Instrumentation and Physics
Instrumentation and physics questions test how the gamma camera and SPECT/CT system form an image, how collimators and energy windows are selected, and how quality-control failures appear on images.
Core rules
- Camera chain: collimator → NaI(Tl) crystal → photomultiplier tubes → amplifier → pulse-height analyzer → computer.
- Collimator choice trades sensitivity for resolution: low-energy high-resolution (LEHR) for Tc-99m, high-energy for I-131, pinhole for small structures such as thyroid.
- Energy window: a 20% window centered on the 140 keV Tc-99m photopeak accepts 126–154 keV; window width affects scatter and count rate.
- SPECT reconstruction: filtered backprojection is fast but noisy; iterative reconstruction (OSEM) improves noise and supports CT attenuation correction.
- Daily QC includes energy peaking, uniformity flood, and center of rotation (COR).
Worked example
A Tc-99m SPECT bone scan shows a ring artifact on reconstructed images. What is the most likely cause? Reasoning: A ring artifact usually comes from nonuniform detector response - a bad photomultiplier tube, crystal defect, or failed flood correction - because the same detector element contributes at every projection angle. COR error more typically degrades resolution or produces misregistration, and patient motion produces streaks or edge blurring. First check the daily uniformity flood and PMT gain adjustments.
Traps
- Blaming patient motion for a centered ring artifact.
- Using a low-energy collimator for I-131.
- Widening the energy window to gain counts and accepting more scatter.