Fluorodeoxyglucose (FDG) is a molecule structurally similar to glucose, the body’s main energy source. It is tagged with a radioactive isotope, making it a radioactive sugar tracer. A PET scan (Positron Emission Tomography) uses this tracer to create visual maps of metabolic activity. By tracking FDG distribution, the PET scanner highlights areas where cells consume glucose at an unusually high rate. This functional imaging provides insight into how tissues and organs operate on a cellular level, complementing anatomical scans like CT or MRI.
How FDG Works
FDG imaging relies on the fact that rapidly growing cells exhibit altered metabolism. Normal cells use oxidative phosphorylation, but highly active cells often favor a less efficient process called aerobic glycolysis, even when oxygen is plentiful. This metabolic shift causes these cells to significantly increase their glucose uptake. To accommodate this high demand, these cells often overexpress glucose transporter proteins (GLUT) on their surface, pulling in more sugar from the bloodstream.
Once the FDG tracer enters the cell via these transporters, the enzyme hexokinase adds a phosphate group, converting the molecule into FDG-6-phosphate. Unlike natural glucose-6-phosphate, the altered structure of FDG prevents it from being processed further in the glycolytic pathway. This results in “metabolic trapping,” locking the radioactive FDG-6-phosphate inside the cell, where it emits positrons. The PET scanner detects the resulting gamma rays, which appear as bright spots, mapping the locations of intense metabolic activity.
The Patient Experience
Preparation for the FDG-PET scan requires the patient to fast for several hours beforehand. This ensures blood sugar levels are low, as high glucose levels compete with the FDG tracer and reduce image quality. Upon arrival, a blood sugar test is performed, and an intravenous line is placed for the FDG tracer injection.
Following the injection, patients rest quietly for about 60 minutes in a dedicated uptake room. This resting phase allows the FDG time to distribute and accumulate in metabolically active tissues. Patients must remain still and avoid talking or reading to prevent unnecessary tracer uptake in the muscles, which can create false activity. The actual scan takes place with the patient lying on a table and typically lasts between 30 and 45 minutes.
Interpreting High Uptake
High FDG uptake signifies a region of increased glucose consumption, often a feature of aggressive cellular growth. This hypermetabolic activity is associated with malignant tumors, as cancer cells divide rapidly and demand high energy. The intensity of this uptake is quantified using the Standardized Uptake Value (SUV). The SUV provides a numerical ratio comparing the tracer concentration in a specific tissue to the average concentration throughout the body.
The clinical utility of high uptake extends beyond initial diagnosis. Physicians use the intensity and location of FDG accumulation for staging, determining the extent of a known disease. Subsequent scans monitor treatment effectiveness; a decrease in SUV values suggests the therapy is successfully reducing metabolic activity. Since metabolic changes often precede anatomical changes, the PET scan can provide an earlier indication of treatment response than other imaging modalities.
Uptake from Non-Cancerous Sources
High FDG uptake indicates metabolic activity, but it is not a definitive diagnosis of malignancy, as the tracer is not cancer-specific. Many non-cancerous conditions involve highly metabolically active cells that accumulate the FDG tracer. The most common benign causes of increased uptake are inflammation or infection, such as arthritis, recent surgical sites, or lung inflammation from radiation therapy.
Immune cells, including activated macrophages and lymphocytes, significantly increase their glucose metabolism when fighting infection or responding to injury, leading to bright spots. Additionally, some normal organs naturally show physiological FDG uptake because they are major glucose consumers.
Physiological Uptake Sources
The brain
The heart
The kidneys
The urinary bladder
Brown fat, especially when the patient is cold, which can obscure other findings.
Interpreting high uptake requires correlating the PET image with the patient’s full clinical history and anatomical imaging, such as a fused PET/CT scan, to accurately distinguish between malignant and non-malignant activity.