Nuclear Medicine: Radiopharmaceuticals, Imaging, and Radiation Safety
Core concepts
Nuclear medicine questions combine radiopharmacy, detector physics, and safety. Know that a radiopharmaceutical pairs a radionuclide with a biodistribution agent. Technetium-99m dominates because of its 6-hour half-life, 140 keV gamma emission, and generator availability. Understand gamma camera components: collimator (defines resolution and sensitivity), scintillation crystal, photomultiplier tubes, and pulse-height analysis. Distinguish planar imaging, SPECT, and PET, and know that PET relies on annihilation photons detected in coincidence. Study time, distance, and shielding for dose reduction, plus decay correction, half-life calculations, and radiopharmaceutical quality control.
Traps to expect
Watch for confusion between physical half-life and biological clearance, and between sensitivity and resolution in collimator choice. A high-sensitivity collimator improves count rate but degrades resolution. Questions may misstate PET as using single-photon detection or confuse SPECT tracers with PET tracers. Also separate patient dose from occupational dose and remember that shielding must match photon energy.
Worked example
A source has a 6-hour half-life and an initial activity of 800 MBq. After 18 hours, three half-lives have passed, so activity is 800 divided by 2 cubed, or 100 MBq. If a protocol needs at least 200 MBq, the dose must be prepared no later than 12 hours after calibration. Apply the same decay logic to generator eluate and waste hold times.