NMTCB Instrument Operations & Quality Control 4 — Questions and Answers
Question 1: A technologist measures the sensitivity of a gamma camera using a point source in air. What does sensitivity measure in this context?
- The count rate detected per unit of source activity (cps/MBq) (Correct answer)
- The minimum activity detectable above background
- The energy resolution of the NaI crystal
- The spatial resolution at the center of the field of view
Correct answer: The count rate detected per unit of source activity (cps/MBq)
Sensitivity is defined as the detected count rate per unit activity (cps/MBq or cpm/µCi) and reflects how efficiently the camera detects emitted photons.
Question 2: When performing a dose calibrator geometry test, what variable is being assessed?
- The effect of sample volume and container shape on measured activity (Correct answer)
- The instrument's ability to measure multiple isotopes simultaneously
- The linearity of response from micro- to millicurie levels
- The reproducibility of sequential measurements of the same source
Correct answer: The effect of sample volume and container shape on measured activity
The geometry test determines correction factors needed when measuring activity in containers of different sizes or volumes than the calibration standard.
Question 3: What is the accepted frequency for performing accuracy (accuracy/constancy) testing of a dose calibrator according to NRC/AAPM recommendations?
- Daily (Correct answer)
- Weekly
- Monthly
- Annually
Correct answer: Daily
NRC regulations (10 CFR 35.60) require that dose calibrators be tested for constancy before each day of use.
Question 4: Which artifact in planar nuclear medicine imaging is caused by non-uniform photomultiplier tube gain across the gamma camera crystal?
- Non-uniformity (hot or cold spots) (Correct answer)
- Star artifact
- Ringing artifact
- Mach band artifact
Correct answer: Non-uniformity (hot or cold spots)
Variations in PMT gain produce areas of artificially increased or decreased counts (hot or cold spots) in the flood image, indicating non-uniformity.
Question 5: A PET scanner's coincidence timing resolution is measured in nanoseconds. Why does better timing resolution improve image quality?
- It narrows the coincidence time window, reducing random coincidences and improving signal-to-noise ratio (Correct answer)
- It increases the scanner's sensitivity to low-energy photons
- It reduces the attenuation correction factor for deep structures
- It widens the axial field of view of the scanner
Correct answer: It narrows the coincidence time window, reducing random coincidences and improving signal-to-noise ratio
Tighter coincidence timing windows reduce random (accidental) coincidences that arise from uncorrelated photon pairs, improving SNR and quantitative accuracy.
Question 6: During routine QC, a technologist observes a 'tuning fork' pattern in the gamma camera flood image. What is the most likely cause?
- A cracked or damaged NaI(Tl) crystal (Correct answer)
- A misaligned collimator septa
- Incorrect energy window setting
- A failed photomultiplier tube
Correct answer: A cracked or damaged NaI(Tl) crystal
A characteristic tuning-fork or linear crack pattern in the flood image strongly suggests a fracture in the NaI(Tl) scintillation crystal.
Question 7: What is the function of the pulse height analyzer (PHA) in a scintillation counting system?
- To select pulses within a specified voltage range corresponding to photons of the desired energy (Correct answer)
- To amplify weak signals from the photomultiplier tube before digitization
- To convert analog detector signals into a digital image matrix
- To measure the time interval between two detected photons
Correct answer: To select pulses within a specified voltage range corresponding to photons of the desired energy
The PHA (energy window) accepts only those voltage pulses that fall within the set lower and upper level discriminators, corresponding to the photon energy of interest.
NMTCB Instrument Operations & Quality Control 4
A technologist measures the sensitivity of a gamma camera using a point source in air.
What does sensitivity measure in this context?