Positron Emission Tomography (PET)
Core concepts
PET tests physics, radiopharmaceuticals, and clinical indications. F-18 FDG dominates oncology, but know F-18 NaF for bone, Ga-68 DOTATATE for neuroendocrine tumors, and amyloid and perfusion agents for brain.
Understand annihilation: a positron meets an electron, producing two 511 keV photons emitted about 180 degrees apart. Coincidence detection defines true, scattered, and random events. Time-of-flight and iterative reconstruction improve image quality; know how randoms and scatter correction affect quantitation.
Patient preparation
For FDG oncology, fasting glucose control matters; elevated glucose competes with uptake. Quiet uptake period reduces muscle and brown fat activity. Know SUV as a semiquantitative index and why it depends on lean body mass, uptake time, and scanner calibration. Standardized uptake value is not a diagnosis.
Traps
Questions test physiologic FDG distribution: brain, myocardium (variable), kidneys and bladder, liver, and bowel. Distinguish physiologic from pathologic uptake. Brown fat, muscle, and infection/inflammation can mimic tumor. Understand that spatial resolution limits small-lesion detection and that partial-volume effects lower measured activity in small structures.
Rule of thumb
Two 511 keV photons back-to-back define a coincidence line; anything off that line is scatter or random and should be corrected. For SUV, always compare same-time, same-scanner, same-protocol studies.