2019 AAPM Annual Meeting
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Session Title: CT Systems Course
Question 1: Assuming the filament length is constant, which one of the following statements regarding the anode angle of X-ray tube in CT is true?
Reference:A smaller anode angle provides a smaller effective focal spot (hence better spatial resolution) for the same actual focal area. However, a smaller anode angle limits the size of usable x-ray field owning to cutoff of the beam. Smaller anode angle also can cause stronger heel effect because of the steeper intensity falloff across the beam. Finally, a larger anode angle reduces the actual focal area and results in poor power loading. Reference: Reference: J. Bushberg, J. Seibert, E. Leidholdt, J. Boone, “The essential physics of medical imaging”, Lippincott Williams & Wilkins.
Choice A:Smaller anode angle provides better spatial resolution, larger field coverage, less heel effect, and higher power loading.
Choice B:Smaller anode angle provides worse spatial resolution, smaller field coverage, stronger heel effect, and less power loading.
Choice C:Larger anode angle provides better spatial resolution, larger field coverage, stronger heel effect, and higher power loading.
Choice D:Larger anode angle provides worse spatial resolution, larger field coverage, less heel effect, and lower power loading.
Choice E:None of above is correct.
Question 2: The CT detector scintillator converts X-rays into?
Reference:Reference: J. Hsieh, “Computed Tomography: Principles, designs, artifacts, and recent advances”, SPIE Press, Bellingham, WA.
Choice A:Visible light
Choice B:Electrons
Choice C:X-ray photons
Choice D:Current
Choice E:Gas
Question 3: The most likely detector-introduced artifact produced by a third-generation CT geometry is a?
Reference:With third-generation geometry in CT, each individual detector gives rise to an annulus (ring) of image information. When a detector becomes mis-calibrated, the tainted data can lead to ring artifacts in the reconstructed image. Reference: J. Bushberg, J.
Choice A:Doubloon
Choice B:Ring
Choice C:Vase
Choice D:Statue
Choice E:Shard
Question 4: A set of parallel projections was acquired with a CT gantry that rotates clockwise over 2. A student developed a parallel-beam reconstruction algorithm and made one mistake by assuming the gantry rotates counterclockwise. What is the relationship, if any, between the image reconstructed by the student and the correctly reconstructed image?
Reference:For parallel geometry, the reconstructed image will be flipped with respect to the iso-center if the rotation direction is reversed. Otherwise, the reconstructed images are identical. J. Hsieh, Computed Tomography: principles, design, artifacts, and recent advances, 3rd ed., SPIE press, 2015
Choice A:It's rotated 180-degree
Choice B:It is flipped
Choice C:There is no relationship between the two images
Question 5: 2. An image was original reconstructed to 50cm FOV and then zoomed to 25cm FOV with image-space interpolation to produce the final image, f. Another image, fâ￿™, was generated with a targeted reconstruction to 25cm FOV (all other parameters were kept the same). Which of the following statement is true?
Reference:Image-space interpolation generally reduces spatial resolution as compared to the original image. Image generated by such approach generally exhibits less sharpness. On the other hand, noise is reduced during the interpolation process on top of the smooth
Choice A:f’ has a positive CT number shift compared to f
Choice B:f' is sharper than f
Choice C:f' is less noisy than f
Question 6: Beam-hardening artifact is caused by:
Reference:Beam-hardening refers to the phenomenon that the x-ray spectrum is shifted toward higher energy after passing through an object. Because of the low-energy (softer) x-ray photons are preferentially absorbed in most material, the x-ray beam becomes “harder”. 1. H. E. Johns and J. R. Cunningham, The Physics of Radiology, Charles C. Thomas Publisher Ltd., Springfield, IL (1983).
Choice A:Polychromatic x-ray spectrum from the x-ray tube
Choice B:Energy-dependent nature of attenuation coefficient
Choice C:Patient motion during data acquisition
Choice D:A and B
Choice E:A, B, and C
Question 7: A patient was scanned on a third-generation CT scanner in a step-and-shoot full scan mode (360o) at a 1 sec scan speed. Image artifacts appeared in the reconstructed image and you suspect that they are related to a patient’s jerky motion. How do you convince the radiologist that they are indeed motion artifacts (The original projection data are available and re-scanning the patient is not an option)?
Reference:Because of the redundant samples in the 2 rotation, we can use halfscan to reconstruct images at different starting angles. For the halfscan image that does not contains the projection of the jerky motion should be free of motion artifact. Alternatively, one can use conjugate samples to replace the corrupted projections to obtain artifact-free image. D. L. Parker, “Optimal Short Scan Convolution Reconstruction for Fan-Beam CT,” Med. Phys. 9, 1982; 254:257.
Choice A:Use half-scan to reconstruct images at different starting angle to obtain artifact-free image
Choice B:Use conjugate samples to replace the motion corrupted samples to obtain artifact-free image
Choice C:Inspecting the sinogram to identify discontinuities along the view angle direction
Choice D:All of the above
Question 8: Low Contrast Detectability (LCD) for CT is affected by the following data acquisition and reconstruction parameters except?
Reference:Image reconstruction interval has no direction impact on the low contrast detectability for CT. The other parameters have direct impact on either image noise or lesion contrast. Reference: J. Hsieh, Computed Tomography: principles, design, artifacts, and
Choice A:mAs (tube current x gantry rotation period)
Choice B:kVp
Choice C:Image slice thickness
Choice D:Image reconstruction interval
Choice E:Reconstruction kernel
Question 9: In CT, the radiation dose to individual patient is better described by “organ dose”. Which of the following CT dose metrics is the best approximation to the dose received by liver from a routine abdominal CT?
Reference:CTDIvol measures the scanner output, not the organ dose. Effective dose is used to estimate the population risk from a CT exam. Dose length product can be used to estimate the effective dose for a standard sized patient. SSDE is based on CTDIvol, but corrected by a factor related to the patient size. SSDE is a better dose metric to approximate the organ dose, for example, the liver dose from an abdominal CT scan. Reference: Moore BM, Brady SL, et al. “Size-specific dose estimate (SSDE) provides a simple method to calculate organ dose for pediatric CT examinations”, Medical Physics: 41(7), July, 2014
Choice A:CTDIvol
Choice B:Size specific dose estimate (SSDE)
Choice C:Dose length product (DLP)
Choice D:Effective dose
Choice E:None of the above
Question 10: For CT low dose lung cancer screening, ACR recommends a set of requirements for a designated lung cancer screening site. Which of the following is NOT one of the ACR requirements?
Reference:ACR recommends CTDIvol <= 3mGy, not 3mSv (effective dose), for a standard size patient of 155lbs. The other acquisition and reconstruction parameters are recommended by ACR. Reference: ACR-STR PRACTICE PARAMETER FOR THE PERFORMANCE AND REP
Choice A:Single breathold
Choice B:CTDIvol <= 3 mSv
Choice C:Image Thickness <= 2.5 mm
Choice D:Gantry rotation period <= 0.5 sec
Choice E:Number of physical detector rows >= 16
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