Physics MCQs
Test your knowledge of nuclear medicine physics concepts.
Which collimator provides the HIGHEST sensitivity but LOWEST resolution?
Explanation: Diverging collimators have higher sensitivity because they accept photons from a larger area, but this reduces spatial resolution. LEHR collimators prioritize resolution over sensitivity.
Technetium-99m decays primarily by which mechanism?
Explanation: Tc-99m decays by isomeric transition (IT), emitting a 140 keV gamma ray. This is the transition from a metastable nuclear isomer to a lower energy state.
In PET imaging, what is the energy of each annihilation photon?
Explanation: When a positron annihilates with an electron, two 511 keV photons are produced traveling in opposite directions (180° apart). This corresponds to the rest mass energy of the electron.
If a radioactive sample has an activity of 80 mCi and decays to 10 mCi over 18 hours, what is the half-life?
Explanation: Activity decreases: 80 → 40 → 20 → 10 mCi = 3 half-lives. Therefore: 18 hours ÷ 3 = 6 hours (Tc-99m half-life).
What is the primary function of the photomultiplier tube (PMT) in a gamma camera?
Explanation: The PMT converts light photons from the scintillator into an amplified electrical signal. The scintillator converts gamma rays to light, and the PMT amplifies this by ~10⁶.
A narrower photopeak (better energy resolution) primarily helps to:
Explanation: Better energy resolution allows tighter energy windows, which more effectively rejects scattered photons that have lower energy and degrade image quality.
Which type of detector is used in dedicated cardiac CZT cameras?
Explanation: Cadmium Zinc Telluride (CZT) is a semiconductor detector that directly converts gamma rays to electrical signals without needing PMTs, offering superior energy resolution (~5%).
In SPECT, what does ‘center of rotation’ (COR) error cause?
Explanation: COR errors occur when the assumed center of rotation doesn’t match the actual mechanical center. This causes image blurring and ring artifacts in reconstructed images.
The ‘star artifact’ in 2D filtered back projection is caused by:
Explanation: Backprojection smears values across the image matrix. Without filtering (ramp filter), intense hot spots create radiating spokes resembling a star.
The daily uniformity flood source for a gamma camera should have an activity variance of less than:
Explanation: For extrinsic (with collimator) or intrinsic (without collimator) uniformity floods, the count uniformity (integral/differential) should typically be < 3-5% for SPECT systems.
Geiger-Muller (GM) counters are most useful for:
Explanation: GM counters are highly sensitive gas-filled detectors ideal for detecting low-level contamination (surveying). They are poor at creating images or measuring high dose rates (dead time).
What effect does increasing the matrix size (e.g., 64x64 to 128x128) have on the image?
Explanation: Higher matrix size (smaller pixels) improves spatial resolution but increases noise (fewer counts per pixel) unless acquisition time is increased.
Time-of-Flight (TOF) in PET imaging improves:
Explanation: TOF uses the difference in arrival times of coincidence photons to localize the event along the line of response, improving Signal-to-Noise Ratio (SNR) and image contrast.
Study Tips:
- Know sensitivity vs resolution trade-offs for collimators
- Memorize common radionuclide energies and half-lives
- Understand the detection chain: gamma → light → electrical signal
- Review quality control limits (uniformity, COR, linearity)