What is PET and how does it work?
PET uses specialized positron-emitting radiopharmaceuticals and equipment to detect areas of increased metabolic activity, a characteristic commonly seen in malignancies. The most commonly used radiopharmaceutical in PET imaging is 18 F-fluorodeoxyglucose ( 18 F-FDG), which is a marker for glucose metabolism. Unlike other commonly used radiopharmaceuticals in nuclear medicine, 18 F-FDG has a short half-life (110 minutes) and requires a cyclotron for production. Because of the complexity and cost of operating a cyclotron, the vast majority of nuclear medicine clinics do not have an on-site cyclotron and require the use of a separate PET radiopharmacy to provide the PET radiopharmaceutical. In addition, the short half-life of 18 F-FDG and the need to transport the 18 F-FDG from an outside facility to the imaging site can limit the overall accessibility of PET imaging.
Tumors demonstrate increased 18 F-FDG avidity because of increased expression of glucose transporters and hexokinase, which is responsible for phosphorylating normal glucose and radioactive 18 F-FDG. After 18 F-FDG is phosphorylated, it is not metabolized any further and becomes effectively trapped intracellularly.
Patient preparation for a study using 18 F-FDG entails fasting for 4 to 6 hours prior the examination to optimize the uptake of 18 F-FDG into malignant cells. At the time of the administration of 18 F-FDG, blood glucose levels less than 150 mg/dL are optimal, although images obtained in patients with blood glucose levels up to 200 mg/dL can still yield diagnostic results. Use of insulin can interfere with the biodistribution of 18 F-FDG, which can complicate the preparation of insulin-dependent diabetics.