A Positron Emission Tomography (PET) scan uses a radioactive tracer called fluorodeoxyglucose (FDG) to create images of the body’s metabolic activity. Physiologic FDG uptake refers to the normal and expected absorption of this radiotracer in various organs and tissues. Understanding these normal patterns is important for accurate interpretation, as not all FDG uptake indicates disease; some reflects healthy bodily functions, allowing differentiation between normal metabolic processes and potential abnormalities.
How FDG Works in the Body
FDG is a radioactive sugar analog that mimics glucose, the body’s primary energy source. After injection into a vein, FDG travels through the bloodstream and is absorbed by cells that use glucose for energy. Cells absorb FDG via glucose transporters on their membranes, entering the glycolytic pathway similar to regular glucose.
Unlike normal glucose, FDG cannot be fully metabolized and becomes trapped inside the cells. The amount of FDG taken up by cells directly reflects their metabolic activity; areas with higher glucose consumption will accumulate more FDG. The radioactive fluorine atoms in FDG decay and emit positrons, which collide with electrons to produce gamma rays. A PET scanner detects these gamma rays, creating three-dimensional images that show where the FDG has accumulated in the body.
Common Areas of Normal FDG Uptake
Several organs and tissues naturally exhibit FDG uptake due to their high metabolic demands or specific functions. The brain, for instance, shows intense FDG uptake, particularly in the cerebral cortex, basal ganglia, and thalami, because glucose is its main energy source for neural activity.
The heart muscle also shows variable uptake, as it can utilize glucose for energy, especially if the patient is not in a completely fasted state or is experiencing stress. Skeletal muscles can also show FDG accumulation, particularly after recent physical activity or if the patient is not fully relaxed during the scan.
The kidneys and bladder show significant FDG uptake because the radiotracer is excreted from the body via the renal system. The liver demonstrates diffuse or heterogeneous uptake due to its role in glucose metabolism. The spleen shows uptake that is less intense or similar to that of the liver.
The gastrointestinal tract exhibits variable and diffuse uptake throughout the bowel, including the esophagus, stomach, and small and large intestines. Bone marrow can also show diffuse uptake, especially in active marrow. Lymphoid tissues, such as the tonsils, adenoids, and thymus (particularly in children and young adults), may show mild to moderate uptake due to their activity.
Salivary glands and the thyroid gland can also demonstrate variable FDG uptake. Brown adipose tissue, a type of fat that generates heat, can show intense uptake in symmetrical sites like the neck, supraclavicular regions, and axillae, especially when activated by cold temperatures.
Factors Affecting Normal Uptake Patterns
Several patient and procedural factors can influence the appearance of normal FDG uptake. Dietary status plays a role; for example, not adhering to fasting can increase FDG uptake in muscles and the heart due to higher circulating glucose. Recent physical activity can also lead to increased FDG uptake in skeletal muscles.
Environmental temperature can affect brown fat uptake; colder conditions activate this tissue, leading to more intense FDG accumulation in areas like the neck and supraclavicular regions. Certain medications, such as insulin or steroids, can also impact glucose metabolism and FDG distribution. High blood glucose levels can compete with FDG for cellular uptake, potentially reducing tracer accumulation.
Benign processes like inflammation or infection can cause increased FDG uptake in specific areas, which might be mistaken for pathological findings. This includes post-surgical changes or dental inflammation, where heightened immune activity leads to increased metabolic demand. Factors like age, sex, and bowel conditions, such as constipation, can also influence intestinal FDG uptake.
Clinical Significance of Physiologic Uptake
Understanding physiologic FDG uptake is important for healthcare professionals interpreting PET scans, particularly in oncology. Recognizing these normal patterns prevents misdiagnosis and unnecessary investigations or treatments. PET/CT imaging, which combines metabolic information from PET with anatomical details from CT, helps attribute FDG activity to normal organs.
Radiologists and nuclear medicine physicians differentiate normal physiologic uptake from abnormal disease uptake by considering several aspects. They integrate the patient’s history, conduct a physical examination, and correlate PET findings with other imaging modalities like CT or MRI. For example, diffuse uptake in certain organs is more likely to be physiological, while focal uptake often warrants closer scrutiny for potential malignancy. This approach ensures accurate interpretation, guiding patient care and reducing false positives.