NMTCB Instrument Operations & Quality Control 3 — Questions and Answers
Question 1: A dose calibrator linearity test uses a vial of Tc-99m measured repeatedly over several days. What does this test confirm?
- That the instrument measures accurately across a range of activity levels (Correct answer)
- That the detector is insensitive to photon energy variations
- That the well geometry is consistent between measurements
- That isotope selector settings are calibrated correctly
Correct answer: That the instrument measures accurately across a range of activity levels
The linearity test confirms that the dose calibrator reads accurately from high activity (millicuries/GBq) down to low activity levels as the source decays.
Question 2: What is the purpose of a constancy test performed on a dose calibrator?
- To verify day-to-day reproducibility using a long-lived sealed reference source (Correct answer)
- To confirm linearity across a wide activity range
- To measure geometry-dependent calibration factors
- To determine the lower limit of detection
Correct answer: To verify day-to-day reproducibility using a long-lived sealed reference source
Constancy testing uses a sealed long-lived source (e.g., Cs-137 or Co-57) measured daily to confirm the dose calibrator's reproducibility over time.
Question 3: Which parameter defines the minimum separation between two point sources that a gamma camera can still resolve as distinct?
- Spatial resolution (Correct answer)
- Spatial linearity
- Sensitivity
- Energy resolution
Correct answer: Spatial resolution
Spatial resolution is the ability to distinguish two closely spaced radioactive sources as separate, typically measured in mm FWHM.
Question 4: In PET/CT, a technologist performs a well-counter cross-calibration. What is the purpose of this procedure?
- To ensure that PET scanner activity measurements agree with those of the dose calibrator (Correct answer)
- To verify CT Hounsfield unit accuracy for attenuation correction
- To synchronize the PET and CT gantry rotation speeds
- To calibrate the TOF timing resolution of the PET detectors
Correct answer: To ensure that PET scanner activity measurements agree with those of the dose calibrator
Cross-calibration ensures that the PET scanner's measured activity concentrations are consistent with dose calibrator readings, enabling accurate quantification.
Question 5: What is dead time in a radiation detector?
- The period after detecting one event during which the detector cannot register another event (Correct answer)
- The delay between photon emission and crystal luminescence
- The time required to reposition the camera head between projections
- The decay constant of the scintillation crystal
Correct answer: The period after detecting one event during which the detector cannot register another event
Dead time is the brief interval after a detected event during which the system is unable to process additional incoming events, causing count loss at high count rates.
Question 6: A technologist performs a SPECT/CT quality control scan and finds a misregistration between the SPECT and CT images. Which QC procedure directly assesses this alignment?
- SPECT/CT co-registration (alignment) phantom test (Correct answer)
- Center of rotation correction
- Uniformity flood acquisition
- Sensitivity measurement
Correct answer: SPECT/CT co-registration (alignment) phantom test
A co-registration or alignment phantom test uses a fillable phantom with radioactive and CT-visible markers to verify spatial alignment between the SPECT and CT components.
Question 7: Which of the following best describes the purpose of the energy window in gamma camera operation?
- To accept only pulses within a defined range around the photopeak, rejecting scattered photons (Correct answer)
- To limit the maximum count rate to prevent dead time losses
- To set the spatial matrix size for image acquisition
- To control the high voltage applied to the photomultiplier tubes
Correct answer: To accept only pulses within a defined range around the photopeak, rejecting scattered photons
The energy window is centered on the photopeak (e.g., ±10% for Tc-99m at 140 keV) to accept true unscattered photons and reject lower-energy Compton scatter.
NMTCB Instrument Operations & Quality Control 3
A dose calibrator linearity test uses a vial of Tc-99m measured repeatedly over several days.
What does this test confirm?