Positron Emission Tomography (PET) is a medical imaging procedure that offers a functional view of the body’s internal processes. Unlike traditional imaging methods that capture anatomical structures, the PET scan measures metabolic activity within tissues and organs. This technology utilizes a small amount of a radioactive substance, called a radiotracer, to map the biochemical function of cells. The resulting images reveal areas where cells are most active, providing an early indication of disease by detecting changes in function before they cause structural changes visible on other scans.
The Science of the “Glow”
The visual signal, or “glow,” that appears on a PET scan represents a cell’s energy consumption. The most common radiotracer used is Fluorodeoxyglucose (FDG), a molecule chemically similar to glucose, the body’s primary sugar source. Cells with high metabolic rates rapidly absorb this FDG tracer from the bloodstream, just as they would regular glucose.
Once inside the cell, an enzyme phosphorylates the FDG, trapping the molecule because it cannot be further metabolized. The radioactive component, Fluorine-18, emits a positron. When this positron collides with a nearby electron, the particles annihilate each other, generating two gamma rays that travel in opposite directions.
The PET scanner detects these coincident gamma rays, pinpointing the location of the annihilation event. A computer uses this data to reconstruct a three-dimensional image. The brightest areas indicate the highest concentration of trapped FDG, confirming which tissues are consuming glucose at the fastest rate.
Tissues That Light Up Normally
Certain healthy organs naturally exhibit high metabolic activity and show significant tracer uptake, which is considered physiological. The brain is the most consistently active organ, consuming a large fraction of the body’s total glucose supply, resulting in a diffuse, intense signal. The myocardium, or heart muscle, also shows intense uptake because it requires substantial energy for continuous pumping; this uptake can be reduced if the patient is properly prepared by fasting.
The kidneys and bladder show high activity because unabsorbed FDG is filtered out of the blood and excreted through the urinary tract. This excretion causes the tracer to pool in the renal collecting system and the bladder. Skeletal muscles are another common source of physiological uptake, particularly if the patient moved or tensed muscles during the tracer uptake period. Even minor activities can cause localized, bright signals that require careful notation to avoid misinterpretation.
Pathological Conditions Causing Abnormal Uptake
Unexpectedly bright spots outside of normal physiological regions suggest a pathological condition driving increased metabolic demand. The most frequent cause is cancer, where many malignant cells exhibit the Warburg effect, relying heavily on glycolysis for energy production. This high rate of glucose metabolism causes tumor masses to appear as intensely bright, focal areas of FDG accumulation.
Infection and inflammation are the two other major categories causing abnormal high uptake, often creating a diagnostic challenge because they can appear as bright as a tumor. During these responses, the body recruits immune cells, such as neutrophils and macrophages, to the affected area. These activated immune cells significantly ramp up their glucose metabolism to fuel processes like phagocytosis and the production of signaling molecules.
Immune cells express high levels of glucose transporters, dramatically increasing FDG uptake to support their active state. Understanding the difference between cancer, infection, and inflammation is paramount, as an inflammatory lymph node might appear visually similar to a metastatic deposit. Careful correlation with the patient’s history and other imaging is necessary to distinguish the underlying cause.
Interpreting the Results
Interpreting PET scan images requires a quantitative assessment of metabolic activity, not just looking for bright spots. Radiologists use the Standardized Uptake Value (SUV) to quantify the intensity of tracer uptake in a specific region. The SUV is a ratio comparing the tracer concentration in the tissue to the injected dose distributed throughout the body, providing a numerical metric for metabolic rate.
A high SUV alone does not confirm cancer, given the overlap in uptake intensity between malignant and benign processes like infection. This creates a risk of false positives, where an inflammatory response is mistaken for a tumor. Conversely, some slow-growing tumors may not exhibit a high metabolic rate, leading to a false negative result. Therefore, the final interpretation integrates the SUV data and the visual pattern of uptake with anatomical information, typically provided by a concurrently performed CT scan.