
means “an act, process, or methodology of marking something
(as a design, system, or decision) as fully perfect, functional,
or effective as possible” (Reduction, Optimization. Merriam‐
Webster Dictionary. Online English Language Dictionary. www.
merriam‐webster.com/dictionary).
An article in the journal Radiation Protection Dosimetry dedicated to optimization strategies in medical imaging (Mattsson
2005) identified at least four important requirements for dose
and image‐quality optimization research. First, ensure patient
safety. Second, determine the level of image quality required
for a particular diagnostic task. The third point to keep in mind
is to acquire images at various exposure levels from high to
low and in such a manner that accurate diagnosis can still be
made. Finally, reliable and valid methodologies for the dosimetry, image acquisition, and evaluation of image quality using
human observers must be used, keeping in mind the nature of
the detection task.
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10
Pregnancy
Essential Radiation
Protection
Considerations
Chapter at a Glance
Rationale for Radiation Protection in Pregnancy
Factors Affecting Dose to the Conceptus
Estimating the Dose to the Conceptus
Continuing/Terminating a Pregnancy
After Exposure
Dose Reduction Techniques for
Pregnant Patients
The Pregnant Worker
Rights to Privacy
Dose Limits and Personnel Monitoring
Protective Aprons for Pregnant Workers
Policies for Pregnant Workers
B
eing of childbearing age and being pregnant are two
situations that warrant special radiation protection considerations, especially for in utero exposure in diagnostic radiology.
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© 2020 John Wiley & Sons Ltd. Published 2020 by John Wiley & Sons Ltd.
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This topic is of vital significance to the radiologic technologist
for two reasons:
◼◼ There are known bioeffects of radiation exposure to the
conceptus; that is, any product of conception (embryo
or fetus).
◼◼ Many technologists are at the childbearing age and several others may be pregnant while working in the radiology department.
The purpose of this chapter is to summarize radiation protection considerations for pregnant patients and pregnant workers.
Specifically, the effects of prenatal exposure will be listed and
the factors affecting fetal dose in radiology, fetal dose estimation,
and recommendations following in utero exposure will be
highlighted. Additionally, regulations for pregnant workers,
including rights to privacy, dose limits, and maternity aprons
and policies, will be outlined.
Rationale for Radiation Protection
in Pregnancy
It is well known that radiation can cause injury to the conceptus.
The effects of prenatal exposure to radiation is as a function
of gestational age (ACR‐SPR 2018). Data from animal experiments and survivors of the nuclear bombing of Hiroshima and
Nagasaki (HN) (classified as a high‐dose event) suggest the following (Seeram and Brennan 2017; Sreetharan et al. 2017):
◼◼ Prenatal death results before embryonic implantation.
◼◼ Head size is small in the children of HN survivors.
◼◼ The most sensitive period for this effect is from 2 to
15 weeks after conception.
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Pregnancy
119
◼◼ Mental retardation.
◼◼ Radiation‐induced malignancy
As noted by Bushong (2017), “… these abnormalities are
based on doses greater than 1 Gy (in tissue) with minimal
reported doses in animal experiments at approximately 100
mGy. No evidence at the human or animal level indicates that
the levels of radiation exposure experienced occupationally or
medically are responsible for any such effects on fetal growth
or development.”
If a pregnant patient must be imaged, then special precautions must always be taken. These precautions are listed later
in this chapter.
Factors Affecting Dose
to the Conceptus
The same factors that affect patient dose in radiography and
fluoroscopy also affect dose to the conceptus. However, as early
as 1999, Parry et al. emphasized two factors that deserve special
attention:
◼◼ Direct exposure. For certain examinations such as the
abdomen, pelvis, and lumbar spine, the fetus can be
exposed directly to the primary beam. In this case, the
dose to the fetus is extremely high. Lead shielding will
not serve any useful purpose.
◼◼ Indirect exposure. For examinations such as the extremities and skull, the fetus will receive radiation scattered
inside the womb. In this case, the dose to the fetus is less
compared to direct exposure. A lead shield will not serve
a useful purpose because the dose is from internal scattered radiation
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Estimating the Dose
to the Conceptus
During situations in which the patient is concerned about the
actual dose to her conceptus, the dose can be estimated and
appropriate actions may be taken. These actions include at least
two choices available to the patient:
◼◼ Continue the pregnancy to term.
◼◼ Terminate the pregnancy.
The fetal dose can be estimated and requires a physicist to
perform the calculations. It is not within the scope of this chapter
to provide details of such calculations; the interested reader
should refer to Solomon et al. (2014) for such details. Several
factors must be known to provide a reasonable estimation of the
dose, such as the output intensity, half‐value layer, location and
number of views for radiography, and beam‐on time for fluoroscopy, among others. Additionally, the fetal age at the time of
the exposure, patient position, patient size (thickness), and fetal
depths must also be known.
Continuing/Terminating a Pregnancy
After Exposure
The notion of continuing or terminating a pregnancy is a matter
between the patient and her physician and requires a benefit–
risk analysis. Such risk assessment is based on knowing the
absorbed dose and the gestational age. These two quantities are
needed before any communications with the patient (ACR‐SPR
2018). Absorbed doses are discussed for radiographic, fluoroscopic, and computed tomography (CT) examinations elsewhere
(ACR‐SPR 2018)
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121
Dose Reduction Techniques
for Pregnant Patients
The following techniques are intended to minimize the dose to
a patient who is pregnant or potentially pregnant:
◼◼ Always check for pregnancy.
◼◼ Elective radiography (examinations that do not contribute
to the diagnosis with respect to the patient’s current health
problems) must not be performed.
◼◼ Use signs and questionnaires to help the patient inform
operators about her state of pregnancy.
◼◼ If a pregnant patient must be irradiated, then the technologist should:
◼◼ Collimate the beam only to the clinical area
of interest.
◼◼ Use high‐kV techniques.
◼◼ Ensure that the radiation beam has the appropriate
filtration.
◼◼ Place gonadal shielding effectively so as not to compromise image quality.
◼◼ Execute all related departmental protocols
regarding in utero exposure such as:
◼◼ Keeping the f luoroscopy time as short
as possible.
◼◼ Reducing the number of images taken.
◼◼ Checking for the date of the last menstrual period.
The Pregnant Worker
There are specific radiation protection regulations and guidelines for pregnant workers in radiology. This section will consider rights to privacy, dose limits, protective clothing, and
maternity policies.
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122 Rad Tech’s Guide to Radiation Protection
Rights to Privacy
Regulations from the National Research Council (NRC)
regarding a pregnancy “have found their way into state regulations for X‐ray workers.” Foremost in consideration is the right
to privacy for the individual:
◼◼ A pregnant technologist should notify the department of
her pregnancy.
Dose Limits and Personnel Monitoring
There are special rules and dose limits for the conceptus once
the pregnancy is declared.
◼◼ For a pregnant worker, the dose limit to an embryo or fetus
is 5 mSv (0.5 rem) (Bushong 2017) over the entire gestational period. This is a dose level “that most technologists
will not reach regardless of pregnancy” (Bushong 2017).
◼◼ The reading from an unshielded dosimeter (worn on the
trunk of the individual) is used to determine the dose to
the conceptus.
◼◼ For pregnant workers who wear protective aprons, a
dosimeter can be worn outside the apron, as well as one
worn under the apron.
◼◼ A dosimeter worn beneath the shielding can provide a
reliable estimate of the dose to the conceptus.
Protective Aprons for Pregnant Workers
Specially designed protective lead aprons are commercially
available for pregnant workers. The following points are important with respect to maternity aprons:
◼◼ Wrap‐around aprons are preferable to attenuate any radi-
ation striking the back of the mother. When the dosimeter
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Pregnancy
123
is in the front of the mother, an unshielded back would
result in the radiation being attenuated by the embryo or
fetus before it is detected by the dosimeter.
◼◼ A lead apron with a 0.5 mm lead‐equivalent thickness will
attenuate the radiation striking the apron by 90% at 75 kV.
◼◼ Lead aprons with a 1 mm lead‐equivalent thickness are
currently available.
Policies for Pregnant Workers
Employer policies for pregnant workers should be available and
should focus on job expectations so as to eliminate any notion
of special treatment that may subject pregnant workers to job
discrimination.
◼◼ Policies should be intended to minimize the dose to the
conceptus during normal periods of work.
◼◼ Policies should ensure, through education and counseling,
that the pregnant worker perform her duties with a clear
understanding of the risks of working while pregnant.
Resources are currently available to help with education and
counseling. One such report is that of the American College of
Radiology (ACR) and the Society for Pediatric Radiology (SPR)
entitled ACR–SPR Practice Parameter for Imaging Pregnant
or Potentially Pregnant Adolescents and Women with Ionizing
Radiation (ACR–SPR 2018).
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1
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Index
absorbed dose, quantifying ionizing
radiation 32
acceptance criteria, quality
control (QC) 115
accessory protective clothing,
equipment specifications for
fluoroscopy 108–109
American College of Radiography
(ACR), diagnostic reference
levels (DRLs) 63
antiscatter grids
digital radiography (DR) 74–75
fluoroscopy 81, 86
‘as low as reasonably
achievable’ (ALARA)
see also dose optimization –
ALARA
radiation protection principle
4, 5, 10–11, 58–59
ATCM see automatic tube current
modulation
attenuation, X‐ray 20–23
beam hardening 22
Compton scattering 24–26
definition 20–22
factors affecting attenuation
22–23
heterogeneous beams 21–22
automatic brightness control (ABC),
fluoroscopy 84
automatic exposure‐rate control
(AERC), fluoroscopy 84
automatic tube current modulation
(ATCM), computed
tomography (CT) 98
automatic voltage selection,
computed tomography
(CT) 98
beam alignment, equipment
specifications for
radiography 105
beam collimation see collimation
beam energy
digital radiography (DR) 71–72
fluoroscopy 82–83
beam hardening, diagnostic
X‐rays 22
beam‐on time, fluoroscopy
83–84
BEIR (Biological Effects of Ionizing
Radiation Committee) 57
bioeffects types 55–56
Biological Effects of Ionizing
Radiation Committee
(BEIR) 57
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130 Index
biologic factors, radiation
protection 3
biologic risks, quantifying 33–36
dose equivalent 34
effective dose 35
equivalent dose 35
radiation weighting factor 34
tissue weighting factor 35–36
bremsstrahlung (brems) radiation,
diagnostic X‐rays 15, 16, 17
Bucky‐slot shielding, equipment
specifications for
fluoroscopy 108
CCD see charge‐coupled device
characteristic radiation, diagnostic
X‐rays 15, 17
charge‐coupled device (CCD), image
intensifier fluoroscopy 80–81
collimation
computed tomography (CT) 97
digital radiography (DR) 73
equipment specifications for
fluoroscopy 107
equipment specifications for
radiography 105
fluoroscopy 84–85
Compton scattering
see also attenuation, X‐ray
diagnostic X‐rays 24–26
computed tomography (CT) 89–100
automatic tube current
modulation (ATCM) 98
automatic voltage selection 98
collimation 97
computed tomography dose index
(CTDI) 94–95
definition 90
dose distribution 93–94
dose factors 96–99
dose length product (DLP) 95
dose metrics 94–96
dose optimization – ALARA
99–100
0004429202.INDD 130
effective dose (ED) 96
effective mAs 97
exposure technique factors
96–97
image quality index 98
iterative reconstruction
algorithms 98–99
multislice CT scanners (MSCT) 93
multislice CT technology 93
Nobel Prize 91
overbeaming 97–98
overranging 97–98
physics 91–93
pitch 97
principles 91–93
scheme for patient exposure 6–8
size‐specific dose estimate
(SSDE) 95
slice thickness 97
technology 91–93
computed tomography dose index
(CTDI), computed tomography
(CT) 94–95
computed tomography factors
affecting dose, diagnostic
radiology 10
computer, digital fluoroscopy 82
cosmic radiation, natural
radiation source 29
criteria and standards, radiation
protection 58–60
CT see computed tomography
CTDI see computed tomography
dose index
cumulative timer, equipment
specifications for
fluoroscopy 107
definition of terms in safety
reports 103
detective quantum efficiency (DQE),
digital radiography (DR) 75–76
deterministic effects, bioeffects
type 55–56
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Index
diagnostic radiology
computed tomography factors
affecting dose 10
dose factors 8–10
fluoroscopic factors
affecting dose 9
modalities 3
radiographic factors affecting dose 9
reasons for protection 3–4
diagnostic reference levels
(DRLs) 62–64
see also dose optimization –
ALARA
American College of
Radiography (ACR) 63
definition 63
dose minimizing 111
guidelines 64
International Commission
on Radiologic Protection
(ICRP) 63, 64
optimization of radiation
protection (ORP) 62–64
diagnostic X‐rays 13–26
bremsstrahlung (brems)
radiation 15, 16, 17
characteristic radiation 15, 17
Compton scattering 24–26
controlling beam quality and
quantity 18–19
filtration 19
generator type 19
increasing kV and scatter
production 26
mechanisms for creating
X‐rays 14–16
photoelectric absorption
(photoelectric effect) 23–24
scattering 24–26
source‐to‐image receptor
distance (SID) 19
target material 19
X‐ray attenuation 20–23, 24–26
X‐ray interactions 23–26
0004429202.INDD 131
131
X‐ray production 13–14
X‐ray quality 18–19
X‐ray quantity 18–19
X‐ray spectrum 16–19
digital fluoroscopy 81–82
see also fluoroscopy
components 81–82
computer 82
flat‐panel digital detector 81–82
image display monitor 82
scattered radiation grid 81, 86
X‐ray tube and generator 81
digital radiography (DR) 65–76
beam energy 71–72
collimation 73
definition 66
detective quantum efficiency
(DQE) 75–76
dose factors 70–76
exposure indicator/index
(EI) 67–68
exposure technique factors 72–73
vs film‐screen radiography
(FSR) 66–68
filtration 72
image detector sensitivity
or speed 75
kilovoltage or kV 71
patient size 73
radiographic antiscatter
grids 74–75
scheme for patient exposure 6–8
source‐to‐image receptor distance
(SID) 73–74
X‐ray generator 71–72
direct exposure
dose factors 119
pregnancy 119
distance
see also source‐to‐image receptor
distance; source‐to‐skin distance
radiation protection
action 6, 59–60
DLP see dose length product
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132 Index
dose considerations
dose factors in diagnostic
radiology 8–10
pregnancy: radiation protection
considerations 12
dose distribution, computed
tomography (CT) 93–94
dose equivalent, quantifying
biologic risks 34
dose estimation, pregnancy 120
dose factors
computed tomography
(CT) 10, 96–99
diagnostic radiology 8–10
digital radiography (DR) 65–76
direct exposure 119
fluoroscopy 9, 82–87
indirect exposure 119
pregnancy 119
radiographic factors 9
dose length product (DLP), computed
tomography (CT) 95
dose limitation
see also recommended dose limits
radiation protection principle 5, 59
dose management regulations and
optimization 101–116
definition of terms in safety
reports 103
dose minimizing 109–112
equipment specifications for
fluoroscopy 106–109
equipment specifications for
radiography 104–106
federal regulations 103
quality assurance (QA)
114–116
shielding 112–114
dose management techniques 10–11
dose metrics, computed tomography
(CT) 94–96
dose minimizing 109–112
see also dose optimization –
ALARA
0004429202.INDD 132
diagnostic reference levels
(DRLs) 111
fluoroscopy 110–112
procedural factors 111–112
radiography 109–110
dose optimization – ALARA
see also dose minimizing
computed tomography
(CT) 99–100
procedural factors 111–112
quality assurance (QA) 116
radiation protection principle 4, 5,
10–11, 58–59
dose reduction 11, 116
see also dose minimizing; dose
optimization – ALARA
pregnancy 121
DQE see detective quantum efficiency
DR see digital radiography
DRLs see diagnostic reference levels
earth sources (terrestrial radiation),
natural radiation source 29
ED see effective dose
education and training, dose
management technique 11
effective dose, quantifying
biologic risks 35
effective dose (ED), computed
tomography (CT) 96
effective mAs, computed
tomography (CT) 97
EI see exposure indicator/index
equipment design and performance,
dose management technique 10
equipment specifications for
fluoroscopy 106–109
accessory protective
clothing 108–109
Bucky‐slot shielding 108
collimation 107
cumulative timer 107
exposure switch 107
filtration 106–107
7/8/2019 2:09:19 PM
Index
protective curtains 108
source‐to‐skin distance (SSD) 107
equipment specifications for
radiography
beam alignment 105
collimation 105
dose management regulations and
optimization 104–106
exposure switch for
radiography 106
filtration 105
leakage radiation from the
X‐ray tube 104
source‐to‐image receptor
distance (SID) 105
source‐to‐skin distance (SSD) 106
X‐ray control panel 104
equivalent dose, quantifying
biologic risks 35
exposure, quantifying ionizing
radiation 31–32
exposure indicator/index (EI)
digital radiography (DR) 67–68
standardized EI 68–69
exposure switch
equipment specifications for
fluoroscopy 107
equipment specifications for
radiography 106
exposure technique factors
computed tomography (CT) 96–97
digital radiography (DR) 72–73
factors affecting dose see dose factors
FDA see Food and Drug
Administration
federal regulations
see also dose management
regulations and optimization
dose management 103
f‐factor, quantifying ionizing
radiation 33
film dosimetry, radiation
measurement 36–37
0004429202.INDD 133
133
film‐screen radiography (FSR) 65
vs digital radiography (DR) 66–68
filtration
diagnostic X‐rays 19
digital radiography (DR) 72
equipment specifications for
fluoroscopy 106–107
equipment specifications for
radiography 105
flat‐panel digital detector, digital
fluoroscopy 81–82
fluoroscopic factors affecting dose,
diagnostic radiology 9
fluoroscopy 77–88
see also digital fluoroscopy
antiscatter grids 81, 86
automatic brightness
control (ABC) 84
automatic exposure‐rate
control (AERC) 84
beam energy 82–83
beam‐on time 83–84
collimation 84–85
components of fluoroscopic
imaging systems 78–82
digital fluoroscopy 81–82
dose factors 82–87
dose minimizing 110–112
equipment specifications 106–109
image intensifier fluoroscopy
79–81
image magnification 86–87
last image hold 87
patient size 85–86
pregnancy: radiation protection
considerations 12
pulsed fluoroscopy 87
reasons for importance 78
scattered radiation 87–88
scheme for patient exposure
6–8
source‐to‐skin distance (SSD) 85
tube current 83
wearing a personnel dosimeter 38
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134 Index
Food and Drug Administration
(FDA) 57
framework for radiation protection,
radiation protection actions 6
FSR see film‐screen radiography
generator type, diagnostic X‐rays 19
heterogeneous beams, diagnostic
X‐rays 21–22
high‐kVp techniques and increased
filtration, pregnancy: radiation
protection considerations 12
ICRP see International Commission on
Radiologic Protection
ICRU see International Commission
on Radiological Units and
Measurements
image detector sensitivity or speed,
digital radiography (DR) 75
image display monitor, digital
fluoroscopy 82
image distributor, image intensifier
fluoroscopy 80
image intensifier fluoroscopy 79–81
charge‐coupled device
(CCD) 80–81
components 79–81
image distributor 80
image intensifier tube 80
spot‐film recording devices 81
television camera 80–81
television monitor 81
X‐ray tube and generator 79–80
image intensifier tube, image
intensifier fluoroscopy 80
image magnification,
fluoroscopy 86–87
image quality index, computed
tomography (CT) 98
indirect exposure
dose factors 119
pregnancy 119
0004429202.INDD 134
International Commission on
Radiological Units and
Measurements (ICRU) 56–57
International Commission on
Radiologic Protection (ICRP) 56
diagnostic reference levels
(DRLs) 63, 64
recommended dose limits 61
International System of Units (SI
units), National Council of
Radiation Protection and
Measurements (NCRP) 31
ionization chamber, radiation
measurement 36
iterative reconstruction algorithms,
computed tomography (CT) 98–99
justification, radiation protection
principle 5, 58
KERMA see kinetic energy released
per unit mass
kilovoltage or kV, digital
radiography (DR) 71
kinetic energy released per unit mass
(KERMA), quantifying ionizing
radiation 32
last image hold, fluoroscopy 87
leakage radiation from the X‐ray tube,
equipment specifications for
radiography 104
linear energy transfer (LET),
quantifying ionizing
radiation 33
medical exposure to radiation 5, 30
medical X‐ray exposure, source of
radiation exposure 30
members of the public see public
exposure to radiation
minimizing dose see dose minimizing
multislice CT scanners (MSCT) 93
multislice CT technology 93
7/8/2019 2:09:19 PM
Index
National Council of Radiation
Protection and Measurements
(NCRP) 4, 12, 57
International System of Units
(SI units) 31
NCRP Report No. 172, Reference
Levels and Achievable Doses in
Medical and Dental Imaging:
Recommendations for the United
States (NCRP 2015) 111
recommended dose limits 61
wearing a personnel dosimeter 38
National Radiological Protection
Board (NRPB) 4
natural radiation sources 29
cosmic radiation 29
earth sources (terrestrial
radiation) 29
radon 29
NCRP see National Council of
Radiation Protection and
Measurements
NRPB see National Radiological
Protection Board
nuclear medicine examinations,
source of radiation exposure 30
objectives, radiation protection
57–58
occupational exposure to
radiation 5, 30
recommended dose limits 60–61
optically stimulated luminescence
(OSL) dosimetry, radiation
measurement 37–38
optimization, dose see dose
management regulations and
optimization; dose minimizing;
dose optimization – ALARA
optimization of radiation
protection (ORP)
see also dose optimization – ALARA
diagnostic reference levels
(DRLs) 62–64
0004429202.INDD 135
135
organizations, radiation
protection 56–57
ORP see optimization of radiation
protection
OSL dosimetry see
optically stimulated
luminescence dosimetry
overbeaming, computed tomography
(CT) 97–98
overranging, computed tomography
(CT) 97–98
patient size
digital radiography (DR) 73
fluoroscopy 85–86
personnel monitoring, pregnant
workers 122
personnel practices, dose
management technique 10–11
photoelectric absorption
(photoelectric effect), diagnostic
X‐rays 23–24
physical factors, radiation
protection 2–3
pitch, computed tomography (CT) 97
policies, pregnant workers 123
pregnancy 117–123
continuing/terminating 120
direct exposure 119
dose considerations 12
dose estimation 120
dose factors 119
dose reduction 121
effects of radiation 118–119
fluoroscopy 12
high‐kVp techniques and
increased filtration 12
indirect exposure 119
radiation protection
considerations 11–12
rationale for radiation
protection 118–119
shielding 12
ultrasound 12
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136 Index
pregnant workers 121–123
dose limits 122
personnel monitoring 122
policies 123
privacy rights 122
protective aprons 122–123
recommended dose limits 61–62
privacy rights, pregnant workers 122
procedural factors, radiation
protection 3
protective barriers
see also shielding
design 112–114
dose management regulations and
optimization 112–114
protective aprons, pregnant
workers 122–123
protective clothing, equipment
specifications for fluoroscopy
108–109
protective curtains, equipment
specifications for
fluoroscopy 108
public exposure to radiation 5, 31
recommended dose limits 62
pulsed fluoroscopy 87
QA see quality assurance
QC see quality control
quality assurance (QA)
dose management regulations and
optimization 114–116
dose optimization – ALARA 116
goals 114
quality control (QC)
acceptance criteria 115
dose management regulations and
optimization 114–116
goals 114
tolerance limits 115
quantifying biologic risks see biologic
risks, quantifying
quantifying ionizing radiation
absorbed dose 32
0004429202.INDD 136
exposure 31–32
f‐factor 33
importance 28
kinetic energy released per unit
mass (KERMA) 32
linear energy transfer (LET) 33
quantities and units 31–33
quantities and units, quantifying
ionizing radiation 31–33
radiation exposure
sources 29–30
types 30–31
radiation measurement 36–38
film dosimetry 36–37
ionization chamber 36
Optically Stimulated Luminescence
(OSL) dosimetry 37–38
thermoluminescent
dosimetry (TLD) 37
wearing a personnel dosimeter 38
radiation protection
actions 6, 59–60
biologic factors 3
criteria and standards 58–60
objectives 57–58
physical factors 2–3
principles 5, 58–59
procedural factors 3
scope 2–3
technical factors 3
Radiation Protection Bureau, Health
Canada (RPB‐HC) 4, 57
recommended dose limits 61
radiation protection framework 4–6
dose limitation 5
dose optimization – ALARA 5
exposure types 5
justification 5
radiation protection principles 5
radiation protection
organizations 56–57
radiation weighting factor,
quantifying biologic risks 34
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Index
radiobiology 39–54
radiographic antiscatter grids, digital
radiography (DR) 74–75
radiographic factors affecting dose,
diagnostic radiology 9
radiography
see also digital radiography
dose minimizing 109–110
radon, natural radiation source 29
reasons for protection in diagnostic
radiology 3–4
recommended dose limits 60–62
International Commission on
Radiologic Protection (ICRP) 61
members of the public 62
National Council of
Radiation Protection and
Measurements (NCRP) 61
occupational exposure 60–61
pregnant workers 61–62, 122
Radiation Protection Bureau,
Health Canada (RPB‐HC) 61
regulations, dose see dose management
regulations and optimization
risks, biologic see biologic risks,
quantifying
RPB‐HC see Radiation Protection
Bureau, Health Canada
scattered radiation,
fluoroscopy 87–88
scattered radiation grids
digital fluoroscopy 81, 86
digital radiography (DR) 74–75
schemes for patient exposure 6–8
scope of radiation protection 2–3
shielding
see also protective barriers
dose management regulations and
optimization 112–114
dose management technique 11
pregnancy: radiation protection
considerations 12
radiation protection action 6, 59
0004429202.INDD 137
137
SID see source‐to‐image
receptor distance
SI units see International
System of Units
size‐specific dose estimate (SSDE),
computed tomography (CT) 95
slice thickness, computed
tomography (CT) 97
sources of radiation exposure 29–30
human origin sources 29–30
medical X‐ray exposure 30
natural radiation sources 29
nuclear medicine examinations 30
source‐to‐image receptor
distance (SID)
diagnostic X‐rays 19
digital radiography (DR) 73–74
equipment specifications for
radiography 105
source‐to‐skin distance (SSD)
equipment specifications for
fluoroscopy 107
equipment specifications for
radiography 106
fluoroscopy 85
spot‐film recording devices, image
intensifier fluoroscopy 81
SSD see source‐to‐skin distance
SSDE see size‐specific dose estimate
standardized EI, exposure indicator/
index (EI) 68–69
standards and criteria, radiation
protection 58–60
stochastic effects, bioeffects type 56
target material, diagnostic X‐rays 19
technical factors, radiation
protection 3
television camera, image intensifier
fluoroscopy 80–81
television monitor, image intensifier
fluoroscopy 81
thermoluminescent dosimetry (TLD),
radiation measurement 37
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138 Index
time, radiation protection action
6, 59
tissue weighting factor, quantifying
biologic risks 35–36
TLD see thermoluminescent
dosimetry
tolerance limits, quality
control (QC) 115
tube current, fluoroscopy 83
ultrasound, pregnancy 12
United Nations Scientific Committee
on the Effects of Atomic
Radiation (UNSCEAR) 57
0004429202.INDD 138
wearing a personnel dosimeter,
radiation measurement 38
X‐ray control panel, equipment
specifications for
radiography 104
X‐ray generator, digital radiography
(DR) 71–72
X‐rays see diagnostic X‐rays; medical
X‐ray exposure
X‐ray tube and generator
digital fluoroscopy 81
image intensifier fluoroscopy
79–80
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