PET/CT and Molecular Imaging
PET physics and quantification
PET detects coincidence 511-keV annihilation photons produced when a positron meets an electron. Time-of-flight (TOF) improves localization, and attenuation correction is essential for quantification. FDG uptake reflects glucose transport and hexokinase activity; it is not tumor-specific. Standardized uptake value (SUV) normalizes activity concentration to injected dose and body size, but it is affected by scanner calibration, reconstruction, uptake time, blood glucose, and lesion size. Partial-volume effects lower SUV in small lesions.
Patient preparation and traps
For FDG, fasting, glucose control, and uptake time matter. Elevated blood glucose or recent insulin can alter biodistribution; insulin can drive FDG into muscle and myocardium. Brown fat, muscle, bowel, urinary activity, and infection or inflammation can mimic malignancy. Attenuation-correction artifacts from oral contrast, metal, or misregistration can create false hotspots or cold spots. For PSMA and dotatate PET, know physiologic uptake in lacrimal glands, salivary glands, liver, spleen, kidneys, and bowel.
Worked example
A lesion has 10 kBq/mL on a scan performed 60 minutes after injection of 370 MBq, with body weight 70 kg. SUV = (activity concentration × weight)/(injected dose) = (10 × 70)/370 ≈ 1.9 when units are matched. Rule of thumb: SUV is a relative metric; compare lesions on the same scan and protocol, not across different protocols.