Nuclear Medicine Instrumentation and Radiation Detection
What the exam tests
Instrumentation questions test whether you can translate detector physics into image-quality decisions: which collimator, what energy window, why a flood is nonuniform, and how count rate affects a study. Expect calculation items on energy resolution, dead time, and collimator performance.
Core rules
- A NaI(Tl) crystal converts gamma energy into light; photomultiplier tubes amplify it; the pulse-height analyzer accepts events inside the photopeak window (typically a 20% window centered on 140 keV for Tc-99m).
- Energy resolution is the photopeak FWHM divided by photopeak energy, expressed as a percent. Better resolution means a smaller FWHM and better scatter rejection.
- The collimator sets spatial resolution and sensitivity. Parallel-hole low-energy high-resolution (LEHR) collimators suit Tc-99m; I-131 at 364 keV requires a high-energy collimator to limit septal penetration.
- Dead time causes count loss at high activity. The nonparalyzable model is m = n/(1 + nτ), where m is observed and n true rate; correction is needed for first-pass or high-dose studies.
- SPECT depends on uniform response and accurate center of rotation; PET detects 511 keV annihilation photon pairs in coincidence, and time-of-flight improves signal-to-noise.
Worked example
A Tc-99m photopeak is centered at 140 keV with FWHM of 14 keV. Energy resolution = 14/140 × 100 = 10%. If the FWHM widens to 21 keV, resolution becomes 21/140 × 100 = 15%; the broader peak admits more Compton scatter, lowering contrast. The calculation shows why a detuned PMT or drifted energy peak degrades images even when the crystal is intact.
Traps that cost marks
- Confusing sensitivity (counts per unit activity) with resolution (FWHM).
- Using a low-energy collimator for I-131, then blaming the camera for septal penetration.
- Forgetting that a 20% window at 140 keV is 28 keV wide, not 20 keV.
- Ignoring dead-time loss when a high-activity bolus arrives.