Physics, Instrumentation, and Radiation Safety
Core physics and instrumentation
A candidate must connect decay kinetics to imaging performance. Activity follows A = A0e−λt with λ = ln2/T½; mean life is 1/λ, not T½. In a gamma camera, a NaI(Tl) crystal converts gamma energy to light, photomultiplier tubes localize and amplify, and a pulse-height analyzer accepts counts within an energy window, typically ±10% around the 140-keV Tc-99m photopeak. Collimator choice sets the resolution–sensitivity trade-off: finer septa and longer holes improve spatial resolution but reduce sensitivity; high-energy collimators are needed for I-131. SPECT adds tomographic reconstruction, where attenuation, scatter, and motion degrade images.
Radiation safety
Use time, distance, and shielding. Dose rate falls with the inverse square of distance; shielding depends on photon energy and material. Know Gy (absorbed dose) versus Sv (equivalent/effective dose), and the ALARA framework. Traps include confusing half-life with mean life, ignoring decay between preparation and imaging, and assuming a low-energy collimator is acceptable for high-energy photons. Also watch for Mo-99 breakthrough, which increases patient dose and degrades image quality, and for dead-time losses at high count rates.
Worked example
A Tc-99m source reads 800 MBq at 08:00. What is it at 20:00? Twelve hours is two half-lives, so activity = 800 × (1/2)^2 = 200 MBq. Rule of thumb: for each half-life, halve the activity; for distance, double the distance and quarter the dose rate.