radiation protection in diagnostic radiology

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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
RADIATION PROTECTION IN
DIAGNOSTIC AND
INTERVENTIONAL RADIOLOGY
L 20: Optimization of Protection in Digital
Radiology
IAEA
International Atomic Energy Agency
Topics
Introduction
Basic concepts
Relation between diagnostic information
and patient dose
Quality Assurance
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Overview
• To become familiar with the digital imaging
techniques in projection radiography and
fluoroscopy, to understand the basis of the
DICOM standard and the influence of the
digital radiology on image quality and patient
doses
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 20: Digital Radiology
Topic 1: Introduction
IAEA
International Atomic Energy Agency
Transition from conventional to digital radiology
 Many conventional fluoroscopic and
radiographic equipment have recently
been replaced by digital techniques in
industrialized countries
 Digital radiology has become a
challenge which may have advantages
as well as disadvantages
 Changing from conventional to digital
radiology requires additional training
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Transition from conventional to digital radiology
 Digital images can be numerically processed
This is not possible in conventional
radiology!!.
 Digital images can be easily transmitted through
networks and archived
 Attention should be paid to the potential increase
of patient doses due to tendency of:
producing more images than needed
producing higher image quality not
necessarily required for the clinical purpose
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Radiation dose in digital radiology
 Conventional films allow to detect
mistakes if a wrong radiographic
technique is used: images are too
white or too black
 Digital technology provides user
always with a “good image” since
its dynamic range compensates for
wrong settings even if the dose is
higher than necessary
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What is “dynamic range”?
Wide dose range to the detector, allows a
“reasonable” image quality to be obtained
Flat panel detectors (discussed later) have
a dynamic range of 104 (from 1 to 10,000)
while a screen-film system has
approximately 101.5
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Characteristic curve of CR system
3.5
HR-III
3
CEA Film-Fuji Mammofine
Density
2.5
2
1.5
CR response
1
0.5
0
0.001
0.01
0.1
1
Air Kerma (mGy)
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Intrinsic digital techniques
• Digital radiography and digital fluoroscopy
are new imaging techniques, which
substitute film based image acquisition
• There are intrinsic digital modalities which
do not have any equivalent in conventional
radiology (CT, MRI, etc).
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Digitizing conventional films
 Conventional radiographic images can be
converted into digital information by a
“digitizer”, and therefore electronically
stored
 Such a conversion also allows some
numerical post-processing
 Such a technique cannot be considered as
a “ digital radiology” technique.
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 20: Digital Radiology
Topic 2: Basic concepts
IAEA
International Atomic Energy Agency
Analogue versus digital
Analogue: A given
parameter can have
continuous values
Digital: A given
parameter can only
have discrete values
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C1
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What is digital radiology?
 In conventional radiographic images, spatial




position and blackening are analogue values
Digital radiology uses a matrix to represent an
image
A matrix is a square or rectangular area divided
into rows and columns. The smallest element
of a matrix is called ”pixel”
Each pixel of the matrix is used to store the
individual grey levels of an image, which are
represented by positive integer numbers
The location of each pixel in a matrix is
encoded by its row and column number (x,y)
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Different number of pixels per image: original was 3732 x 3062 pixels
x 256 grey levels (21.8 Mbytes). Here, resized at 1024 x 840 (1.6 MB).
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Different number of pixels per image: original was 3732 x 3062 pixels
x 256 grey levels (21.8 Mbytes). Here, resized at 128 x 105 (26.2 kB).
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Different number of pixels per image: original was 3732 x 3062 pixels
x 256 grey levels (21.8 Mbytes). Here, resized at 64 x 53 (6.6 kB)
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The digital radiology department
 In addition to the X-ray rooms and
imaging systems, a digital radiology
department has two other components:
 A Radiology Information
management System (RIS) that can
be a subset of the hospital
information system (HIS)
 A Picture Archiving and
Communication System (PACS).
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DICOM
• DICOM (Digital Imaging and Communications in
Medicine) is the industry standard for transferal of
radiological images and other medical information
between different systems
• All recently introduced medical products should
therefore be in compliance with the DICOM
standard
• However, due to the rapid development of new
technologies and methods, the compatibility and
connectivity of systems from different vendors is
still a great challenge
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DICOM format images:
 Radiology images in DICOM format contain
in addition to the image, a header, with an
important set of additional data related with:
 the X ray system used to obtain the image
 the identification of the patient
 the radiographic technique, dosimetric details,
etc.
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Digital radiology process
 Image acquisition
 Image processing
 Image display
 Importance of viewing conditions
 Image archiving (PACS)
 Image retrieving
 Importance of time allocated to retrieve
images
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Outline of a basic PACS system
Radiotherapy
Department
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Image acquisition (I):
 Phosphor photostimulable plates (PSP).
• So called CR (computed radiography)
• Conventional X-ray systems can be
used
 Direct digital registration of image at the
detector (flat panel detectors).
• Direct conversion (selenium)
• Indirect conversion (scintillation)
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Computed Radiography (CR)
• CR utilises the principle of photostimulable
phosphor luminescence
• Image plate made of a suitable phosphor
material are exposed to X-rays in the same way
as a conventional screen-film combination
• However unlike a normal radiographic screen,
which releases light spontaneously upon
exposure to X-rays, the CR image plate retains
most of the absorbed X-ray energy, in energy
traps, forming a latent image
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Computed Radiography (CR)
 A scanning laser is then used to release the
stored energy producing luminescence
 The emitted light, which is linearly proportional
to the locally incident X-ray intensity over at
least four decades of exposure range, is
detected by a photo multiplier/ADC
configuration and converted to a digital image
 The resultant images have a digital
specification of 2,370 x 1,770 pixels (for
mammograms) with 1,024 grey levels (10 bits)
and a pixel size of 100 mm corresponding to a
24 x 18 cm field size
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The principle of PSP
PMT
ADC
CB
Trap
Excitation
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Storage
Emission
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Cassette and PSP
PSP digitizer
Workstation
(Images courtesy of AFGA)
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Digital detector
(Images courtesy of GE Medical Systems)
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Image acquisition (II)
 Other alternatives are:
 Selenium cylinder detector (introduced
for chest radiography with a vertical
mounted rotating cylinder coated with
selenium)
 Charge Coupled Devices (CCD)
 The image of a luminescent screen is
recorded with CCD cameras or devices
and converted into digital images
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Digital fluoroscopy
• Digital fluoroscopic systems are mainly based on the
•
•
•
•
use of image intensifiers (I.I.)
In conventional systems the output screen of the I.I. is
projected by an optical lens onto a film. In digital
systems the output screen is projected onto a video
camera system or a CCD camera
The output signals of the camera are converted into a
digital image matrix (1024 x 1024 pixel in most
systems).
Typical digital functions are “last image hold”, “virtual
collimation”, etc.
Some new systems start to use flat panel detectors
instead of image intensifier.
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 20: Digital Radiology
Topic 3: Relation between diagnostic information
and patient dose
IAEA
International Atomic Energy Agency
Image quality and dose
• Diagnostic information content in digital
radiology is generally higher than in
conventional radiology if equivalent dose
parameters are used
• The wider dynamic range of the digital
detectors and the capabilities of post
processing allow to obtain more information
from the radiographic images
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Tendency to increase dose ?
 In digital radiology, some parameters that
usually characterize image quality (e.g. noise)
correlate well with dose
 For digital detectors, higher doses result in
a better image quality (less “noisy” images)
 Actually, when increasing dose, is the signal to
noise ratio which is improved
 Thus, a certain tendency to increase doses
could happen specially in those examinations
where automatic exposure control is not usually
available (e.g. in bed patients).
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Computed radiography versus film screen
• In computed radiography (CR) the “image density” is
automatically adjusted by the image processing, no
matter of the applied dose.
• This is one of the key advantages of the CR which helps to
reduce significantly the retakes rate, but at the same time
may hide occasional or systematic under or overexposures.
• Underexposures are easily corrected by radiographers
(too noisy image).
• Overexposures cannot be detected unless patient dose
measurements are performed
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 Underexposure results in a “too noisy” image
 Overexposure yields good images with
unnecessary high dose to the patient
 Over range of digitiser may result in uniformly
black area with potential loss of information
Exposure level 2,98
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Exposure level 2,36
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An underexposed image is “too noisy”
Exposure level 1,15
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Exposure level 1,87
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Exposure level
Some digital systems provide the user with
a so called “exposure level” index which
expresses the dose level received at the
digital detector and orientates the operator
about the goodness of the radiographic
technique used
The relation between dose and exposure
level is usually logarithmic: doubling the
dose to the detector, will increase the
“exposure level” to a factor of 0.3 = log(2).
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Risk to increase doses:
 The wide dynamic range of digital
detectors allows to obtain good
image quality while using high dose
technique at the entrance of the detector
and at the entrance of the patient
 With conventional screen film systems such
a choice is not possible since high dose
technique always results in a “too black”
image.
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Digital fluoroscopy:
In digital fluoroscopy there is a direct link
between diagnostic information (number of
images and quality of the images) and
patient dose
Digital fluoroscopy allows producing very
easily a great number of images (since
there is no need to introduce cassettes or film
changers as in the analogical systems).
As a consequence of that: dose to the
patient is likely to increase without any
benefit
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Difficulty to audit the number of
images per procedure
• Deleting useless images before sending
them to the PACS is also very easy in digital
fluoroscopy
• This makes difficult any auditing of the dose
imparted to the patient
• The same applies to projection radiography
to audit the retakes.
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Actions that can influence image quality
and patient doses in digital radiology (1)
• Ask for a significant reduction of noise
(detector saturation in some areas, e.g. lung
in chest images)
• Avoid bad viewing conditions (e.g. lack of
monitor brightness or contrast, poor spatial
resolution, etc)
• Improve insufficient skill to use the
workstation capabilities to visualize images
(window level, inversion, magnification, etc).
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Actions that can influence image quality
and patient doses in digital radiology (2)
• Eliminate post-processing problems, digitizer
problems, local hard disk, fault in electrical
power supply, network problems during
image archiving etc.
• Avoid loss of images in the network or in the
PACS due to bad identification or others
• Reduce artifacts due to incorrect digital postprocessing (creation of false lesions or
pathologies)
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Actions that can influence image quality
and patient doses in digital radiology (3)
• Promote easy access to the PACS to look previous
images to avoid repetitions.
• Use easy access to teleradiology network to look
previous images.
• Display dose indication at the console of the X ray
system.
• Availability of a workstation for post-processing
(also for radiographers) additional to hard copy to
avoid some retakes.
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Influence of the different image
compression levels
 Image compression can:
• influence the image quality of stored images in the
PACS
• modify the time necessary to have the images
available (transmission speed in the intranet)
 A too high level of image compression may
result in a loss of image quality and,
consequently, in a possible repetition of the
examination (extra radiation dose to the
patients)
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Digital radiography: initial pitfalls (1)
• Lack of training (and people reluctant to
computers)
• Mismatching of image density on the monitor and
dose level (and as a consequence, to increase
doses).
• Lack of knowledge of the viewing possibilities on
the monitors (and post-processing capabilities).
• Drastic changes in radiographic techniques or
geometric parameters without paying attention to
patient doses (image quality are usually good
enough with the post-processing).
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Digital radiography: initial pitfalls (2)
• The radiologist advice on the image quality should
be taken into consideration before printing the
images
• Lack of a preliminary image visualization on the
monitors (made by the radiologist) may result in a
loss of diagnostic information (wrong contrast and
window levels selection made by the radiographer)
• The quality of the image to be sent (Tele-radiology)
has to be adequately determined , in particular
when re-processing is not available
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 20: Digital Radiology
Topic 4: Quality Assurance
IAEA
International Atomic Energy Agency
Important aspects to be considered for the
QA programs in digital radiology(1)
• Availability of requirements for different digital
•
•
•
•
systems (CR, digital fluoroscopy, etc).
Availability of procedures avoiding loss of images
due to network problems or electric power supply
Information confidentiality
Compromise between image quality and
compression level in the images
Recommended minimum time to archive the
images
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Important aspects to be considered for the
QA programs in digital radiology(2)
• Measurement of dosimetric parameters and
records keeping
• Specific reference levels
• How to avoid that radiographers delete
images (or full series in fluoroscopy
systems)
• How to audit patient doses
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Displaying of dose related parameters
(1)
• Medical specialists should take care of the dose
delivered to the patients referring to the physical
parameters displayed (when available) at the
control panel level (or inside the X-ray room, for
interventional procedures)
• Some digital systems offer a color code or a bar in
the previsualization monitor. This code or bar
indicates the operator whether the dose received
by the detector is in the normal range (green or
blue color) or whether it is too high (red color).
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• Example of bar in
the image showing
the level of dose
received by the
digital detector
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Displaying of dose related parameters
(2)
• The use of the radiographic and dosimetric data
contained in DICOM header can also be used to
auditing patient doses
• If radiographic (kV, mA, time, distances, filters, field
size, etc) and dosimetric data (entrance dose,
dose area product, etc) are transferred to the
image DICOM header, some automatic on-line or
retrospective analysis of patient doses can be
performed and assessed against the image quality.
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Reference levels
• In digital radiology, the evaluation of patient doses
should be performed more frequently than in
conventional radiology:
• Easy improvement of image quality
• Unknown use of high dose technique
• Re-assessment of local reference levels when new
digital techniques are introduced is recommended
to demonstrate the optimization of the systems and
to establish a baseline value useful for future
patient dose assessment
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Initial basic quality control
• A first tentative approach could be:
• to obtain images of a test object under different
radiographic conditions (measuring the
corresponding doses)
• to decide the best compromise considering both
image quality and patient dose aspects
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Optimisation technique
TOR(CDR) plus ANSI
phantom to simulate
chest and abdomen
examinations and to
evaluate image quality
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Optimization technique for Abdomen AP
Simulation with TOR(CDR) + ANSI phantom
12
3
10
2.5
8
2
6
1.5
4
1
2
0.5
1.6 mGy
0
0
20
High cont. (n)
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lp/mm
number of objects
81 kVp, 100 cm (focus-film distance)
0
40
60
Low cont. (n)
80
mAs
Resol. (lp/mm)
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Optimisation technique for Chest PA
14
12
10
8
6
4
2
0
3.5
3
2.5
2
1.5
1
0.5
0
0
0.2510
mGy
High cont. (no.)
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30
40
lp/mm
number of objects
Simulation with TOR(CDR) + ANSI phantom
125 kVp, 180 cm (focus-film distance)
* Grid focalised at 130 cm
50
mAs
Low cont. (no.)
Resol. (lp/mm)
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Image quality comparison
Exam.
Type
Resolution
(lp/mm)
Low contrast
sensitivity
threshold
High contrast
sensitivity
threshold
Conv
2.50
7
9
CR
3.15
9
9
Conv
3.55
8
6
CR
2.24
7
6
Conv
7.10
11
14
CR
2.80
16
16
Abdomen
Chest
TOR(CDR)+
1.5 mm Cu
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Routine QC programme
• Not affected by change to CR
• Patient dose evaluation (when optimised)
• Tube-generator controls (except. AEC)
• Affected by change to CR
• Image quality evaluation with test object
• Image quality evaluation with clinical
criteria
• Image receptors (film-screen, viewing...)
• Automatic processors
• Image processing
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QC equipment
• Available
• TOR(CDR) image quality
test
• Photometer
• Densitometer
• Dosimeters
• Needed
• CR image quality test
object
• SMPTE image test
• Pencil type photometer
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Workload with CR
High
• Image quality with test object
• CRT evaluation (monitors)
Low
• Rejection rate analysis
• Image devices: film-screen,
dark rooms,...
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Summary
• Digital radiology requires some specific training to
benefit of the advantages of this new technique.
• Image quality and diagnostic information are
closely related with patient dose.
• The transmission, archiving an retrieving of images
can also influence the workflow and patient doses
• Quality assurance programs are specially
important in digital radiology due to risk of
increasing patient doses
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Where to Get More Information (1)
• Balter S. Interventional fluoroscopy.
Physics, technology and safety. WileyLiss, New York, 2001.
• Radiation Protection Dosimetry. Vol 94
No 1-2 (2001). Dose and image quality
in digital imaging and interventional
radiology (DIMOND) Workshop held in
Dublin, Ireland. June 24-26 1999.
• ICRP draft on Dose Management in
Digital Radiology. Expected for 2003.
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Where to Get More Information (2)
• Practical Digital Imaging and PACS.
Seibert JA, Filipow LJ, Andriole KP,
Editors. Medical Physics Monograph No.
25. AAPM 1999 Summer School
Proceedings.
• PACS. Basic Principles and Applications.
Huang HK. Wiley – Liss, New York, 1999.
• Vañó E, Fernandez JM, Gracia A,
Guibelalde E, Gonzalez L. Routine Quality
Control in Digital versus Analog Radiology.
Physica Medica 1999; XV(4): 319-321.
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Where to Get More Information (2)
• http://www.gemedicalsystems.com/rad/x
r/education/dig_xray_intro.html (last
access 22 August 2002).
• http://www.agfa.com/healthcare/ (last
access 22 August 2002).
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