A Positron Emission Tomography (PET) scan is a sophisticated medical imaging tool that visualizes the body’s function at the molecular level. Unlike X-rays or CT scans, which show structure, the PET scan measures the metabolic activity of cells within tissues and organs. The phrase “physiologic activity” is fundamental to interpreting the scan results, as it describes the normal, expected biological processes occurring within a healthy body.
How PET Scans Measure Metabolism
PET scans rely on a radioactive tracer, most commonly fluorodeoxyglucose (FDG), to capture an image of metabolic function. FDG is a glucose analog, meaning it chemically mimics glucose, the primary fuel source for most cells. After the tracer is injected into the bloodstream, it circulates and is taken up by cells that are actively consuming glucose for energy.
The tracer is “trapped” inside the cells because its altered structure prevents it from being fully metabolized, causing it to accumulate. The PET scanner detects the energy emissions from the tracer, and a computer uses this information to create a three-dimensional image. Areas with high metabolic activity appear as brighter spots, or “hot spots,” on the final scan.
Defining Physiologic Activity
Physiologic activity, in the context of a PET scan, is the normal, anticipated uptake of the FDG tracer by healthy tissues and organs. This activity reflects the routine biological processes necessary for cellular maintenance and organ function. Recognizing this pattern establishes a baseline for what is expected in a healthy individual.
The presence of physiologic activity is not an indication of disease; rather, it confirms that a tissue is functioning normally. Radiologists must be familiar with these common patterns to avoid mistaking them for an abnormal finding that might suggest a health problem.
Common Sites of Normal Tracer Uptake
Several organs consistently show high physiologic activity because of their high energy requirements.
- The brain is a constant and intense consumer of glucose, resulting in very bright tracer uptake across the cerebral cortex and basal ganglia. This intense uptake is normal because the brain relies almost exclusively on glucose for its complex neuronal functions.
- The heart muscle exhibits variable but often significant uptake, reflecting its continuous work of pumping blood throughout the body.
- Muscles may show activity, particularly if the patient has moved, spoken, or tensed muscles before or during the scan. Muscles in the head and neck, such as the laryngeal muscles, may show uptake if a patient was talking or tense.
- The kidneys filter the circulating FDG out of the bloodstream, and the tracer subsequently collects in the urinary tract. The ureters and the bladder routinely show intense tracer accumulation as the radioactive material is prepared for excretion.
- The liver and spleen show moderate, homogeneous uptake, which is a normal result of their roles in glucose metabolism and blood cell processing.
Distinguishing Normal from Pathologic Uptake
The ability to differentiate between physiologic and pathologic (abnormal) uptake is the central challenge of interpreting a PET scan. Pathologic uptake is often a sign of disease, such as cancer, infection, or inflammation. These conditions involve cells that are rapidly dividing or highly active and therefore consume excessive amounts of glucose.
Clinicians evaluate three main factors to make this distinction: location, pattern, and intensity. Location is a primary indicator, as activity in an area where it is not normally expected, such as a specific part of the lung, immediately raises suspicion.
The pattern of uptake is also considered; physiologic activity is typically diffuse and symmetrical, while pathologic activity is often focal, intense, and asymmetrical. For example, normal tonsillar tissue will show symmetrical uptake, but a tumor would likely be asymmetrical.
The intensity of the tracer accumulation is quantified using the Standardized Uptake Value (SUV). This provides a numerical measure of how concentrated the FDG is in a specific tissue region. A higher SUV number indicates a greater degree of metabolic activity in that area compared to the surrounding tissue. While there is no single cutoff, an unexpectedly high SUV in a focal, abnormal location is a strong indicator of a hypermetabolic process that may be malignancy, infection, or inflammation.