Nuclear Medicine Physics
Radioactivity and decay
Understand decay modes relevant to imaging-beta-minus, beta-plus, electron capture, isomeric transition-and how each produces gamma photons or annihilation radiation. Know the decay equation and the concept of physical half-life, biological half-life, and their combination into effective half-life. Be comfortable with activity units (becquerel, curie) and the relationship between activity and count rate.
Detectors and imaging systems
Know how a gamma camera works: collimator geometry (parallel-hole, pinhole, converging), scintillation crystal (NaI(Tl)), photomultiplier tubes, and pulse-height analysis to reject scatter. Understand spatial resolution versus sensitivity tradeoffs set by collimator hole size and length. For PET, know coincidence detection of 511 keV annihilation photons, the need for attenuation correction, and time-of-flight concepts. For SPECT, understand backprojection and filtered backprojection, plus iterative reconstruction.
Traps to watch
Questions often test that sensitivity and resolution move in opposite directions when collimator parameters change, or that effective half-life is always shorter than either component half-life. Watch for confusion between energy resolution (crystal and PMT) and spatial resolution (collimator-dominated). Also expect items on why scatter correction and attenuation correction are separate problems.
Worked example / rule of thumb
Effective half-life: 1/Teff = 1/Tphys + 1/Tbio. For Tphys = 6 h and Tbio = 3 h, 1/Teff = 1/6 + 1/3 = 0.5, so Teff = 2 h. Rule of thumb: effective half-life is less than the smaller of the two.
Study guidance
Practice predicting count-rate changes over several half-lives and reasoning about how collimator choice trades sensitivity for resolution.