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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
RADIATION PROTECTION IN
DIAGNOSTIC AND
INTERVENTIONAL RADIOLOGY
Part 18: Optimization of protection in CT scanner
Practical exercise - Quality Control
IAEA
International Atomic Energy Agency
Contents
Quality control tests
• CT accuracy, uniformity, linearity. and noise,
• Low and high contrast resolution
• Z-axis sensitivity
• Alignment, table top travel accuracy
• Gantry tilt measurement
• Dosimetry
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 18: Optimization of protection in CT
scanner
Topic 1: Quality Control
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International Atomic Energy Agency
Physical parameters of CT image
Image quality
• May be expressed in terms of physical parameters such
as uniformity, linearity, noise, spatial resolution, low
contrast resolution
• Image quality depends on the technical characteristics of
the CT scanner, the exposure factors used and image
viewing conditions.
• Quality may be assessed by quantitative measurement
using test phantoms, and by the appearance of artifacts.
• Measurements should be conducted regularly
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Scanner performance: technical
parameters (I)
Test Phantoms:
• Test phantoms of a standardized human shape
or test objects of a particular shape, size and
structure, are used for the purposes of
calibration and evaluation of the performances
of CT scanners
• Phantoms should allow for evaluation of CT
number; uniformity; noise; spatial resolution;
slice thickness; dose; positioning of table top
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Scanner performance: technical
parameters (II)
CT Number Accuracy
• CT number depends on tube voltage, filtration, object thickness
• CT number of water is by definition equal to 0
• Measured CT number should be < ± 4 HU in the central ROI
CT Number Linearity
• The linear relationship between the calculated CT number and the
linear attenuation coefficient of each element of the object
• Deviations from linearity should be < ± 5 HU
CT Number Uniformity
• The CT number of each pixel in the image of an homogeneous
object should be the same over the image area
• The difference in the CT number between a peripheral and a central
region of an homogeneous test object should be < 8 HU
• Differences are largely due to beam hardening
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Scanner performance: technical
parameters (III)
Noise
• The local statistical fluctuation (standard deviation) of CT
numbers in a homogeneous Region Of Interest (ROI)
• Noise strongly affects the low contrast resolution
• Noise is dependent on the radiation dose
Noise =
1
dose
• Image noise should be measured over an area of about
10% of the cross-sectional area of the test object.
• Goal— obtain an image with an acceptable level of
noise while keeping the patient dose as low as
reasonably achievable
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Scanner performance: technical
parameters (IV)
Spatial Resolution
• The high contrast resolution determines the minimum
size of details visualized in the plane of the slice with a
contrast >10%.
It is affected by:
• the reconstruction algorithm
• the detector width
• the effective slice thickness
• the object to detector distance
• the x-ray tube focal spot size
• the matrix size.
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Scanner performance: technical
parameters (V)
Spatial Resolution
• The low contrast resolution determines the size
of detail that can be visibly reproduced when
there is only a small difference in density relative
to the surrounding area
• Low contrast resolution is limited by noise.
• The perception threshold in relation to contrast and
detail size can be determined by creating contrastdetail curve.
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Scanner performance: technical
parameters (VI)
Slice Thickness
• The slice thickness is determined in
•
the center of the field of view as the
distance between the two points on
the sensitivity profile along the axis
of rotation at which response has
fallen to 50%.
The use of post-patient collimation
to reduce the width of the image
slice leads to very significant
increases in the patient dose
Positioning of couch
• The accuracy of positioning of the
patient couch is evaluated by
moving the loaded couch a defined
distance and moving it back to the
start position
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Minimum requirements: CT scanner
(I)
• Image noise
The Standard Deviation of CT numbers in the central
500 mm2 ROI for a water or tissue equivalent phantom
should not deviate more than 20% from the baseline.
• CT number values
The deviation in the CT number values for water or
tissue equivalent material and materials of different
densities should <± 20 HU or 5%.
• CT number uniformity
The SD of the CT number over a 500 mm2 region of
interest for water or tissue equivalent material at the
center and at the periphery of phantom should vary by
less than 1.5% of the baseline
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Minimum requirements: CT scanner
(II)
• Computed tomography dose index (CTDI)
The CTDI for a single slice for each available beam shaping filter and
for each available slice thickness should not deviate more than ± 20%
from the baseline.
• Irradiated slice thickness
The FWHM of the dose profile should not differ more than ± 20% from
baseline.
• High contrast resolution (spatial resolution)
The FWHM of the point spread function of a pin, or the edge response
function of an edge should not differ more than ± 20% from baseline.
• Low contrast resolution
Polystyrene pins of 0.35 cm diameter inserted in a uniform body water
phantom should be visible in the image.
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 18: Optimization of protection in CT
scanner
Topic 2: Noise
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International Atomic Energy Agency
Imaging performance (Noise)
• Noise is assessed using cylindrical
phantoms, which are either filled with water
or made of a tissue equivalent material
• Once an axial image of the phantom has
been acquired, noise is measures as the
standard deviation in CT number in a region
of interest (ROI) placed in the center of the
image
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Imaging performance (Noise)
Region of
interest
(ROI)
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Imaging performance (Noise)
• Noise values are given in manufacturers’
specifications are quoted for a specific phantom
(e.g., manufacturer’s QA phantom) and for
specified scan techniques
• These conditions must be matched exactly for the
purposes of the acceptance test
• Manufacturers quote noise at a particular surface
dose
• If this is the case, dose for axial scans can be
measured by taping an ion chamber to the surface
of the phantom
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Imaging performance (Noise)
• Baseline noise values should be obtained for
several scan protocols that will be used
clinically, using the routine QC noise
phantom
• To ensure that noise figures are both
accurate and representative, it is essential to
find the mean value from several scans (10
scans.)
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 18: Optimization of protection in CT
scanner
Topic 3: CT uniformity and linearity
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International Atomic Energy Agency
CT number uniformity
• CT number uniformity can be assessed at the
same time as measuring noise, by placing four
additional ROI (N, E, S and W) at positions near
the edge of the image of a uniform phantom
• The mean CT number is then noted for these four
regions, as well as the center
• The deviation from the central value should be
calculated
• The CT number uniformity should be measured for
large fields of view
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CT number uniformity
Axial image of
a homogenous
phantom
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CT number uniformity
• CT number linearity is assessed using a
phantom containing inserts of a number of
different materials (materials should cover a
wide range of CT numbers)
• One example of a suitable phantom to use
at acceptance is the Catphan (The Phantom
Laboratory, Salem, NY), which contains four
inserts with CT numbers ranging from
approximately -1000HU to +1000HU
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CT number uniformity
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 18: Optimization of protection in CT
scanner
Topic 4: Low and high contrast resolution
IAEA
International Atomic Energy Agency
Low contrast resolution
• Low contrast resolution (or low contrast
detectability) is often quoted in specification
documentation as the smallest visible object at a
given contrast for a given dose
• Since this measurement relates directly to imaging
performance, it is an important value to verify at
acceptance testing
• At least 20 images of the low contrast insert (LCR)
should be acquired and then viewed by at least 3
observers under optimal viewing conditions to
obtain an average
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Low contrast resolution
Typical image
of the
Catphan LCR
insert
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Spatial resolution (high contrast)
• There exist two broad categories of measuring
techniques :
• analysis of the point spread function, usually by
calculation of the modulation transfer function (MTF)
• either objective analysis or visual assessment of images
of a resolution bar phantom.
• The resolution is quoted as the spatial frequency (in
line pairs/cm) at which the modulation falls to 50%,
10% or 2% MTF.
• These figures are often given for more than one
reconstruction algorithm, e.g., for standard and highresolution scans.
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Spatial resolution (high contrast)
• The number of line
pairs per cm just
visible in the image is
approximately
equivalent to the 2%
value of the MTF
• This result can then be
compared with the 2%
MTF, as quoted in the
manufacturer’s
specifications
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 18: Optimization of protection in CT
scanner
Topic 5: Z-axis sensitivity
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International Atomic Energy Agency
Z-Sensitivity (Imaged slice width)
Axial measurements
• Phantoms used for axial measurements may
contain thin metal plates, wires or arrays of air
holes, inclined at an angle to the image plane
• Manufacturers should be able to supply an
appropriate phantom or, alternatively, the Catphan
contains an insert suitable for this test
Note: to obtain meaningful measurements, the
thickness of the plates, wires or holes cannot be
greater than the nominal slice width concerned.
This may create problems for the sub-millimetre
slice widths offered on multi-slice scanners.
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Z-Sensitivity (Imaged slice width)
Axial measurements
• Phantoms manufacturers may quote the tolerance
for each nominal slice width setting in their
specification documentation
• Z-sensitivity measurements in axial mode can be
used to check that imaged slice widths are within
the tolerances given
• This can also be used in conjunction with irradiated
slice width measurements to assess the accuracy
of post patient collimation and to calculate the
geometric efficiency for the scanner
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Z-Sensitivity (Imaged slice width)
Plan view of a test object used to measure imaged
slice widths for axial scans
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 18: Optimization of protection in CT
scanner
Topic 6: Alignment
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International Atomic Energy Agency
Alignment of indicating lights with
scan, coronal and sagittal planes
• Several methods can
be used to perform
these tests
• The techniques
described here are
straightforward to
implement and require
minimal test
equipment.
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Agreement between internal and
external scan plane lights
• Use an envelope-wrapped film (therapy verificaiton
film) for the measurement. However, a piece of
paper or card can also be used
• The film is placed flat on the table and illuminated
by the external scan plane light
• The position of the light is marked on the film
envelope and the table is moved automatically to
the scan plane
• If the distance between the internal and external
lights is correct, the internal light should now
coincide with the mark on the film envelope.
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Co-incidence of internal scan plane
lights and scan plane
• Pin pricks are made in a piece of therapy
verification film (or similar) along the line of the
internal scan plane light
• The film is wrapped around a phantom and
scanned with a narrow axial and developed
• Coincidence between the pin pricks and the x-ray
beam exposure indicates alignment between the
alignment lights and the scan plane
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Co-incidence of internal scan plane
lights and scan plane
• For multi-slice scanners, pin pricks (and, thus, the
internal alignment light) are usually found to
coincide with the center of the four slices
• To plan a scan so that the x-ray beam is centered
over the internal scan plane lights at zero, it will be
necessary to center the first and last slices
symmetrically around zero (e.g. four slices from –
7.5 mm to +7.5 mm on a 4 x 5 mm scan)
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Co-incidence of internal scan plane
lights and scan plane
Pin pricks made in
film at position of
scan plane light
Z
X-ray beam
exposure
X
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Coronal and Sagital plane lights
• A long, thin object, with a high CT number relative
to air, such as the ‘lead’ in a pencil or a
straightened paper clip, can be used as a marker
to perform this test
• The marker is supported above the patient table
and aligned, using the indicating lights, so that it is
positioned at the isocentre, parallel to the z-axis
and perpendicular to the scan plane
• If indicating lights are accurately aligned to the
coronal and sagital planes, the marker should
appear as a dot at exactly x = 0, y = 0 on the axial
image.
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 18: Optimization of protection in CT
scanner
Topic 7: Table travel accuracy
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International Atomic Energy Agency
Table travel accuracy
• A ruler or tape measure placed alongside the
table, can be used to check that the amount of
table movement indicated on the gantry agrees
with the actual distance moved.
• A load of approximately 70- 80 kg should be
placed on the table in order to simulate the weight
of a patient.
• The test should be performed twice: by driving the
table top both away from and towards the gantry.
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Table travel accuracy
Assessment of distance indicator accuracy
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Axial scan incrementation accuracy
• Verification of incrementation accuracy between
successive axial slices can be achieved by placing
therapy verification film on the couch (in the
isocentre plane) and exposing it to an axial scan
sequence
• Narrow slices separated by a couch increment
greater than 1 slice width can be used, and the
distance between the lines on the film measured
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Table travel accuracy for helical scans
• In helical scanning, it is not sufficient to use a simple
mechanical test because the distance imaged depends on
couch speed and scanner software
• One method of assessing distance accuracy is to use an
acrylic test object containing two small radio-opaque
markers, separated by a fixed distance (e.g., 20 cm)
• The test object is scanned in Scan Projection Radiography
(SPR) mode and a helical scan is planned to start at the
first marker and to end at a distance x from the first marker
• If couch travel is accurate during the helical scan, the
markers should be clearly seen on the first and final images
of the series.
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Couch travel accuracy for helical
scans
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 18: Optimization of protection in CT
scanner
Topic 8: Gantry tilt measurement
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International Atomic Energy Agency
Gantry Tilt
• The accuracy of displayed gantry tilt can be
assessed by supporting therapy verification
film at the gantry end of the patient table
• The film must be held vertically (e.g., by
taping to an acrylic block), so that it is
parallel to the sagital plane and intersects
scan and coronal planes at right angles
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Gantry Tilt
• Three axial exposures are made using the same
film:
• one for the maximum superior gantry tilt,
• one for the maximum inferior gantry tilt
• one at 0º gantry tilt
• The three scan planes should then be visible on
the developed film
• The angles + and - between scan planes at
maximum tilt relative to that at 0º tilt should equal
tilt angles displayed on the gantry.
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Assessment of accuracy of gantry tilt
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 18: Optimization of protection in CT
scanner
Topic 9: Dosimetry
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International Atomic Energy Agency
Dosimetry - CTDI in air
• The Computed Tomography Dose Index
(CTDI) in air can be measured using a 10
cm pencil ionization chamber, located in the
scan plane at the isocentre
• The ion chamber can be supported using a
laboratory test stand and clamp
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Dosimetry - CTDI in air
Ionization
chamber
Table
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Dosimetry - CTDI in air
• It is important to measure the CTDI in air
under the following conditions When
commissioning a CT scanner:
• For all beam shaping filters
• For all nominal slice widths
• For all clinical kV settings
• For a range of scan times
• For a range of mA settings
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Dosimetry - CTDI in air
•
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Dosimetry - CTDI in air
•
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Dosimetry - CTDI in air (helical)
Axial slice positions
Helical scan (pitch 1)
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Dosimetry - CTDI in Acrylic Phantoms
• The body phantom is placed on the patient table
and the head phantom is supported in the head
rest
• Phantoms are aligned and centred at the scan
isocentre
• The ion chamber is inserted into either the central
or one of the peripheral holes of the phantom (all
other holes being filled with acrylic rods)
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Dosimetry - CTDI in Acrylic Phantoms
• Dose measurements at the center are used to
calculate the central CTDI
• Peripheral CTDI is measured in at least four
positions around the phantom, so as to achieve a
true average
Note: If gantry rotation is initiated from different
angular positions for successive scans it may be
necessary to take a number of measurements at
each position in order to get a representative mean
dose
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Dosimetry - CTDI in Perspex
Phantoms
Insert to plug holes
Head phantom
IAEA
Body phantom (or annulus
to fit over head phantom)
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Dosimetry - CTDI in Perspex
Phantoms
• Central and peripheral CTDI’s are used to
calculate weighted CTDI, CTDIw:
1
n CTDI w =
C
(
1
2
CTDI100,c + CTDI100,p
3
3
)
• CTDIws can be compared against diagnostic
reference levels for standard scan
examinations
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Example of CTDI Values for some CT
units
MODEL
KV
FAD
S
nCDTI
Siemens
HIQ
133
700
10
0.195
(mGy/mAs)
weighted
Head
Body
0.161 (0.83) 0.093 (0.48)
Siemens
Plus S
120
700
10
0.128
0.110 (0.86)
Siemens
Plus S
137
700
8
0.161
GE Pace
120
525
10
0.344
0.200 (0.58)
0.094 (0.27)
Philips
LX
120
606
10
0.200
0.160 (0.80)
0.081 (0.41)
Air
0.062 (0.48)
0.082 (0.51)
FAD(mm): Focus to Axis distance
S (mm): Slice thickness
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Dose Profiles (Irradiated slice width)
• Measurement of irradiated slice widths, for all
nominal slice width settings, provides a direct test
of pre-patient beam collimation and allows
geometric efficiencies to be calculated for the
scanner
• Geometric efficiency (GE) is defined as:
GE =
width slice irradiated x 100 %
width slice imaged
• It is recommended that GE value is displayed on
the console if it is less than 70%
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Dose Profiles (Irradiated slice width)
• First method : expose therapy verification film, supported in
air at the isocentre, at each of the slice width settings
• Once developed, optical density profiles may be plotted
using a digitizer or the width measured using a ruler
• To provide an accurate dose profile, a calibration curve can
be applied to convert optical density profiles into dose
profiles, from which irradiated slice widths (FWHM of dose
profiles) may be derived
• Alternative technique: measure dose profiles using TLD’s.
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Dose Profiles (Irradiated slice width)
Measurement of
irradiated slice
widths for a range
of nominal slice
width settings
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Quality control program
Example of a quality control program for a CT unit:
Physical parameter
Level of
expertise
Frequency Priority
Remedial level
Baseline ± 20%
*more scans/parameters
Image noise
A
B
D/W
1 y*
H
H
CT number values
A
D/W
H
B
1 Y*
H
CT number uniformity
B
1y
H
> ± 1.5%
CTDI
A
1y
H
Baseline ± 20%
Irradiated slice thickness
B
1y
H
> ± 20% or 1 mm
Image slice thickness
B
1y
M
> ± 20% or 1 mm
High contrast resolution
B
1y
H
Baseline ± 20%
IAEA
Water: ± 10 HU
Other materials ± 20 HU
*more scans/parameters
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