Optimization of Protection in Computed
Tomography (CT)
Introduction
• The subject matter: CT scanner and related
image quality considerations
• The importance of the technological
improvement made in this field
• The quality criteria system developed to
optimize the CT procedure
• Background: medical doctor, medical physicist
2
Topics
CT equipment and technology
Radiation protection rules and operational
consideration
Quality criteria for CT images
3
Overview
• To understand the principles and the
technology of CT
• To be able to apply the principle of radiation
protection to CT scanner including design,
Quality Control and dosimetry.
4
Optimization of protection in CT
scanner
Topic 1: CT equipment and
technology
Introduction
• Computed Tomography (CT) was introduced into clinical practice
in 1972 and revolutionized X Ray imaging by providing high
quality images which reproduced transverse cross sections of the
body.
• Tissues are not superimposed on the image as they are in
conventional projections
• The CT provides improved low contrast resolution for better
visualization of soft tissue, but with relatively high radiation
dose, i.e. CT is a high dose procedure
6
Computed Tomography
• CT uses a rotating X Ray tube, with the beam
in the form of a thin slice (about 1 - 10 mm)
• The “image” is a simple array of X Ray
intensities, and many hundreds of these are
used to make the CT image, which is a “slice”
through the patient
7
The CT Scanner
8
A look inside a rotate/rotate CT
Detector
Array
and
Collimator
X Ray
Tube
9
Helical (spiral) CT
• If the X Ray tube can rotate constantly, the
patient can then be moved continuously
through the beam, making the examination
much faster
10
Helical Scan Principle
• Scanning Geometry
X Ray beam
Direction of patient
movement
• Continuous Data Acquisition and Table Feed
11
Helical CT Scanners
• For helical scanners, the X Ray tube rotates
continuously
• This is obviously not possible with a cable
combining all electrical sources and signals
• A “slip ring” is used to supply power and to
collect the signals
12
A Look Inside a Slip Ring CT
X Ray
Tube
Detector
Array
Note:
how most
of the
electronics
are
placed on
the rotating
gantry
Slip Ring
13
New CT Features
• The new helical scanning CT units allow a
range of new features, such as:
– CT fluoroscopy, where the patient is stationary,
but the tube continues to rotate
– multislice CT, where up to 128 slices can be
collected simultaneously
– 3-dimensional CT and CT endoscopy
14
CT Fluoroscopy
• Real Time Guidance
(up to 8 fps)
• Great Image Quality
• High Dose Rate
• Faster Procedures
(up to 66% faster
than non-fluoroscopic
procedures)
• Approx. 80 kVp, 30 mA
15
Multi slice CT collimation
5mm
2,5mm
1mm
0,5mm
16
3D Stereo Imaging
17
CT Endoscopy
18
CT Scanner
– Generator
• High frequency, 30 - 70 kW
– X Ray tube
• Rotating anode, high thermal capacity: 3-7
MHU
• Dual focal spot sizes: about 0.8 and 1.4
– Gantry
•
•
•
•
•
Aperture: > 70 cm of diameter
Detectors: gas or solid state; > 600 detectors
Scanning time: <1 s, 1 - 4 s
Slice thickness: 1 - 10 mm
Spiral scanning: up to 1400 mm
19
Image processing

Reconstruction time:



Reconstruction matrix:
256x256 – 1024x1024
Reconstruction algorithms:


0.5 - 5 s/slice
Bone, Standard, High
resolution, etc
Special image processing
software:




3D reconstruction
Angio CT with MIP
Virtual endoscopy
CT fluoroscopy
20
Spiral (helical) CT
Spiral CT and Spiral multislice CT:
Volume acquisition may be preferred to serial CT
• Advantages:
 dose reduction:
• reduction of single scan repetition (shorter examination times)
• replacement of overlapped thin slices (high quality 3D display) by the
reconstruction of one helical scan volume data
• use of pitch > 1
 no data missing as in the case of inter-slice interval
 shorter examination time
• to acquire data during a single breath-holding period avoiding respiratory
disturbances
• disturbances due to involuntary movements such as peristalsis and
cardiovascular action are reduced
21
Spiral (helical) CT
Drawbacks
– Increasing of dose:
• equipment performance may tempt the
operator to extend the examination area
– Use of a pitch > 1.5 and an image
reconstruction at intervals equal to the
slice width results in lower diagnostic
image quality due to reduced low
contrast resolution
– Loss of spatial resolution in the z-axes
unless special interpolation is performed
– Technique inherent artifact
22
Optimization of protection in CT
scanner
Topic 2: Radiation protection rules
and operational consideration
Contribution to collective dose (I)
• As a result of such technological improvements, the
number of examinations have markedly increased
• Today CT procedures contribute for up to 40% of the
collective dose from diagnostic radiology in all
developed countries
• Special protection measures are therefore required
24
Examination
Mean effective dose (mSv)
Routine head
1.8
Posterior fossa
0.7
Orbits
0.6
Cervical spine
2.6
Chest
7.8
Abdomen
7.6
Liver
7.2
Pelvis
7.1
Lumbar spine
3.3
CT scanners in clinical use in UK
Contribution to collective dose (II)
500
400
300
200
100
0
70
75
80
85
90
95
Years
25
Justification of CT practice
• Justification in CT is of particular importance for RP
• CT examination is a “high dose” procedure
• A series of clinical factors play a special part
– Adequate clinical information, including the records of previous
imaging investigations, must be available
– In certain applications prior investigation of the patient by
alternative imaging techniques might be required
• Additional training in radiation protection is required for
radiologists and radiographers
• Guidelines of EU are available
26
Optimization of CT practice
• Once a CT examination has been clinically justified, the
subsequent imaging process must be optimized
• There is dosimetric evidence that procedures are not
optimized from the patient radiation protection point
of view
CTDIw (mGy)
Examination
Sample
size
Mean
SD
Min
25%
Median 75%
Max
Head
102
50.0
14.6
21.0
41.9
49.6
57.8
130
Chest
88
20.3
7.6
4.0
15.2
18.6
26.8
46.4
Abdomen
91
25.6
8.4
6.8
18.8
24.8
32.8
46.4
Pelvis
82
26.4
9.6
6.8
18.5
26.0
33.1
55.2
27
Optimization of CT practice
• Optimal use of ionizing radiation involves the
interplay of the imaging process:
 Diagnostic quality of the CT image
 Radiation dose to the patient
 Choice of radiological technique
28
Optimization of CT practice
• CT examinations should be performed under the
responsibility of a radiologist according to the national
regulations
• Standard examination protocols should be available.
• Effective supervision may aid radiation protection by
terminating the examination when the clinical
requirement has been satisfied
• Quality Criteria can be adopted by radiologists,
radiographers, and medical physicists as a check on the
routine performance of the entire imaging process
29
Optimization of protection in CT
scanner
Topic 3: Quality criteria for CT images
Quality criteria for CT images: Example of good
imaging technique (brain general examination)
Patient position
Volume of investigation
Supine
Nominal slice thickness
2 - 5 mm in posterior fossa; 5-10 mm in hemispheres
Inter-slice distance/pitch
Contiguous or a pitch = 1
FOV
Head dimension (about 24 cm)
10-12 ° above the orbito-meatal (OM) line to reduce
exposure of the eye lenses
Standard
Gantry tilt
X Ray tube voltage (kV)
Tube current and exposure
time product (mAs)
Reconstruction algorithm
Window width
Window level
From foramen magnum to the skull vertex
As low as consistent with required image quality
Soft
0 - 90 HU (supratentorial brain)
140- 160 HU (brain in posterior fossa)
2000 - 3000 HU (bones)
40 - 45 HU (supratentorial brain)
30 - 40 HU (brain in posterior fossa)
200 - 400 HU (bones)
31
Quality criteria for CT images: brain,
general examination
Image criteria
 Visualization of
• Whole cerebrum, cerebellum, skull base and osseous basis
• Vessels after intravenous contrast media
 Critical reproduction
• Visually sharp reproduction of the
 border between white and grey matter
 basal ganglia
 ventricular system
 cerebrospinal fluid space around the mesencephalon
 cerebrospinal fluid space over the brain
 great vessels and the choroid plexuses after i.v. contrast
Criteria for radiation dose to the patient
• CTDIW 60 mGy
• DLP
1050 mGy cm
32
Image criteria for CT images: brain,
general examination (visualization of)
• Whole cerebrum,
cerebellum, skull
base and
osseous basis
• Vessels after
intravenous
contrast media
33
Image criteria for CT images: brain, general
examination (critical reproduction)
Visually sharp reproduction of
the:
• border between white and
grey matter
• basal ganglia
• ventricular system
• cerebrospinal fluid space
around the mesencephalon
• cerebrospinal fluid space
over the brain
• great vessels and the choroid
plexuses after i.v. contrast
34
Quality criteria for CT images
• A preliminary list of reference dose for the patient are given for
some examinations expressed in term of:
– CTDIw for the single slice
– DLP for the whole examination
Examination
Reference doses
CTDIw (mGy)
DLP (mGy cm)
Routine head
60
1050
Routine chest
30
650
Routine abdomen
35
800
Routine pelvis
35
600
35
Viewing conditions and film processing
Viewing conditions
• It is recommended to read CT images on video display
• Brightness and contrast control on the viewing monitor should
give a uniform progression of the grey scale
• Choice of window width dictates the visible contrast between
tissues
Film Processing
• Optimal processing of the film has important implications for the
diagnostic quality
• Film processors should be maintained at their optimum operating
conditions by frequent (i.e., daily) quality control
36
Summary
• The CT scanner technology and the related
radiation protection aspects
• The ways of implementing the quality criteria
system related to the image quality and to
dosimetry
• The importance of Quality Control
37
Where to Get More Information (II)
• Quality criteria for computed tomography, EUR
16262 report, (Luxembourg, EC), 1997.
http://w3.tue.nl/fileadmin/sbd/Documenten/Leerga
ng/BSM/European_Guidelines_Quality_Criteria_Com
puted_Tomography_Eur_16252.pdf
• Radiation exposure in Computed Tomography; 4th
revised Edition, December 2002, H.D.Nagel, CTB
Publications, D-21073 Hamburg
38
CT Dose Reduction Techniques
A Practical Approach
Outline
•
•
•
•
•
•
•
•
CT Dose Units
Effective Dose
Dose Reference Levels
CT Dose Optimisation Techniques
CT Dose Modulation
Bismuth Shielding
Breast Shields in Practice
Summary
CT Dose Units
• CT Dose Index - measures Absorbed Dose in a CT phantom
(mGy)
• CTDIw = CTDI . tissue weighted factors
• CTDIvol - weighted average of CTDI from within a phantom
and corrected for pitch or table increment
• DLP = CTDIvol (mGy) . L (mGy.cm)
– Where L = Scan Length
– Allows us to calculate Dose
• Effective dose – Estimate of Stochastic Radiation Risk
– Effective Dose (mSv) = DLP . CF
– Where CF is the conversion factor from IRCP table
– Takes Organ Sensitivity weighting factors into account
103 ICRP Tissue Weighting Factors
Tissue
Tissue Weighting ICRP 2007
Gonads
0.08
Bone Marrow (Red)
0.12
Colon
0.12
Lung
0.12
Stomach
0.12
Breast
0.12
Remainder
0.12
Bladder
0.04
Liver
0.04
Oesophagus
0.04
Thyroid
0.04
Skin
0.01
Bone surface
0.01
Brain
0.01
Salivary Glands
0.01
Total
1
ICRRP 103, 2008
Effective Dose Conversion Table
Effective Dose = DLP . CF
Body Region
Conversion Factor
(mSv mGy-1 cm-1)
Head
0.0023
Neck
0.0054
Chest
0.017
Abdomen
Pelvis
0.015
0.019
Ref. European Guidelines on Quality Criteria for Computed Tomography
EUR 16262, May 1999
CT Radiation Sources
1%
US Radiation sources to Population
3%
11%
26%
4%
55%
Radon
Other Natural Sources
Medical X rays
Nuclear Medicine
Consumer Products
Other
From NCRP Report No. 93
CT is 13% of medical x-ray exams, but accounts for 70% of medical
dose (Lee, 04)
In Australia CT accounts for 50% of all medical radiation dose (0607)
ARPNSA looking at establishing national DRLs
DRL’s
 Dose Reference Level
 A reference level of dose likely to be appropriate for
average sized patient undergoing medical diagnosis and
treatment
• DRLs allow us to:
–
–
–
–
Compare CT dose in mSv with other Modalities
Compare our practice with other centers
Realise if we have a certain margin for Optimisation
Detect abnormal situations with high radiological risk to
the patient
Establishing DRLs
• How
– Audit dose reports for range of body sizes of each scan
type
– Record DLP and CTDIvol
– Employ your in house Physicist or Radiation Safety Officer
to develop DRLs - third quartile values of CTDIvol and DLP
• Published DRLs Reference
–
–
–
–
NRPB data survey 1990
ACR Recommendations
European Guidelines 16262
ICRP
UK DRL Guide
Examination
Diagnostic Reference Level
CTDI (mGy)
DLP (mGy . Cm)
Routine Head
60
1060
Face/Sinuses
35
360
Vertebral Trauma
70
460
Routine chest
30
650
HRCT
35
280
Routine Abdomen
35
780
Liver/Spleen
35
900
Routine Pelvis
35
570
Osseous Pelvis
25
520
Ref. European Guidelines on Quality Criteria for Computed Tomography
EUR 16262, May 1999
US Typical Effective Radiation Dose
Values
CT
Head CT
Pelvis CT
Liver CT
Chest CT
Abdopelvis CT
Cardiac CT
mSv
1-2
3-4
5-7
5-7
8-11
5-12
NON CT
Hand X-ray
Chest X-ray
Mammogram
Barium Enema
Coronary Angio
Sestamibi Scan
mSv
<0.1
<0.1
0.3-0.6
3-6
5-10
6-9
Mayo Clinic, 06
What should we be Doing?
• Archiving Dose Reports
• Employ Dose Reduction Techniques
• Ask your Radiologist’s to Accept more Noise in
your Images
• Look at developing your own site related DRL’s
Dose Optimisation Techniques
•
•
•
•
•
•
•
•
Patient Positioning
Scouts
kV
FOV and Filters
Pitch
Image Noise
Rotation Time
Dose Modulation
Patient Positioning
• Take the time to position the patient in
isocentre
• Use different tilt Positions when scanning the
head
• Reduces scan Volume
• Ensure the patient is flat in the Z plane
– This effects optimal dose modulation
Positioning and Dose Modulation
• Correct Alignment can reduce dose up to 56%
(Banghart, 06)
• Centered too high = Increased Dose
• Centered too low = Reduced Dose and
Increased breast dose
Excessive dose
Centering
error
Dose too low
Tube Position for Scouts
• Make sure that tube position is PA when
scanning scouts
• Reference vendor user guide to find out tube
home position
• Kv must be the same
for the scout and scan
acquisitions for optimal
dose modulation
kV
• kV and Dose have an exponential relationship by a
factor of 2
• Lower kV = better image contrast resolution
• Generally standardised at 120 kV
– Try using 100kvp for smaller patients on chest scans
– Isolated Extremities can be scanned at 80-100kV
– Cardiac Scan performed at 100kV for patients <180pds
• Use 80- 100kV for Paediatrics
• When kV is increased from 120 to 140kV = 39% dose
increase
FOV and Filters
• Always choose the smallest FOV possible for
the body part being examined
• Use Appropriate Filters provided by vendor
– Bow tie Filters can reduce skin dose by 50%
– Reduce noise and Artifact
• Use Paediatric Filters if Available
• Post Processing Filters
– Neuro
– Cardiac
Excessive dose
Centering
error
Dose too low
Ref: www.gehealthcare.org
Pitch
• Pitch = table increment per rotation /beam
collimation
• Inversely Proportional to patient dose
• Larger Pitches
– Lower Radiation Dose
– Faster Scan times
– More image Noise
– Decreased Resolution
• Paediatric scans should have pitch of 0.9-1.5
Image Noise
• Noise is related to Dose
Phantom B (40 mAs)
Phantom A (80 mAs)
A
• Overcoming Noise
B
– Increase MPR Thickness
– Use Post Processing Filters
– Use Appropriate Algorithms
• Ask Radiologists to accept more image noise
Rotation Time
• Rotation Time is related to dose in a linear
fashion
• Trade off with image noise
• Shorter Rotation Time Advantages 0.4s RT (200mAs)
– Linear decrease in dose
– Faster scan time
– Less motion/breathing artifact
1s RT (200mAs)
• Use Short Rotation times for Paediatrics
Dose Modulation
• Scanner adjusts the Xray tube mA
automatically with changes in patient
anatomy during the scan and from patient to
patient
• Produces reduced dose scans without image
quality compromise
Ref. Radiographic Journal ,2006
Advantages of Dose Modulation
•
•
•
•
•
•
•
•
More consistent signal to detectors
Image quality is maintained at a constant level
Tube Heat capacity conserved
Reduction in (photon starvation) streak artefact
Dose Optimisation
Dose Reductions from 10-50%
Able to set Reference or noise levels
Some vendors allow you to cap a max and min mA
Bismuth Shielding
• Shielding that can be used on in plane MDCT
scanning
• Has been shown to reduce radiation dose to
skin and superficial organs without
compromising image quality
• Reduces Primary beam Attenuation
Ref. Medscape.com
Bismuth Breast Shielding
• Used to Reduce unwanted radiation to the
breast without degrading image quality
• Can reduce dose to breast from 43-73% for
Thoracic scans
Ref. Medscape.com
Breast Shielding In Practice
• Patient Selection
– All Females of child bearing age ( <50yrs )
– Where the anatomical Thorax is being scanned
• Shield Parameters
– Attenurad Bismuth Shield 0.06mm Pb
– 0.675cm offset – applied to each side
– Covered in plastic for cleaning and reuse
Breast Shielding Protocol
• Shield Placement
– Top of shield is placed on sternal notch to cover
breasts – curve round auxilla
– Shield is positioned after scouts have been
performed
The Resultant Images
Other Bismuth Applications
• Ask your Vendor if Bismuth Shielding is
compatible with your scanner
• Paediatric Breast Shielding
• Thyroid and Eye Shield
Ref. Impactscan.org
Summary
•
•
•
•
•
•
•
•
Know your CT dose Units
Audit CT Doses
Archive Dose Reports
Think about possible site related DRL’s
Review Dose Optimisation Techniques
Use Dose Modulation where possible
Ask your Radiologists to accept more image noise
Use Shielding if available
Any Questions?