Positron Emission Tomography (PET) scanning is a powerful form of functional medical imaging. Unlike structural methods such as X-rays or standard Computed Tomography (CT) scans, PET provides a detailed look at the body’s physiological processes. To capture this functional information, PET scans rely on radiotracers, which are compounds injected into the bloodstream that travel to target tissues to highlight areas of high biological activity. The most common and widely used radiotracer is Fluorodeoxyglucose (FDG). Understanding the composition and mechanism of FDG is key to grasping how this imaging tool provides unique insights into health and disease.
Defining Fluorodeoxyglucose
FDG is classified as a radiopharmaceutical, a chemical compound tagged with a small amount of radioactive material. Its full chemical name is 2-deoxy-2-\[ \({ }^{18}\)F]fluoro-D-glucose. The compound consists of a glucose molecule, which acts as a biological carrier mimicking natural sugar, and the positron-emitting radioisotope Fluorine-18 (\({ }^{18}\)F). This radioisotope is substituted onto the glucose structure in place of a hydroxyl group. Fluorine-18 is ideally suited for PET imaging due to its short half-life of approximately 110 minutes, which minimizes patient radiation exposure while allowing time for production and administration.
The Metabolic Mechanism of FDG
The utility of FDG stems from its ability to imitate natural glucose in the body’s initial metabolic steps. Once injected, FDG travels through the bloodstream and is selectively taken up by cells with a high demand for energy, such as tumor cells, brain cells, and activated immune cells. This uptake uses the same glucose transporter proteins (GLUTs) that cells use to import regular glucose.
Upon entering the cell, the enzyme hexokinase recognizes FDG and adds a phosphate group, converting it into FDG-6-phosphate. This phosphorylation is the final step in FDG’s metabolic journey, known as metabolic trapping. Because the Fluorine-18 atom replaced a specific oxygen atom, the next enzyme in the metabolic pathway cannot break the molecule down further. The resulting FDG-6-phosphate carries an ionic charge, preventing it from passing back out through the cell membrane, thus trapping it inside the cell.
The trapped Fluorine-18 then decays, emitting a positron (an anti-electron). This positron travels a short distance before colliding with a nearby electron, resulting in the annihilation of both particles and the simultaneous emission of two gamma rays traveling in opposite directions. The PET scanner detects these pairs of gamma rays, allowing a computer to precisely map the location of the trapped FDG. Since the concentration of trapped FDG is proportional to the cell’s glucose uptake rate, the resulting PET image maps the body’s relative metabolic activity.
Clinical Applications of FDG PET Scans
The metabolic image generated by the FDG PET scan provides unique diagnostic information across several medical specialties. The ability to visualize glucose metabolism is valuable for detecting and managing diseases that alter cellular activity. Although initially used in neurology and cardiology, oncology indications now account for the majority of PET scans performed worldwide.
Oncology
In oncology, FDG PET is primarily used to identify malignant tumors, which often display a significantly higher rate of glucose metabolism compared to healthy tissue. The scan aids in initial diagnosis, determines the extent of cancer spread (staging), and monitors treatment response by tracking changes in tumor metabolic activity. FDG PET imaging is often more sensitive than conventional imaging for certain cancers, leading to changes in treatment planning.
Neurology
FDG PET is heavily utilized in neurology to assess brain function by mapping regional glucose consumption. It can visualize decreased glucose metabolism in specific brain areas, characterizing conditions like Alzheimer’s disease and other dementias years before structural changes appear. For epilepsy patients, the scan can locate the specific brain region where seizure activity originates by identifying areas of reduced metabolism between seizures.
Cardiology
In cardiology, FDG PET assesses the viability of heart muscle tissue, especially after a heart attack. This application distinguishes between non-functional scar tissue and dormant, yet living, heart muscle that could potentially be revived through procedures like bypass surgery. By showing which areas of the heart are still metabolizing glucose, the scan helps determine the appropriate course of action for revascularization.
Patient Experience and Safety Considerations
The FDG PET scan procedure is non-invasive and generally takes two to three hours. The process begins with an intravenous injection of the FDG radiotracer, typically into an arm vein. Following the injection, a resting period of approximately 60 minutes allows the FDG to distribute throughout the body and be taken up by target cells.
Patients are instructed to fast for several hours before the exam to ensure low blood sugar levels, maximizing tracer uptake. Movement and talking are minimized during the uptake period to prevent the tracer from accumulating in muscles, which could obscure the regions of interest. The patient is then positioned on a table that slides into the PET scanner.
Safety is managed by the small amount of radioactive material used and the short half-life of Fluorine-18. The radiation exposure is modest and quickly diminishes after the scan. Patients are encouraged to drink plenty of fluids afterward to help flush the tracer from their system. As a standard precaution, patients may be advised to limit close contact with pregnant women and young children for a few hours after the procedure.