NCT Quality Control & Equipment Assurance 2 — Questions and Answers
Question 1: Center of rotation (COR) calibration in SPECT imaging is performed to:
- Measure the sensitivity of the detector at multiple angles
- Ensure the mechanical rotation center and the electronic center of the projection data are aligned (Correct answer)
- Verify that collimator holes are parallel to the detector face
- Calibrate the injected activity against a dose calibrator
Correct answer: Ensure the mechanical rotation center and the electronic center of the projection data are aligned
A COR offset causes the projection data to be misregistered during backprojection, producing ring or 'bull's-eye' artifacts in reconstructed SPECT images.
Question 2: How frequently should SPECT center of rotation calibration be performed under standard nuclear cardiology QC protocols?
- Daily
- Weekly (Correct answer)
- Monthly
- Annually
Correct answer: Weekly
Weekly COR calibration is the standard recommendation from ASNC and SNM guidelines, or whenever camera service has been performed, to detect mechanical drift before it affects clinical studies.
Question 3: The recommended energy window for Tc-99m myocardial perfusion SPECT imaging is:
- 10% symmetric window centered at 140 keV
- 20% symmetric window centered at 140 keV (Correct answer)
- 30% symmetric window centered at 140 keV
- 15% symmetric window centered at 159 keV
Correct answer: 20% symmetric window centered at 140 keV
A 20% symmetric window (126–154 keV) centered on the 140 keV photopeak of Tc-99m provides the optimal balance between true-event acceptance and scatter rejection.
Question 4: Dead time in nuclear medicine instrumentation refers to:
- The time required for radioactive tracer to reach peak myocardial uptake
- The period after a detected event during which the system cannot register additional events (Correct answer)
- The delay between patient injection and SPECT acquisition start
- The pause between detector rotation steps in step-and-shoot SPECT
Correct answer: The period after a detected event during which the system cannot register additional events
During dead time, the detection electronics are busy processing a pulse and are unable to detect another incoming photon, leading to count loss at high activity levels.
Question 5: The multi-window spatial registration test evaluates whether:
- Multiple energy peaks from different radionuclides are spatially co-registered on the detector (Correct answer)
- The camera can simultaneously acquire two patients at different energy windows
- Energy window settings shift with changing room temperature
- Flood field uniformity is maintained across all energy windows
Correct answer: Multiple energy peaks from different radionuclides are spatially co-registered on the detector
Multi-window spatial registration verifies that images acquired at different energy peaks (e.g., Tl-201's 70 and 167 keV peaks) are spatially aligned, preventing artifactual misregistration in dual-isotope protocols.
Question 6: Gamma camera system sensitivity is typically expressed as:
- Counts per second per megabecquerel (cps/MBq) or counts per minute per microcurie (cpm/µCi) (Correct answer)
- Millimeters of spatial resolution at 10 cm distance
- Percentage of photons transmitted through the collimator
- Ratio of scatter to primary counts in the energy window
Correct answer: Counts per second per megabecquerel (cps/MBq) or counts per minute per microcurie (cpm/µCi)
Sensitivity describes the detection efficiency of the system — how many counts are recorded per unit of radioactivity — which directly affects how much activity is needed to achieve adequate image statistics.
Question 7: A SPECT uniformity correction flood is acquired to:
- Determine how much radioactivity to inject in each patient
- Generate a pixel-by-pixel correction matrix that compensates for non-uniformities during tomographic reconstruction (Correct answer)
- Measure the geometric efficiency of the collimator at each rotation angle
- Verify correct table height during patient acquisition
Correct answer: Generate a pixel-by-pixel correction matrix that compensates for non-uniformities during tomographic reconstruction
Non-uniformities that are minor in planar imaging become amplified during SPECT backprojection into ring artifacts; a high-count flood correction map normalizes sensitivity across all pixels before reconstruction.
NCT Quality Control & Equipment Assurance 2
Center of rotation (COR) calibration in SPECT imaging is performed to: