“Why do you have to fast for a PET scan?”

SUGAR LEVEL

The radioactive radiotracer used is attached to a glucose molecule which is a sugar seen in food. Fasting prevents uptake within the stomach or bowel which could make the radiologist think there is a concerning tumor or it can result in decrease of the tracer that any other tumor may pick up which could lead to a false negative exam.

Dr Cox

Basically, you want to lower your blood sugar level so that the pet CT agent will not compete with glucose…

The radioisotope that is used in PET is a glucose transporter. This PET radioisotope which also contains glucose will go to any metabolically active areas in the body which sucks up the glucose. However, if the glucose levels are elevated from food or drink taken before the exam, then the level of insulin will increase. When insulin levels increase, our muscles will take up the glucose or sugar instead of the cancer hungry cells that seek the glucose in the PET radiotracer. So basically the PET radioactive tracer may not go to the areas of interest which results in a suboptimal exam.

The reason of a 15 year old who needs a PET scan is most likely to detect tumors or to localize culprit site of seizures. In both scenarios, fasting for at least 6 hours before the intended PET scan is to reduce competitions from ordinary blood sugar (glucose molecules) with PET scanning agent Fluorine-18 Fluorodeoxyglucose (FDG) that is an analogue of glucose, meaning that fasting before PET scan with FDG can greatly facilitate the uptake of FDG by the targeted cells/tissues, thus to better visualize the lesions your doctors are looking for.