PERFORMANCE EVALUATION OF COMPUTED TOMOGRAPHY (CT) SCANNERS

1. Mrs. BHUVANESHWARI.P
M.Sc. MEDICAL PHYSICS
LECTURER
GANGA COLLEGE OF NURSING
COIMBATORE

2. PERFORMANCE EVALUATION OF COMPUTED
TOMOGRAPHY (CT) SCANNERS
American Association of Physicists in Medicine initiated
pioneering R &D activities in the field.
These initial works were brought out through:
1.Phantoms for Performance Evaluation and Quality Assurance
of CT scanners (AAPM Report No.1)
2.Specification and Acceptance Testing of Computed
Tomography Scanners (AAPM Report No.39)
Though CT scanners underwent tremendous design
modifications with respect to source -detector configurations
and their relative movements , the basic principles adopted in
the performance evaluation of these scanners still are based on
the above reports.

3. CT Performance evaluation tests can be broadly
classified into:
1.Electromechanical Tests
2.X-ray Generator (Electrical ) Tests
3.Image Quality Tests
4.Radiation Dose Tests
5.General Tests Related to CT Number

4. ELECTRO MECHANICAL TESTS
1.Scan Localization laser light accuracy
Scan localization laser lights are provided in the gantry to
locate the scan plane and to position the isocentre of the gantry
at the centre of the slice being imaged.
Modern units are provided with external and internal
localization lights at fixed distance apart (60-70 cm).
External for positioning the patient and internal to define the
plane of slice being imaged.
Test Tool : Laser Light Test Tool

5. 2. Vertical Alignment of Table to Imaging Plane
If alignment fails, isocentre of different slices would be at different
locations. This situation affects the accuracy of treatment plan on the
basis of virtual simulation.
Test Tool : Laser Light Test Tool
3. Table increment /backlash
Table increment or backlash is controlled from computer console.
Hence correspondence between the applied increment/backlash and
the effected increment or backlash is necessary as it determines the
relative location of image slices.
Test Tool : Film (Kodak X-Omat V / industrial films Agfa D2 or D4
pre packed in a light proof envelop so as to get a film density of 1 to
1.5 under exposure parameters 120-140 kVp and 50 –100 mAs)

6. 4. Gantry Tilt
To determine the accuracy of tilt indicators and to ensure that
specified tilt can be accomplished under clinical condition.
Test Method : Test 3 and 4 can be combined using the same
film.
5. Slice Localization from Radiographic (scout) image
To determine the correspondence of the slice localization at
computer consol with the actual slice position and angle.
Test Tool : 450 cross wire tool as described in AAPM report No.
39.

7. 6. Collimation Tests
Collimation determines the slice thickness.
Pre patient collimation/ post patient collimation or both can be
present depending upon the make and model of the scanner.
In pre patient collimation system, slice thickness is defined at
isocentre.
In post patient collimation system, slice thickness is determined
by detector collimators.
In multi channel detector array systems, slice thickness is
determined by the detector array being activated for the scan.

8. Test Methods:
Slice thickness defined at isocentre can be checked with film.
FWHM of the radiation profile width gives slice thickness.
If slice thickness is determined by detector collimation or
activated detector channels 
slice thickness has to be measured by the response of the scanner
to an attenuating impulse along Z axis at an angle to the the
imaging plane (Z sensitivity)
AAPM -39 suggests a RAMP phantom or WIRE HELIX
phantom for this test.

10. Above methods for evaluation of slice thickness, however are not
valid for spiral scanners because of orientation difficulties.
Line response phantom is ideal for evaluating spiral CT Scan
Sensitivity Profile.
Line Response phantom :
Comprises of a small thin circular foil (0.1 mm) of high CT number
material (e.g. PTFE) sandwiched between two discs of rigid foam
having CT number nearly equal to that of air.
ImPACT recommends the use of a phantom containing a tungsten
foil (6 mm diameter , 0.05 mm thick sandwiched ) within a perspex
rod.
Test Method : CT number along Z axis of the images of the phantom
rises from background to maximum at the disc level and again falls
of to background because of partial volume averaging. FWHM of CT
number profile along Z axis provides slice thickness.

11. X-RAY GENERATOR (ELECTRICAL ) TESTS
1. Accelerating Voltage
Non-Invasive Methods for routine evaluation.
Digital kVp meters commercially available are used.
These meters consists of a pair of solid state detectors shielded
with beam hardening filters of different thickness. The ratio of
the signals under different filtration varies with kVp.
Tolerance :  2 kV
2. Measurement of mA linearity
Linearity of mA is inferred by measuring radiation output at
isocentre in air using a dose meter.

12. mA Linearity:
Keep kV , time and slice thickness constant and measure the
radiation output for different mAs
Tabulate the results as follows:
Sr.
No
Radiation output (mGy) Rav/mAs
mGy/mAs
(X)
R1 R2 R3 R4 Rav
mAs1 X1
mAs2 X2
mAs3 X3
mAs4 X4
mAs5 X5
Coefficient of linearity (COL) is estimated as :
(Xmax-Xmin) / (Xmax + Xmin)
COL  0.01

13. 3.Reproducibility of Radiation Output
Verified by analyzing exposure to exposure variation of radiation
output for applied exposure parameters
kV mAs Radiation output (mGy/mAs) (Xi – Xm) 2
X1 X2 X3 X4 X5 Xm
Coefficient of variation (COV ) =  /m = (1/ Xm ) [  (Xi-Xm)2 / (n-1) ] ½
Tolerance  0.05 i.e within  5 %

14. IMAGE QUALITY TEST
1.Low Contrast Resolution
Low contrast resolution refers to the capability of the
scanner to image small objects having attenuation properties not
very much different from that of surrounding tissues.
Test Tool : Consists of polystyrene sheet, on which holes of
different diameters are milled, is placed in a 20 cm diameter
perspex phantom filled with water (or 10% dextrose (C6H12O6) in
water).
CT number of polystyrene = -24
CT number of water = 0
Hence contrast variation between polystyrene and water is 2.4% ,
which is low contrast.

15. Depending upon the selected slice thickness, polystyrene sheet
and water portion of different thickness provides varying low
contrast ( 0.5 to 2.4 %) by partial volume effect.
Low contrast resolution is normally quoted by the minimum hole
size (lp/mm) that could be resolved at a particular low contrast
variation (in %).
Low contrast resolution test
tool developed by RPAD,
BARC
Low contrast is achieved
between polystyrene and
10% dextrose solution
Tolerance: 5 mm hole should
be observable for 1%
contrast change

16. High Contrast Resolution
Refers to the ability of scanner to distinguish small objects of
high subject contrast, situated close by.
A linear attenuation difference of 10 % is considered as high
contrast in CT.
Perspex of CT number 120 HU and water (CT number 0 HU)
provides contrast of this order (12%)
High Contrast Resolution Test Tool
Comprises of a Perspex disc ( 19 cm diameter , 2 cm thick) with
line pair /hole pair pattern milled on it and positioned in the
middle of a cylindrical water phantom so that the line pair/hole
pair patterns are filled with water.

17. High contrast is normally specified by the smallest line/hole size
(lp/mm) resolvable at 10- 12 % contrast variation.
Typically,
1.0 lp/mm (object size 0.5 mm) to 0.06 lp/mm (object size 8mm )
A resolution of 0.64 mm ( 0.78 lp/mm should be typically
distinguishable.
High Contrast Resolution Test Tool
developed by RPAD, BARC
Hole size varies from 2.5 mm
diameter (0.2 lp/mm) to 0.5 mm
diameter (1.0 lp/mm) in eight steps.
Tolerance : 1mm hole should be
resolved at 10% contrast change

18. Noise
The random variation of CT number around a mean value in
the image of a homogeneous body is known as noise.
Test Tool – Water Phantom
Noise is expressed as a percentage
Noise (%) = [  / (CT water – CT air) ] x 100
The dose at which noise is measured should be stated.

19. RADIATION DOSE TESTS
Dose Descriptors : Computed Tomography Dose Index (CTDI)
and Multiple Scan Average Dose (MSAD)
CTDI is equivalent of the dose value inside the irradiated slice
that would result if the absorbed radiation profile were entirely
concentrated to a rectangular profile of width equal to the nominal
slice thickness.
-  + D(z) dz = CTDI x h
CTDI = (1/h) x -  + D(z) dz

20. MSAD
In case of multiple scan, the dose in the central region of the
total dose profile increase as a result of superimposition of all
single dose profiles. This increased value is called the Multiple
Scan Average Dose (MSAD).
If the examination is done with overlapping slices, that is by
using a table feed smaller than the slice thickness the increase in
dose becomes even larger.

21. MSAD = CTDI , if table feed (TF) is equal to slice thickness (h)
i.e , if pitch (p) = 1
Pitch (P) = Table feed (TF) / Slice Thickness (h)
If p is not equal to 1,
MSAD = 1/p (CTDI)
Above relation implies that in order to obtain average dose for a
scan series, it is sufficient to obtain the CTDI from a single scan.

22. Different CTDI definitions:
1. CTDI FDA = (1/h ) -7h  +7h D(z) dz
Measured in Perspex phantom
Disadvantage : With decreasing slice thickness the length over which
the dose contributions are summed becomes shorter . However the
length of the tails of on the dose profile is not reduced to the same
extent. As a consequence, the dose from smaller slice thickness will
be underestimated.
To avoid this underestimation, the definition of CTDI is modified as
2. CTDI 100 =(1/h) -50mm  +50mm K air(z) dz
Instead of dose, air kerma (Kair ) is used and error of about 10% due
to Kair to dose in Perspex (D(z)) is avoided.

23. 3. Normalized CTDI
n CTDI XYZ = CTDI XYZ / mAs
Normalized CTDI represents the capacity of scanner in terms
of its radiation output and does not convey anything about
patient dose.
Reference to patient dose can be made by multiplying n CTDI
with current-time product (mAs).
CTDI measurement to relate it to patient dose is done in Head
and Body phantoms
Head/Body phantoms : Perspex solid cylinder (Head:16 cm
dia. Body: 32 cm dia. and each with15 cm height) with
through holes at centre and at four points at vertically opposite
directions at periphery (Centres of holes at periphery at 1cm
from the edge of phantom)

24. Body and head phantoms for CTDI measurement

25. CTDI was earlier specified as the CTDI at the centre and the
average CTDI at the periphery separately for Head and body
phantoms.
In order to represent both axial and peripheral dose together,
weighted CTDI is introduced.
4. Weighted CTDI
CTDIW = 1/3 CTDI 100 c + 2/3 CTDI 100p
The advantage of CTDI that it enables the use of a single number
instead of two.
The dose display at the operators’ console (in terms of CTDIW)
is presently a mandatory requirement in European Community.

26. If the pitch factor is not equal to 1, CTDI value has to be divided
by the pitch factor to yield effective CTDI.
5. CTDI w.eff = (1/p ) x CTDI w
Dose is not appropriate to indicate the extent of body irradiation.
Dose Length Product (DLP) is introduced to mention the extent
of body irradiation.
6. DLP = CTDI x n x h (mGy.cm)
n  number of slices, h  slice thickness
DLP w = CTDI w.eff .p. n. h = CTDI w.eff . n . TF ( because p.h
is equal to the table feed)
Measured CTDI value should be preferably within  20 % of the
value quoted by the vendor and should not exceed  40%

27. GENERAL TESTS RELATED TO CT NUMBER
1.CT Number uniformity
The mean CT number over 100 pixels over various regions
of an image of a homogeneous medium should not be very much
different.
Test Method : In the image of a water phantom , note down CT
number values at centre and four locations at periphery using an
ROI of 2 cm diameter.
Variation should not be more than  5 HU among 100 pixels
Image uniformity checks can be helpful in identifying perturbations
such as beam hardening artifacts , ring artifacts etc.
CT number variations due to slice thickness, phantom size,
phantom position and reconstruction algorithm also can be checked
with the same experimental set up.

28. 1.CT Number Linearity
CT number implies the relative variation in attenuation
properties of a medium in comparison with that of water.
Ideally, different linear attenuation coefficients and
corresponding CT number should have a linear relationship.
This linearity is important when CT scanner is used for
quantitative studies and virtual simulation.
Test Method
CT number linearity can be assessed by scanning samples of
material of known composition having different attenuation
coefficients.
The plot of CT numbers against  values should be a straight
line.

29. CT Number Linearity Phantom
developed by RPAD, BARC
Built in attenuating materials
are selected to cover the
attenuation coefficients of
lung to bone.
Material used include:
1. Bakelite
2. Perspex
3. Polystyrene
4. Nylon
5. Teflon
6. Bakelite
7. Polypropylene

30. Quantitative Tests
When CT scanner is used for Bone Mineral Analysis
(BMA), CT numbers are used directly for quantification.
Hence linear relationship between mineral concentration and
corresponding CT numbers is necessary for the effective
BMA.
Test Tool : Elliptical body phantom (Perspex) with hole for
mineral vial inserts at spinal position.
Bone mineral
equivalent
K2HPO4 of
different
concentrations (0,
50, 100, 200
mg/ml) in the vials
were scanned