CATPHAN PHANTOM PDF

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Name: Syeda Umme Ayman Roll:201826040 Level-3 ,Term-2 Dept: Biomedical Engineering Course Name: Medical Imaging Sessional Course code: BME312

Catphan Phantom 600 Abstract: In modern medical imaging calibration of the radiation detectors like CT,MRI and different radiation therapy are usually done using different types of phantoms. These tests are done for fulfilling the requirements of Quality Assurance Program for different imaging modalities. Each modalities require different phantoms. With these phantoms we measure sensitometry (linearity), spatial uniformity, noise of CT system etc. to ensure that the system is operating at the right way and we will be able get better image quality and performance.

Introduction: Phantom is a specialized object utilized in medical imaging for quality control, equipment calibration ,dosimetry and education. The medical imaging phantoms are used as a substitute of human tissue to ensure that the system and methods for imaging the human body are operating correctly. Though different QC phantoms are used for testing the image quality on CT are available but most commonly used Catphan Phantom. They are cylindrical in shape. There are 3 modules of the Catphan Phantom that are used a)(Model CTP 404)for pixel value stability and contrast to noise ratio, b)(Model CTP 528) for high contrast resolution test , c)(Model CTP 515) for low contrast resolution test. In Catphan phantom different multi-slice spiral protocols are used for performance evaluation and step and shoot approach is used for bead ramps where the slice geometry and MTF can be measured from each slice and uniformity section where the signal to noise ratio and uniformity of each skice can be evaluated. It is because in spiral mode multi slice image is reconstructed from an ensemble of data taken from the different positions across the beam from detector rows. Then allover images are minimal.In the spiral mode each slice is represents data as seen from all detector rows.It is in sense of averaging the detector row positions. In multi slice step and shoot mode each slice is created from a single detector row with a consistent collimation, then the different between slices will be evident. It is like conventional axial scanner.

Catphan Phantoms is used for CT QC test and in radiation therapy for evaluation scanners’ sensitometry ,uniformity ,geometry of the slice area and low contrast sensitivity performance. Test Summary: CTP404 - sensitometry (linearity) - scan slice geometry (slice width and slice sensitivity profiles) - pixel (matrix) size - circular symmetry - phantom position verification - patient alignment system check -scan incrementation CTP591 - slice width for thin slices - slice width for thick slices - MTF and SSP with two size point sources - test on multiple slices in a multi-slice sequence CTP528 - high resolution measurement -up to 21 line pairs per cm CTP515 - low contrast sensitivity - comparative subslice and supra-slice low contrast sensitivity CTP486 - spatial uniformity - noise (precision) of CT systems These are the different parts of Catphan Phantom 600

Method: Initial Phantom Positioning: First the Catphan phantom is removed from the box and cantilever the phantom off the end of the table that eliminate potential table artifacts.the Catphan Phantom case enables easy mounting .It is ensured that the case is stable with the weight of the phantom and is properly counterweighted to prevet tipping.The Catphan is leveled by adjusting the thumb screw. The Catphan Phantom is aligned using the marker dots on the phantom and the scanner alignment lights .To align the phantom to x position ,slide the phantom along the rim of the case. To align the phantom to y axis the height f the table height is adjusted.To align the first test section with z axis ,light drive the table into the gantry.

Fig: initial phantom positioning Once the phantom is aligned we are ready to scan The Catphan Phantom begin with a scan of the section (CTP 404) at 0 mm table position. The center of this test section can also be located on the scout view where the wire ramps cross 4 opposed ramps allow us to verify the phantom position quickly. When the phantom is aligned properly, the ramps should be centered in the image. Phantom Position Verification: By evaluating the scan image of section1(CTP 404) the phantoms’ position and alignment can be verified. The section contains 4 wire ramps which rise at 23 degree angles from the base to the top of the module.

The above figure shows how the ramp images changes if the scan center is above or below the axis center of test module. The scanner grid selection feature will help us in viewing the axis.Incorrect alignment may indicate the need of recalibration of the alignment of of lights and scout view accuracy test by selectingand scanning the center of the slice. Scanning slice geometry: Slice width: To measure the phantom slice width we establish full width at half maximum of the slice with ramps. For this calculation we use region of interest measurement

to determine the background CT number and subtract it from peak CT number where the ramps disappear with smallest window settings. Then we divide the number by two to get the mean and add it to the background Using the CT number , determine the half maximum: First calculate the net peak... (CT # peak - background = net peak CT #) the 50% net peak... (net peak CT # ÷ 2 = 50% net peak CT #) the half maximum CT number= (50% net peak CT # + background CT # = half maximum CT #) Then we set the level at full width at half maximum number. To determine the length of the wire we use scanner measurement tool. We use 23 degree ramps to increase accuracy. Lastly the measured wire length is multiplied by 0.4 Scan incrementation: The ramps are also used to test the couch incrementation accuracy by driving the table 30mm in and out of the gantry . It is also done by testing to see if phantom is returned to the same position. First we scan section 1 using a given slice width. Then we increment the table one slice width and make a second scan. We establish the x and y coordinates for the center of each ramp image. To calculate the distance between these points and multiply by the 23° ramp angle correction factor of 0.42. 0.42(L1 - L2) = scan incrementation Linear distortion and pixel size verification: The 4 thin rods in the center of the phantom are used to measure linear distortion and pixel size .To test distortion we measure the centers of the rods. Surround the ramps there are 4 cents otamentone umber samples .These are made from air LDPE acrylic and Teflon. The samples allow us to monitor any possible CT measurement problem resulting from scanner drift. This evaluation of CT linearity is extremely important when evaluation Ct scanner used for treatment planning. Another use is to count the number of pixels between the hole centers,

and by knowing the distance and number of pixels. Then the pixel size can be verified. Spherical acrylic contrast targets : The section has five 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. Sensitometry (CT number linearity): There are 4 or 7 high contrast sensitometric targets surround the wire slice thickness ramps. As mentioned before 3 are made from the commercial plastics: Teflon, acrylic and low density polyethylene (LDPE). The fourth is air. These targets range from approximately +1000 H to -1000 H. The monitoring of sensitometry target values over time and can provide valuable information, indicating changes in scanner performance.

Contrast Scale (CS) is formally defined as, CS = um (E) - uw (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 = u1 (E) - u2 (E)/ CT1 (E) – CT2 (E) where 1,2 are two materials with low z effective, similar to water like acrylic & air. Bead Point Source for point spread function and MTF : All Catphan phantom model use a very dense tungsten carbide bead to create point source signal for MTF or Modulation Transfer Function plot. The MTF curve results from the Fourier transform of the Line Spread Function data.

The use of bead eliminates the error which can be caused by misalignment of wires. We moved away from thin wire as a the point source to a small bead bead is much easier to align and we do not have artifacts. Bead point source for slice sensitivity profile: The bead in this module is used to calculate the Slice Sensitivity Profile (SSP).

The figure describes 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.

High Resolution Test: The high resolution sections offer visual tests of resolution in 412 and 424 models. The high resolution test ranges from 5-11 lp/cm on the upper side and 12-21 lp/cm in the lower side. In case of 500 model test gauge ranges from 1-21 lp/cm. In this module is set in a radial layout with a minimum use of material. This reduces the artifact which might distort the data created by MTF which is cast into same section. The gauge accuracy is ± 0.5 line pair at the 21 line pair test and even better at lower line pair tests.

References: 1) https://www.uio.no/studier/emner/matnat/fys/nedlagteemner/FYS4760/h07/Catphan500-600manual.pdf 2) https://www.youtube.com/watch?v=_DAMdhgD9kY&t=298s 3) https://www.phantomlab.com/catphan-600...