Phantom Laboratory Catphan 503 User manual

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T h e P h a n t o m L a b o r a t o r y
C a t p h a n ® 503 M a n u a l
Copyright © 2017
WARRANTY
THE PHANTOM LABORATORY INCORPORATED (“Seller”) warrants that this product
shall remain in good working order and free of all material defects for a period of one
(1) year following the date of purchase. If, prior to the expiration of the one (1) year
warranty period, the product becomes defective, Buyer shall return the product to the
Seller at:
By Truck By Mail
The Phantom Laboratory, Incorporated The Phantom Laboratory, Incorporated
2727 State Route 29 PO Box 511
Greenwich, NY12834 Salem, NY 12865-0511
Seller shall, at Seller’s sole option, repair or replace the defective product. The Warranty
does not cover damage to the product resulting from accident or misuse.
IF THE PRODUCT IS NOT IN GOOD WORKING ORDER AS WARRANTED, THE
SOLE AND EXCLUSIVE REMEDY SHALL BE REPAIR OR REPLACEMENT,
AT SELLER’S OPTION. IN NO EVENT SHALL SELLER BE LIABLE FOR ANY
DAMAGES IN EXCESS OF THE PURCHASE PRICE OF THE PRODUCT. THIS
LIMITATION APPLIES TO DAMAGES OF ANY KIND, INCLUDING, BUT NOT
LIMITED TO, DIRECT OR INDIRECT DAMAGES, LOST PROFITS, OR OTHER
SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, WHETHER FOR
BREACH OF CONTRACT, TORT OR OTHERWISE, OR WHETHER ARISING OUT
OF THE USE OF OR INABILITY TO USE THE PRODUCT. ALL OTHER EXPRESS
OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTY OF MERCHANT ABILITY AND FITNESS FOR PARTICULAR PURPOSE,
ARE HEREBY DISCLAIMED.
WARNING
This product has an FH3-4 mm/min ame rating and is considered to be ammable. It is
advised not to expose this product to open ame or high temperature (over 125° Celsius
or 250° Fahrenheit) heating elements.
CTP503
1/12/17

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T h e P h a n t o m L a b o r a t o r y
Catphan® Manual
Contents
Warranty 1
Introduction 4
Initial phantom positioning 5
Illustration of Catphan® 503 model 6
Phantom position verication 7
Incremental phantom module positioning 8
Drawings in the manual 8
CTP404 module 9
Patient alignment system check 10
Scan slice geometry (slice width) 11
Scan incrementation 12
Circular symmetry 13
Spatial linearity of pixel size verication 13
Spherical acrylic contrast targets 13
CT or Hounseld Numbers by David Goodenough, Ph.D. 14
Sensitometry (CT number linearity) 15
CTP528 High resolution module with 21 line pair per cm gauge and point source 17
Bead point source for point spread function and MTF 17
Use of automated scanner MTF programs 18
Bead point source (slice sensitivity prole) 19
21 Line pair per centimeter high resolution gauge 20
CTP486 Image uniformity module 21
Installation and removal of test modules 23
Optional phantom annuli 24
Dose Phantoms 25
Sample quality assurance program 26
Automated computer analysis program 27
Bibliography 28

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Introduction
The Catphan® 503 phantom conguration has been selected by Elekta. This manual
is intended to supplement the Elekta Procedures and Manuals by offering additional
details regarding the use of the Catphan® phantom. The Phantom Laboratory and
physicist, David J. Goodenough, Ph.D., are continually developing and researching new
tests and modications for the Catphan® phantoms. The test objects that make up the
current Catphan® models embody more than a quarter century of scientic evaluation
and eld experience. This manual outlines the applications of each module contained in
the Catphan® 503 phantom.
We do not make specic recommendations on the content of your quality assurance
program, because each medical imaging facility has its own unique set of requirements.
A sample program is provided to give you ideas for possible program content. We suggest
a review of local governing regulations, manufacturers’ specications and the needs of
your radiologists and physicists before developing your CT quality assurance program.
If you would like a pdf version of this manual they can be downloaded from the
Catphan® page of our website. www.phantomlab.com
If you have any additional questions please contact The Phantom Laboratory at:
Phone: [phone] or 518-692-1190
Fax: [phone]
email: [email protected]
Additional product information is available at: www.phantomlab.com

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Initial phantom positioning
The Catphan® phantom is positioned in the CT scanner by mounting it on the case.
Place the phantom case on the gantry end of the patient table with the box hinges away
from the gantry. It is best to place the box directly on the table and not on the table pads.
Open the box, rotating the lid back 180°. If you are using an annulus, additional weight
will need to be placed in the box to counterweigh the phantom. The patient straps can be
used for additional stability.
Remove the phantom from the box and hang the Catphan® from the gantry end of the
box. Make sure the box is stable with the weight of the phantom and is adequately
counterweighed to prevent tipping.
Use the level and adjust the black thumb screws to level the Catphan®. Once the
phantom is level, slide the phantom along the end of the box to align the section center
dots on the top of the phantom with the x axis alignment light. Raise or lower the
patient table so the laterial height dot on the side of the phantom aligns with the y axis
alignment light.
Use the z axis table drive to position the phantom for scanning.
Depending on the type of scanner being tested and the selected protocols there are
different ways to align the phantom along the z axis.
Cone beam CT: the most common approach is to center the scanner between the second
and third alignment dots and do a single scan that will provide slices of all test modules.
However because of beam divergence there will be variations between images taken of
a module that is in the center of the beam verses images taken on the edge of the beam.

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Because of beam divergence a physicist may develop specic alignment protocols.
Single slice axial CT: the phantom can be centered on the CTP404 module and then
after verifying the alignment, the table can be indexed to the center of the other modules.
Spiral CT: the scanner can be aligned with the rst module and then the phantom can
be scanned in one sequence.
Review the scans of the CTP404 module to check the image for proper alignment as
illustrated in the Phantom position verication section.
Illustration of Catphan® 503 model
Note: The brass mount nuts hold the mount to the phantom housing. Only the black
adjusting thumb screws should be used to level the phantom.

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Phantom position verication
By evaluating a scan image of the CTP404 module the phantom’s position and alignment
can be veried. The section contains 4 wire ramps which rise at 23° angles from the base
to the top of the module. The schematic sketches below indicate how the ramp images
change if the scan center is above or below the z axis center of the test module. The use of
the scanner’s grid image function may assist in evaluation of phantom position.
If misalignment is indicated by the scan image, the phantom should be repositioned to
obtain proper alignment and then rescanned. If the images of the repositioned phantom
duplicate the original misalignment indications, the scanner’s alignment lights may
require adjustment (contact your local service engineer).
Once correct alignment has been established, you can proceed with the tests.

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Incremental phantom module positioning
The Catphan® 503 phantom is designed so all test sections can be located by precisely
indexing the table from the center of module CTP404 to the center of the other test
modules. The indexing distances from the center of the CTP404 module are listed below.
Catphan® 503 test module locations:
Module Distance from the center of CTP404
CTP528, 21 line pair high resolution 30mm
CTP486, Solid image uniformity module 110mm
Drawings in the manual
The drawings in this manual show a schematic view of the modules as they would be
seen in a typical head rst CT scan where the right side of the phantom is shown on the
left side of the image.
Most of the schematic drawings show the 15cm diameter module without the 20cm
diameter housing. Please note that the modules are pressed into the housing and
sometimes there is a small air gap that creates a black crescent line between the test
module and the housing. This is shown on the above illustration running from the 10:00
to 12:30 positions. Since this is a radial air gap it will not affect the internal image
measurements if the phantom is centered in the scan eld.

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CTP404 Module with slice width, sensitometry and pixel size

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Patient alignment system check
The laser, optical, and mechanical patient alignment system can be checked for accuracy.
Align the white dots on the phantom housing with the alignment lights as discussed
in Initial phantom positioning. The scanned image should show good alignment as
discussed in Phantom position verication.
For measuring the z axis alignment accuracy, measure from the center of the ramp image
to the part of the ramp which aligns with the center of the phantom and sensitometry
samples. Multiply the distance A by 0.42 to determine the z axis alignment light
accuracy. To evaluate x and y accuracy, measure from the center of the phantom to the
center of the scan eld by use of the grid function or knowledge of the central pixel
location.
The accuracy of the localizer, pilot or scout view can be checked. To check this function
perform a localization scan of the phantom. Align an axial scan at the crossing point of
the wire ramps. Scan this axial cut and check the misalignment as discussed above.

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Scan slice geometry (slice width)
Section 1 has two pair of 23° wire ramps: one pair is oriented parallel to the x axis;
the other pair to the y axis. These wire ramps are used to estimate slice width
measurements and misalignment errors as previously discussed.
The 23° wire ramp angle is chosen to improve measurement precision through the
trigonometric enlargement of 2.38 in the x-y image plane.
To evaluate the slice width (Zmm), measure the Full Width at Half Maximum (FWHM)
length of any of the four wire ramps and multiply the length by 0.42:
(Zmm) = FWHM * 0.42
To nd the FWHM of the wire from the scan image, you need to determine the CT
number values for the peak of the wire and for the background.
To calculate the CT number value for the maximum of the wire, close down the CT
“window” opening to 1 or the minimum setting. Move the CT scanner “level” to the
point where the ramp image just totally disappears. The CT number of the level at this
position is your peak or maximum value.
To calculate the value for the background, use the region of interest function to identify
the “mean” CT number value of the area adjacent to the ramp.
Using the above CT values, determine the half maximum:
First calculate the net peak... (CT # peak - background = net peak CT #)
Calculate the 50% net peak... (net peak CT # ÷ 2 = 50% net peak CT #)
Calculate the half maximum CT number...
(50% net peak CT # + background CT # = half maximum CT #)

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Now that you have determined the half maximum CT number, you can measure the
full width at half maximum of the ramp. Set the CT scanner level at the half maximum
CT value and set your window width at 1. Measure the length of the wire image to
determine the FWHM. Multiply the FWHM by 0.42 to determine the slice width.
Schematic illustration of two sequential 5mm scans superimposed. L1 is
the center point on the 23° ramp in the rst scan image and L2 is the
center point on the 23° ramp on the second image.
Scan incrementation
Use the wire ramps to test for proper scanner incrementation between slices, and for
table movement.
Scan section 1 using a given slice width, (e.g. 5mm). Increment the table one slice width
(e.g. 5mm) and make a second scan. Establish the x and y coordinates for the center of
each ramp image. Calculate the distance between these points and multiply by the 23°
ramp angle correction factor of 0.42.
0.42(L1 - L2) = scan incrementation
This test can also be used to test table increment accuracy. Scan the section and
increment the table 30mm in and out of the gantry and scan again. The ramp centers
should be the same on both images.
0.42(L1 - L2) = 0

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Circular symmetry of display system
The circular phantom sections are used to test for circular symmetry of the CT image,
including calibration of the CT display system. If an elliptical image is produced, the x-y
balance of the image display system should be adjusted.
Measuring spatial linearity in x and y axes.
Spatial linearity of pixel size verication
This section has four holes (one with a Teon pin). These 3mm diameter holes are
positioned 50mm on center apart. By measuring from center to center the spatial
linearity of the CT scanner can be veried. Another use is to count the number of pixels
between the hole centers, and by knowing the distance (50mm) and number of pixels, the
pixel size can be veried.
The Teon pin is used for identication and orientation only. The ability to change the
Teon pin position enables organizations with more than one Catphan® phantom to
identify their phantoms by images of the rst section.
Spherical acrylic contrast targets
The section has ve acrylic spheres located in a 30mm diameter circular pattern. These
spheres are used to evaluate the scanner’s ability to image volume averaged spheres.
The sphere diameters are 2, 4, 6, 8, and 10mm.

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CT or Hounseld Numbers
by David Goodenough, Ph.D.
Users of CT systems are often surprised when the CT number of a given tissue or
substance is different from what they expect from previous experience. These differences
do not usually indicate problems of a given CT scanner, but more likely arise from the
fact that CT numbers are not universal. They vary depending on the particular energy,
ltration, object size and calibration schemes used in a given scanner. One of the
problems is that we are all taught that the CT number is given by the equation:
CT# = k(µ - µw)/µw,
where k is the weighting constant (1000 is for Hounseld Scale), µ is the linear
attenuation coefcient of the substance of interest, and µwis the linear attenuation
coefcient of water. Close review of the physics reveals that although the above
equation is true to rst order, it is not totally correct for a practical CT scanner. In
practice, µ and µw are functions of energy, typical x-ray spectra are not monoenergetic
but polychromatic, and a given spectrum emitted by the tube is “hardened” as it is
transmitted (passes) through lter(s) and the object, nally reaching the detector. More
accurately, µ=µ(E), a function of energy. Therefore:
CT#(E) = k(µ(E) - µw(E))/µw(E)
Because the spectrum is polychromatic we can at best assign some “effective energy” Ê
to the beam (typically some 50% to 60% of the peak kV or kVp). Additionally, the CT
detector will have some energy dependence, and the scatter contribution (dependent on
beam width and scanned object size, shape, and composition) may further complicate
matters. Although the CT scanner has a built in calibration scheme that tries to correct
for beam hardening and other factors, this is based on models and calibration phantoms
that are usually round and uniformly lled with water, and will not generally match the
body “habitus” (size, shape, etc.).
The situation is really so complicated that it is remarkable that tissue CT numbers are in
some rst order ways “portable”!
In light of the above we can examine a parameter of CT performance, the “linearity
scale”, as required by the FDA for CT manufacturer’s performance specications.
The linearity scale is the best t relationship between the CT numbers and the
corresponding µ values at the effective energy Ê of the x-ray beam.
The effective energy Ê is determined by minimizing the residuals in a best-t straight
line relationship between CT numbers and the corresponding µ values.
In review, we will encounter considerable inter and intra scanner CT number variability.
CT numbers can easily vary by 10 or more based on kVp, slice thickness, and object size,
shape, and composition. There is some possibility of the use of iterative techniques and/
or dual energy approaches that might lessen these effects, but certainly CT numbers are
not strictly portable and vary according to the factors listed above.
More complete scientic references are contained in the bibliography. In particular,
references 2, 13, 14, and 20 are recommended for those with greater interest in the
topic.

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Sensitometry (CT number linearity)
The CTP404 module has sensitometry targets made from Teon®, Delrin®, acrylic,
Polystyrene and low density polyethylene (LDPE), polymethylpentene (PMP) and air.
The Catphan® 600 is also equipped with a small vial which can be lled with water
and inserted into the top hole of the CTP404 module. The CTP401 module has Teon,
acrylic and low density polyethylene (LDPE) and air targets. These targets range from
approximately +1000 H to -1000 H.
The monitoring of sensitometry target values over time can provide valuable information,
indicating changes in scanner performance.
Nominal material formulation and specic gravity
Material Formula Zeff1 Specic Gravity2HU range3
Air .78N, .21O, .01Ar 8.00 0.00 -1046 : -986
PMP [C6H12(CH2)] 5.44 0.83 -220 : -172
LDPE [C2H4] 5.44 0.92 -121 : -87
Water [H2O] 7.42 1.00 -7 : 7
Polystyrene [C8H8] 5.70 1.03 -65 : -29
Acrylic [C5H8O2] 6.47 1.18 92 : 137
Delrin® Proprietary 6.95 1.42 344 : 387
Teon® [CF2] 8.43 2.16 941 : 1060
Electron density and relative electron density
Material Electron Density Electron Density Relative Electron
(1023e/g) (1023e/cm3) Density4
Air 3.002 0.004 0.001
PMP 53.435 2.851 0.853
LDPE 6 3.435 3.160 0.945
Water 3.343 3.343 1.000
Polystyrene 3.238 3.335 0.998
Acrylic 3.248 3.833 1.147
Delrin® 3.209 4.557 1.363
Teon® 2.890 6.243 1.868
1Zeff, the efective atomic number, is calculated using a power law approximation.
2For standard material sensitometry inserts The Phantom Laboratory purchases a multiple year supply of
material from a single batch. Samples of the purchased material are then measured to determine the
actual specic gravity. The specic gravity of air is taken to be .0013 at standard temperature and
pressure. For custom cast materials the specic gravity of each cast batch is noted and supplied
with the phantom.
3These are maximum and minimum measured values from a sample of 94 scans using different
scanners and protocols. HU can vary dramatically between scanners and imaging protocols and
numbers outside of this range are not unusual.
4Relative Electron Density is the electron density of the material in e/cm3
divided by the electron density of water (H2O) in e/cm3.
5 Polymethylpentene
6Low Density Polyethylene

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An excel le with the linear attenuation coefcient µ [units cm-1] for the sensitometry
materials can be downloaded from our web site.
Contrast Scale (CS) is formally dened as
CS = µm(E) - µw(E)
CTm (E) – CTw (E)
where m is reference medium, and w is water, and E is the effective energy of the CT
beam.
Alternatively, CS = µ1(E) - µ2(E)
CT1 (E) – CT2 (E)
where 1,2 are two materials with low z effective, similar to water (eg. acrylic & air).
Linear attenuation coefcient µ [units cm-1]

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CTP528 High resolution module with 21 line pair per cm gauge and point
source
This section has a 1 through 21 line pair per centimeter high resolution test gauge
and two impulse sources (beads) which are cast into a uniform material. The beads are
positioned along the y axis 20mm above or below the phantom’s center and 2.5 and 10mm
past the center of the gauge in the z direction. On older CTP528 modules the bead is
aligned in the z axis with the gauge.
Bead Point Source for point spread function and MTF
Use the impulse source to estimate the point source response function of the CT
system. Print out a digitized image of the area surrounding the impulse source. Use the
numerical data to determine the two-dimensional array of the CT values arising from the
impulse source.
The FWHM of the point spread function is determined from the best-t curve of the point
spread function numerical data.
The average of several different arrays of impulse response functions is calculated to
obtain the average point spread function of the system. These numerical values are
used in conjunction with the Fourier Transform Program to provide an estimate of the
two-dimensional spatial frequency response characteristics of the CT system (MTF).
Illustration is on the next page.
The tungsten carbide bead has a diameter of 0.011” or 0.28mm. Because the bead is
subpixel sized it is not usually necessary to compensate for it. However, some MTF
programs are designed to compensate for its size.

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The above illustration shows how by summing the columns (y axis) of numbers in the
point spread function (PSF) the line spread function (LSF) for the x axis is obtained.
The MTF curve results from the Fourier transform of the LSF data. Generally it
is easiest to use automated software for this operation. Some CT scanners are
supplied with software which can calculate the MTF from the Catphan® bead images.
Independent software is listed in the Current automated programs available section
of the manual.
Use of automated scanner MTF programs
Many manufacturers include automated MTF software in the standard scanner software
packages. Because the bead is cast into an epoxy background which has a different
density than water, the software must accept an input for the background. The point size
of .28mm must also be selected. While a sphere does produce a different density prole
than a cross section of a wire or cylinder, the actual difference is not usually signicant in
current CT scanners.

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Bead point source for slice sensitivity prole
The bead in this module can be used to calculate the slice sensitivity prole (SSP).
The above image illustrates how the bead will produce an ovoid object in a 3 dimensional
reconstruction. The length of the object at the Full Width at Half Maximum signal
indicates the SSP. This measurement can be easily obtained on some systems, by
making a sagittal or coronal reconstruction through the bead. The bead image in these
reconstructions will appear as a small line. By setting the FWHM (use the same
technique described in the Scan slice geometry section) measuring the z axis length of
the bead image to obtain the SSP.
If the scanner does not have the ability to measure z axis lengths in the sagittal or
coronal planes, a SSP can be made by incrementing or spiraling the slice through the
bead and reconstructing images in positive and negative table directions from the bead
(using the smallest available increments) and plotting the peak CT number of the bead
image in each slice. The FWHM measurement can then be made from the plotted CT
values of the bead as a function of z axis table position.

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21 Line pair per centimeter high resolution gauge
The 21 line pair/cm gauge has resolution tests for visual evaluation of high resolution
ranging from 1 through 21 line pair/cm. The gauge accuracy is ± 0.5 line pair at the 21
line pair test and even better at lower line pair tests.
The gauge is cut from 2mm thick aluminum sheets and cast into epoxy. Depending on the
choice of slice thickness, the contrast levels will vary due to volume averaging.