Why is the Preparation so important?
The radiotracer used in clinical Neurologic and Whole Body Oncologic PET is F-18 fluorodeoxyglucose, essentially a radioactive sugar. As the name implies, this is a glucose analogue and it is governed by the effects of insulin. If the patient is hyperglycemic or hyperinsulinemic at the time of the exam, tumor uptake may be reduced by competitive inhibition or insulin effect, respectively. Therefore, it is imperative that the patient be both relatively normoglycemic (under 200 mg/dL) and normoinsulinemic at the time of exam.
To achieve this, the patient should not have had any calories for at least 4 hours prior to the exam. In general, nothing to eat or drink except plain tap water is the recommended method. For those patients on IV fluids, no glucose, dextrose, or TPN should be administered for at least 4 hours. We will also check a finger stick blood sugar to make sure the patient does not have occult hyperglycemia. If the sugar is above 200 mg/dL, we will consider rescheduling the exam after the sugar is under control.
Diabetic patients can present a particular challenge. They also need to be NPO, have a sugar below 200 mg/dL, and have no active injected insulin in their system (they may stay on their oral agents unless they are receiving a contrast CT as well). For most diabetics, this means no injected insulin past midnight but patients on longer lasting agents may have to be off their insulin longer. In general it is best to choose a PET time slot in the day where the patient’s sugar is the lowest (again, off insulin). Sometimes insulin given with a small meal in the early morning followed by a PET exam in the afternoon works well. Close coordination with the patient’s endocrinologist is useful although some very brittle diabetics simply are not good candidates for PET imaging.
Although a PET scanner looks like a cross between a CT and MRI machine, it imposes none of the limitations of MRI. Just like a CT, patients with implanted metallic devices, including pacemakers, stents, and clips, can be safely imaged.
Immediately following a PET exam the patient can resume normal activities and diet.