What instructions do patients need to prepare for FDG PET imaging?
The most important issue regarding an FDG PET scan is that FDG competes with nonradioactive glucose. If a patient has recently eaten or has diabetes mellitus with a serum glucose level >150 mg/dl, the sensitivity of the scan to detect malignancy may be diminished. Typically, serum glucose levels greater than 200 mg/dl should exclude a patient from having an FDG PET scan. Nondiabetic patients should have nothing by mouth overnight for a scan scheduled in the morning or else for at least 4 to 6 hours for a scan scheduled later in the day. Diabetic patients should be managed carefully because some have difficulty forgoing eating. Also, insulin drives glucose (and FDG) into the muscles, so care should be taken regarding the administration of insulin too close to the scan time. It is preferred that the patient not inject insulin, eat, or exercise before the PET scan, but if this is impossible, the referring physician must balance glucose levels, eating, and insulin in the diabetic patient. The scan itself is typically acquired starting 1 hour after intravenous FDG administration from the skull base to the proximal thighs and takes about 30 minutes to perform. Whole body scanning from skull vertex to toes, which takes about 45 minutes to acquire, is performed in some patients with malignancies that more commonly involve the head, skin, or extremities such as melanoma and cutaneous lymphoma.