NCT Quality Control & Equipment Assurance 1 — Questions and Answers
Question 1: What is the primary purpose of performing a daily flood field uniformity test on a gamma camera?
- To calibrate the injected radiopharmaceutical dose
- To detect non-uniformities in detector sensitivity that could degrade image quality (Correct answer)
- To verify patient positioning accuracy on the imaging table
- To measure collimator hole size and alignment
Correct answer: To detect non-uniformities in detector sensitivity that could degrade image quality
Flood field uniformity tests reveal variations in detector sensitivity across the field of view, which can produce artifactual hot or cold regions in clinical images.
Question 2: An intrinsic flood field uniformity test is performed by:
- Placing a sheet source against a collimator attached to the camera
- Removing the collimator and using a distant point source to irradiate the bare crystal uniformly (Correct answer)
- Using a bar phantom positioned at isocenter with the collimator on
- Injecting a known activity into a water-filled phantom placed on the detector
Correct answer: Removing the collimator and using a distant point source to irradiate the bare crystal uniformly
Without a collimator, a distant point source provides a uniform flux of photons across the NaI crystal, isolating intrinsic detector performance from collimator effects.
Question 3: Which term describes the maximum count-rate difference between any pixel and the mean count rate across the entire detector field of view in a uniformity test?
- Differential uniformity
- Integral uniformity (Correct answer)
- Spatial linearity
- Energy resolution
Correct answer: Integral uniformity
Integral uniformity expresses the global maximum deviation from the mean count rate across the full useful field of view, while differential uniformity measures local pixel-to-pixel variation.
Question 4: The 'peaking' procedure performed before gamma camera imaging refers to:
- Adjusting the photomultiplier tube high voltage to center the energy window on the radionuclide photopeak (Correct answer)
- Maximizing patient dose to achieve the highest possible count rate
- Setting the SPECT rotation arc to 360 degrees
- Calibrating collimator attenuation at peak energy
Correct answer: Adjusting the photomultiplier tube high voltage to center the energy window on the radionuclide photopeak
Peaking ensures the energy window is correctly positioned at the photopeak of the radionuclide in use, maximizing true coincidence detection and minimizing scatter acceptance.
Question 5: A bar phantom placed in front of a gamma camera is used primarily to assess:
- Detector sensitivity and count rate capability
- Spatial resolution and spatial linearity (Correct answer)
- Energy resolution of the NaI crystal
- Uniformity across the flood field
Correct answer: Spatial resolution and spatial linearity
Bar phantoms contain alternating lead strips of known widths; the ability to distinguish individual bars evaluates spatial resolution, and any curvature of bar images indicates spatial non-linearity.
Question 6: The acceptable limit for integral uniformity on a modern digital gamma camera, as recommended by NEMA standards, is:
- Less than 2%
- Less than 5% (Correct answer)
- Less than 10%
- Less than 15%
Correct answer: Less than 5%
NEMA standards specify that integral uniformity should be less than 5% for clinical cameras; values exceeding this threshold can introduce significant image artifacts.
Question 7: Which of the following best describes the energy resolution of a gamma camera?
- The ability to differentiate two point sources separated by a known distance
- The full-width at half-maximum (FWHM) of the photopeak expressed as a percentage of the peak energy (Correct answer)
- The count rate at which dead time losses reach 20%
- The maximum field of view size available for imaging
Correct answer: The full-width at half-maximum (FWHM) of the photopeak expressed as a percentage of the peak energy
Energy resolution is expressed as the FWHM of the photopeak divided by the peak energy, expressed as a percentage; better resolution means the camera more effectively rejects scattered photons.
NCT Quality Control & Equipment Assurance 1
What is the primary purpose of performing a daily flood field uniformity test on a gamma camera?