A Positron Emission Tomography (PET) scan is a medical imaging technique that provides detailed pictures of organs and tissues at a molecular level. Unlike methods showing physical structure, a PET scan reveals function by measuring metabolic activity. This process uses a radioactive tracer, a substance injected into the patient that emits positrons. The most common tracer is Fluorodeoxyglucose (FDG), a modified form of glucose that tracks sugar consumption. By mapping where FDG accumulates, the scan identifies areas of disease or abnormality characterized by unusually high or low metabolic rates.
Understanding the Core Mechanism of FDG
The diagnostic power of FDG stems from its chemical resemblance to glucose, the primary energy source for nearly all cells. FDG is chemically known as 2-deoxy-2-[fluorine-18]-D-glucose, making it an analog of glucose where a radioactive fluorine atom replaces a hydroxyl group. This structural similarity allows FDG to be transported into cells via the same glucose transporter proteins (GLUTs) that move glucose across the cell membrane.
Once inside, the enzyme hexokinase attempts to phosphorylate the FDG molecule, just as it would with regular glucose. This phosphorylation step converts the FDG into FDG-6-phosphate. However, because the radioactive fluorine atom blocks the next step of glycolysis, the metabolic pathway is halted.
This results in “metabolic trapping,” where FDG-6-phosphate is held captive inside the cell, unable to be broken down further or easily exit. Tissues with high glucose consumption, such as rapidly growing tumors, often have increased glucose transporters and higher hexokinase activity. This elevated metabolic demand causes them to accumulate significantly more FDG than surrounding normal tissue, making them brightly visible. The resulting image maps glucose metabolism, highlighting areas of increased cellular activity.
Preparing for and Undergoing the PET Scan Procedure
Proper preparation ensures the FDG tracer is directed primarily to the area of clinical interest.
Patient Preparation
Patients must fast for at least four to six hours before the scan, consuming only plain water. Fasting lowers circulating blood glucose levels, which reduces competition between the FDG tracer and natural glucose for cell uptake. Patients should also avoid strenuous physical activity for up to 24 hours prior, as muscle movement can cause temporary FDG accumulation that may obscure findings. Before the injection, blood sugar levels are checked; if too high, the scan may be rescheduled to ensure accuracy.
Injection and Imaging
Once preparation is complete, a measured dose of the FDG tracer is injected intravenously. Following the injection, there is an uptake phase, typically lasting between 45 and 90 minutes. During this time, the patient rests quietly and remains still, allowing the FDG to distribute and accumulate in the target tissues. The patient then lies on a table that slides into the PET scanner, which detects the energy emitted by the decaying fluorine-18 atoms in the trapped FDG molecules to create the final image.
Interpreting FDG Uptake Results
The results of an FDG PET scan are visualized as a three-dimensional image where areas of tracer accumulation appear brighter, or “hotter,” indicating higher metabolic activity. The degree of FDG uptake is quantified by the Standardized Uptake Value (SUV). This metric is a ratio that compares the tracer concentration in a specific tissue region to the average concentration expected based on the injected dose and the patient’s body size. A higher SUV signifies greater metabolic activity in that area.
Physiological vs. Pathological Uptake
Interpretation requires differentiating between normal (physiological) and abnormal (pathological) uptake. Physiological uptake occurs in organs that naturally consume large amounts of glucose, such as the brain, heart, kidneys, and bladder. Low-level uptake is also seen in muscles and the gastrointestinal tract.
Pathological uptake is associated with disease processes that accelerate glucose metabolism. While high FDG uptake often signals malignant tumors due to their rapid growth, it is also seen in non-cancerous conditions. Active inflammation and infection involve immune cells that become highly metabolically active, leading to significant FDG accumulation that can mimic cancer. Therefore, interpreting a hot spot must be correlated with anatomical imaging and the patient’s full clinical history for an accurate diagnosis.
Clinical Applications of FDG PET Imaging
FDG PET imaging is widely used across several medical disciplines because it provides functional information that complements traditional anatomical imaging.
Oncology
Its most frequent application is in cancer care. It helps determine the stage of various cancers by identifying primary tumors and detecting metastatic spread. The scan also monitors treatment response, showing decreased FDG uptake if chemotherapy or radiation is successful.
Neurology
FDG PET assesses brain metabolism, aiding in the diagnosis of conditions like Alzheimer’s disease, which shows characteristic patterns of decreased glucose metabolism. It is also used to localize the seizure focus in epilepsy patients by detecting areas of reduced metabolism between seizures.
Cardiology
The scan assesses myocardial viability in patients who have experienced heart damage. FDG uptake in the heart muscle indicates the tissue is metabolically alive, or “viable,” even with reduced blood flow. This suggests the muscle may recover function if revascularized, guiding decisions about procedures like bypass surgery.