A Positron Emission Tomography (PET) scan is a medical imaging procedure that provides a functional view of the body’s metabolism, unlike an X-ray or CT scan which shows anatomy. A PET scan highlights areas of high biological activity within tissues and organs. To achieve this metabolic mapping, the procedure requires the injection of a special radioactive substance called a radiotracer. Because this imaging method is highly sensitive to the body’s internal chemistry, strict adherence to a pre-scan fast is required for obtaining accurate results.
Understanding the Scan’s Energy Source
The vast majority of PET scans rely on fluorodeoxyglucose (FDG), a specific radiotracer. FDG is chemically engineered to be an analog of regular glucose, the primary energy source for nearly all cells. Once injected, cells recognize FDG as real glucose and actively transport it across their membranes.
The principle of the PET scan relies on metabolically active cells, such as cancer cells or high-energy organs like the brain, taking up glucose much faster than normal cells. After FDG enters the cell, phosphorylation traps the tracer inside. The trapped radioactive FDG emits signals that the PET scanner detects to create detailed images of metabolic activity. By mapping where this “fake” glucose accumulates, radiologists can pinpoint areas of abnormally high energy demand that may indicate disease.
Why Glucose Interferes with Image Quality
Fasting before a PET scan is required due to biological competition within the body. If a patient has recently eaten, the digestive process introduces a flood of real, non-radioactive glucose into the bloodstream. This surge of real glucose competes directly with the injected FDG tracer for uptake into the body’s cells.
Eating also causes the pancreas to release insulin, which lowers blood sugar. Insulin stimulates tissues like muscle and fat cells to aggressively pull glucose out of the blood. This diverts the FDG tracer away from intended target areas, such as a tumor, and causes it to be soaked up by surrounding normal tissues like skeletal muscles.
This misdirection impairs image quality, often making results blurry or inconsistent. Radiologists rely on a clear contrast between high uptake in a lesion and low background uptake in normal tissue. If background tissues are bright with diverted FDG, the area of interest becomes obscured, rendering the scan results inaccurate or unusable for diagnosis.
Detailed Instructions for Fasting
To ensure optimal image clarity, patients must fast for a minimum of four to six hours before the scheduled PET scan. The restriction on consumption is absolute, extending beyond solid food. Items like chewing gum, mints, hard candies, and even sugar-free drinks are prohibited because they can stimulate the digestive tract and trigger an insulin response.
The only permitted substance during the fasting window is plain, unflavored water, which aids hydration and tracer distribution. Patients must also avoid strenuous exercise for at least 24 hours prior to the scan, as muscle exertion increases FDG uptake. Patients with diabetes require special preparation, often involving consultation to adjust medication schedules and ensure safe blood glucose levels. Non-diabetic prescribed medications can usually be taken with plain water, but this must be confirmed with the imaging center.