Nuclear Medical Physics: Instrumentation, Radiopharmaceuticals, and Dosimetry
Core concepts to master
Candidates must be fluent in the physics of radionuclide production (reactor vs. cyclotron), decay schemes (isomeric transitions, beta-plus/beta-minus, electron capture, parent-daughter equilibria), and how photon energy and abundance drive imaging quality. Know the gamma camera chain: collimator geometry and resolution-sensitivity trade-off, NaI(Tl) scintillation, photomultiplier tubes, pulse-height analysis, and position logic. Understand SPECT reconstruction (filtered backprojection vs. iterative OSEM, attenuation and scatter correction) and PET fundamentals: annihilation, coincidence timing/energy windows, randoms, scatter fraction, NECR, and time-of-flight. Be solid on dosimetry: MIRD schema (S-values, cumulated activity, residence time), the difference between absorbed dose and effective dose, and typical administered activities.
Traps that appear in questions
- Confusing spatial resolution contributors: collimator resolution usually dominates system resolution at distance, while intrinsic resolution dominates close to the collimator face.
- Mixing up sensitivity vs. resolution - finer collimator = better resolution, worse sensitivity.
- Forgetting that Tc-99m comes from a Mo-99/Tc-99m generator and that transient equilibrium applies; Sr-82/Rb-82 is a generator too, but F-18 is cyclotron-produced.
- PET randoms scale with the square of activity while trues scale linearly; scatter and randoms are corrected, dead time is not fully recoverable.
- Confusing physical half-life with biological and effective half-life in dosimetry problems (1/Teff = 1/Tp + 1/Tb).
- Attributing bremsstrahlung to beta-minus emitters without noting it matters most for high-energy betas and for Sr-90/Y-90 therapy.
Worked example / rule of thumb
A radionuclide has physical half-life 6 h and biological half-life 12 h. Effective half-life = (6×12)/(6+12) = 72/18 = 4 h. Rule of thumb: for collimators, resolution improves (smaller FWHM) as hole diameter decreases and hole length increases; sensitivity moves the opposite way. For PET, always ask whether the question is about trues, randoms, or NECR before choosing an answer.
Exam strategy
Read each item for the quantity being asked (resolution, sensitivity, dose, or count rate) before doing arithmetic. Eliminate options that violate conservation or the resolution-sensitivity trade-off.