Positron Emission Tomography (PET) scans are an advanced medical imaging technique that offers insights into the body’s functions. Unlike imaging methods that primarily show anatomical structures, PET scans visualize metabolic activity at a molecular level. A radiopharmaceutical called FDG is central to enabling these functional images by highlighting areas of increased cellular activity.
What is FDG?
FDG stands for Fluorodeoxyglucose, a radiopharmaceutical used in PET scans. Chemically, FDG is a glucose analog, closely resembling glucose, the body’s primary sugar and energy source. This similarity allows cells to take up FDG as if it were regular glucose.
FDG contains a radioactive isotope, Fluorine-18 (¹⁸F). This component makes FDG a “radiotracer,” enabling its detection by the PET scanner. While cells absorb FDG, they cannot fully metabolize it due to the Fluorine-18 substitution. This allows it to accumulate within cells that have high glucose metabolism.
How FDG Lights Up the Scan
FDG highlights areas in a PET scan through cellular metabolism. Cells with high metabolic rates, like cancer cells or active brain regions, actively take up glucose for energy. Because FDG mimics glucose, these metabolically active cells also absorb FDG.
Once inside the cell, FDG cannot proceed through the entire metabolic pathway of glucose. It gets trapped within the cell. The Fluorine-18 isotope in FDG then undergoes radioactive decay, emitting positrons.
When a positron interacts with an electron, they annihilate each other, producing two gamma rays that travel in opposite directions. The PET scanner detects these gamma rays. A computer uses this information to create 3D images showing where FDG has accumulated, effectively “lighting up” areas of high metabolic activity.
What FDG PET Scans Can Show
FDG PET scans provide functional information by revealing metabolic activity, complementing anatomical details from other imaging techniques like CT or MRI. By showing where glucose is used most actively, these scans help identify and monitor medical conditions. They are widely used in oncology to detect cancer, determine its spread (staging), and monitor treatment effectiveness.
Beyond cancer, FDG PET scans are used in neurology for evaluating brain disorders. They help diagnose conditions like Alzheimer’s disease by showing reduced glucose metabolism in specific brain regions, and localize seizure foci in epilepsy. In cardiology, these scans assess heart conditions, such as determining heart muscle viability after a heart attack. This functional imaging capability makes FDG PET useful in disease management.
Patient Considerations for an FDG PET Scan
Undergoing an FDG PET scan involves preparations. Patients fast for at least 6 hours before the appointment, consuming only plain water. They should avoid strenuous physical activity for at least 24 hours prior to the scan to prevent unwanted FDG uptake in muscles.
Upon arrival, the FDG radiotracer is administered intravenously. After injection, there is a waiting period, typically between 60 to 90 minutes, allowing the FDG to circulate and be absorbed by cells. During this uptake phase, patients rest quietly. The actual scan takes 20 to 45 minutes, during which the patient lies still. The radiation dose from FDG is minimal and quickly leaves the body, often aided by drinking plenty of fluids after the scan.