A Positron Emission Tomography (PET) scan helps medical professionals visualize and measure changes in the body’s metabolic processes. Fluorodeoxyglucose, known as FDG, is a key component in most PET scans, enabling detailed insights. This article will explain the composition of FDG, its function during a PET scan, the diagnostic information it yields, and what a patient can expect during the procedure.
The FDG Tracer Explained
FDG, or Fluorodeoxyglucose, is a radioactive sugar analog, closely resembling glucose, the body’s primary energy source. One hydroxyl group in a glucose molecule is replaced by a radioactive fluorine-18 isotope. This structural similarity allows FDG to be absorbed by cells that actively consume glucose, such as cancer cells or highly active brain cells. Unlike regular glucose, once FDG enters a cell and is phosphorylated, it cannot be further metabolized and becomes trapped inside. The Fluorine-18 isotope in FDG is a positron emitter, crucial for PET imaging.
The FDG PET Scan Process
FDG is injected into the bloodstream. It travels throughout the body, accumulating in cells with high metabolic activity, similar to glucose. Since FDG cannot be fully broken down, it remains “trapped” within these cells, highlighting them.
The Fluorine-18 isotope within the trapped FDG then undergoes radioactive decay, emitting positrons. These positrons travel a short distance before encountering an electron, leading to an annihilation event. This produces two gamma rays that travel in nearly opposite directions. The PET scanner detects these coincident gamma rays, using their detection points and timing to reconstruct a three-dimensional image of the FDG distribution. Areas with higher concentrations of FDG appear as bright spots, indicating increased metabolic activity.
What an FDG-PET Scan Shows
Areas with high FDG uptake, called “hot spots,” indicate elevated metabolic activity. This functional information is useful for diagnosing and monitoring various conditions, complementing anatomical details from CT or MRI.
In oncology, FDG-PET scans identify cancerous tumors, which consume more glucose than healthy cells. The scan helps in staging cancer, monitoring the effectiveness of treatment, and detecting recurrence of the disease. Beyond cancer, FDG-PET scans assess brain activity in neurological conditions such as Alzheimer’s disease, where affected areas show reduced glucose metabolism, and in localizing seizure foci in epilepsy. The scans are also effective in locating areas of infection or inflammation, as immune cells also exhibit increased metabolic rates.
What to Expect During an FDG-PET Scan
Preparation for an FDG-PET scan typically involves fasting for a minimum of 4 to 6 hours before the appointment, consuming only plain water. Patients are generally advised to avoid strenuous physical activity for at least 24 hours prior to the scan to prevent FDG uptake in muscles, which could interfere with imaging other areas. It is also recommended to stay warm and avoid chewing gum.
During the procedure, a small amount of FDG is injected into a vein, usually in the arm. Following the injection, there is a resting period, typically lasting about 60 minutes, to allow the tracer to distribute throughout the body and be absorbed by target cells. The patient then lies still on a narrow table that slides into the PET scanner, with the actual scanning taking approximately 20 to 40 minutes. The total duration for the entire process, from arrival to completion, can range from two to four hours. The radiation exposure from the FDG is minimal and the tracer quickly leaves the body, primarily through urine.