Positron Emission Tomography (PET) scans are a powerful diagnostic imaging tool. They provide functional insights into the body’s tissues and organs, complementing anatomical imaging methods. A key component often used is fluorodeoxyglucose (FDG). This article explores what FDG is and its significant role in PET imaging.
What is FDG?
FDG, or Fluorodeoxyglucose F 18, is a radioactive tracer. It is a molecule that closely resembles regular glucose, the body’s primary sugar source. Cells readily take up glucose for energy, and FDG mimics this natural process, allowing its absorption by cells with high metabolic activity.
The “F-18” refers to Fluorine-18, a radioactive isotope that emits positrons. Fluorine-18 has a short half-life of about 109.8 minutes, decaying quickly and limiting patient exposure. This radioactive labeling allows medical professionals to track the FDG’s distribution within the body.
How FDG Works in a PET Scan
An FDG-PET scan typically begins with an intravenous (IV) injection of FDG. After injection, a waiting period of 30 to 60 minutes allows the FDG to circulate and be absorbed by the body’s cells. During this uptake phase, patients are asked to rest quietly to ensure optimal distribution of the tracer.
Cells with high metabolic rates, such as active cancer cells, brain cells, and heart muscle cells, readily take up FDG just as they would normal glucose. This uptake occurs via glucose transporter proteins (GLUTs). After entering the cell, FDG is phosphorylated by an enzyme called hexokinase, forming FDG-6-phosphate. Unlike regular glucose, FDG cannot be further metabolized, effectively “trapping” it inside the cell.
The trapped Fluorine-18 in the FDG undergoes radioactive decay, emitting a positron. This positron travels a short distance before encountering an electron, leading to annihilation. This event produces two gamma rays, each with an energy of 511 keV, that travel in opposite directions. The PET scanner detects these simultaneous gamma rays. A computer reconstructs a detailed three-dimensional image from their arrival times and locations, showing where the FDG has accumulated and indicating areas of increased metabolic activity within the body.
Clinical Applications of FDG-PET
FDG-PET scans are used across medical specialties to visualize metabolic processes. A primary application is in oncology, where FDG-PET helps in the detection, staging, and monitoring of many cancers. Cancer cells often exhibit higher metabolic rates and increased glucose uptake compared to normal cells, leading to greater FDG accumulation, which appears as “hot spots” on the scan. This allows clinicians to identify malignant tumors, assess disease extent, detect recurrence, and evaluate treatment effectiveness.
In neurology, FDG-PET diagnoses conditions affecting brain metabolism. It helps diagnose Alzheimer’s disease by showing reduced glucose metabolism patterns in specific brain regions. The scan also assists in locating seizure foci in patients with epilepsy, as these areas often show altered metabolic activity. FDG-PET offers insights into neuronal activity that can precede structural changes.
FDG-PET is valuable in cardiology for assessing myocardial viability. It identifies heart muscle tissue that is dysfunctional but viable and potentially salvageable through interventions like revascularization. This is achieved by observing the heart’s glucose metabolism, which increases in ischemic (low blood flow) conditions, indicating viable tissue.
FDG-PET also identifies inflammation or infection. Immune cells involved in inflammatory and infectious processes have elevated metabolic demands, similar to cancer cells, leading to increased FDG uptake. This makes FDG-PET useful for diagnosing conditions such as fever of unknown origin, osteomyelitis, and certain autoimmune diseases.