What is the principle of FDG-PET imaging and what does increased uptake signify?

• A. FDG is a lipid tracer; higher uptake indicates better perfusion.
• B. FDG is a glucose analogue; tissues with increased metabolic activity accumulate more FDG, often indicating malignancy or infection; uptake is quantified by SUV.
• C. FDG is a radioactive dye; uptake is qualitative.
• D. FDG-PET uses gamma photons directly; SUV is not used.

Answer: (B)

FDG-PET imaging works by using a glucose analogue that is taken up by cells in proportion to their metabolic activity and then trapped inside the cell. FDG behaves like glucose, entering cells through glucose transporters and being phosphorylated by hexokinase to FDG-6-phosphate. Because it cannot be further metabolized, it becomes trapped, so tissues with higher metabolic rate accumulate more FDG and appear brighter on the scan. The amount of uptake is quantified with the standardized uptake value (SUV), which normalizes the tissue radioactivity to the injected dose and the patient’s size/time since injection, allowing comparison across regions or scans.

Increased uptake typically signals higher metabolic activity, which is common in malignant tumors and in sites of infection or inflammation. However, it is not cancer-specific; physiologic uptake in brain, heart, or active muscles, as well as inflammatory processes, can also show elevated FDG. Clinicians interpret the SUV in the context of anatomy (often with CT or MRI) and clinical information to distinguish malignancy from other causes of increased uptake.