The term “physiologic FDG uptake” refers to the expected, normal absorption of the radioactive tracer, Fluorodeoxyglucose (FDG), within healthy tissues during a Positron Emission Tomography (PET) scan. FDG is a compound that mimics glucose, the body’s primary sugar. Since a PET scan measures metabolic activity, physiologic uptake simply reflects the normal energy demands of various organs. This uptake is not a sign of disease; understanding these normal patterns is important for medical professionals to accurately interpret the scan.
The Basics of FDG and Metabolic Activity
FDG is a glucose analog, a modified sugar molecule labeled with the radioactive isotope Fluorine-18. When injected, FDG enters cells using the same transporters as natural glucose. Inside the cell, an enzyme adds a phosphate group to both glucose and FDG. While normal glucose proceeds through the metabolic pathway for energy, the altered structure of FDG prevents the next step.
This process, known as metabolic trapping, causes the FDG-6-phosphate to accumulate inside the cell, trapping the radioactive signal. The PET scanner detects this radiation, creating a three-dimensional image where brighter areas indicate higher tracer concentrations. Since FDG accumulation is proportional to a cell’s rate of glucose use, the image maps the body’s metabolic activity.
Common Sites of Physiologic FDG Uptake
Several organs consistently show high levels of physiologic FDG uptake because they naturally have high energy demands. The brain demonstrates the most intense uptake, as it relies almost exclusively on glucose for fuel and accounts for approximately 20% of the body’s total glucose metabolism in a fasting state. This activity is typically uniform and symmetrical throughout the gray matter, including the cerebral cortex and basal ganglia.
The myocardium, or heart muscle, also shows variable uptake, as it can switch its primary fuel source between fatty acids and glucose. In a properly fasted patient, myocardial uptake is often suppressed or minimal, but it can become intense if the heart is utilizing glucose for energy. The liver demonstrates homogeneous, moderate uptake due to its ongoing role in glucose metabolism. The spleen typically shows a lower level of uptake compared to the liver, reflecting its general metabolic background.
Because the body eliminates the unused FDG tracer through the urinary system, the kidneys and bladder routinely show intense activity. The FDG is filtered and not reabsorbed, leading to visible tracer concentration in the renal collecting system and urine. Skeletal muscles normally show low-level uptake, but this becomes intensely visible if the muscles were recently active. Lymphatic tissue, such as the tonsils and areas in the gastrointestinal tract, may also show mild to moderate normal uptake due to active immune cells.
Variables That Affect Normal FDG Distribution
The degree of normal FDG uptake is not static and can be altered by a patient’s physical state or preparation before the scan. High blood sugar (hyperglycemia) directly reduces FDG uptake because excess circulating glucose competes with the tracer for cellular transporters. This competition can lead to a lower quality scan where the tracer is less concentrated in both normal and abnormal tissues.
Muscle activity immediately before or during the tracer uptake period causes intense, temporary physiologic signals. Simple actions like talking, chewing gum, or shivering can result in bright uptake in the muscles of mastication, the larynx, or neck muscles. Exposure to cold temperatures can also activate brown adipose tissue (brown fat), which generates heat through metabolic activity. This results in symmetrical, intense tracer accumulation in the neck, supraclavicular regions, and along the spine.
A patient’s insulin status and medications also play a role in tracer distribution. The medication metformin, often prescribed for type 2 diabetes, is known to cause intensely increased FDG uptake in the walls of the large intestine. Similarly, high levels of insulin can shift metabolism toward glucose utilization in muscle and fat cells, which can unexpectedly increase uptake in those tissues. Awareness of these variables is important for imaging specialists to avoid misinterpreting a patient’s pre-scan state as a sign of disease.
Interpreting the Scan: Physiologic vs. Pathologic Uptake
Medical professionals differentiate between physiologic and pathologic uptake by comparing the signal to known patterns and anatomical landmarks. Physiologic uptake is characterized by its predictable location and often symmetrical pattern, such as uniform activity in the brain or bilateral uptake in brown fat. Pathologic uptake, which may indicate malignancy or inflammation, is typically focal, intense, and appears where normal uptake is expected to be low or absent.
A quantitative tool called the Standardized Uptake Value (SUV) measures the intensity of tracer accumulation. While a high SUV can be concerning, it is not specific to cancer, as both malignant and inflammatory processes show intense FDG accumulation due to high metabolic rates. Interpretation relies on correlating the metabolic information from the PET scan with structural details provided by a concurrent CT scan.