The Positron Emission Tomography (PET) scan is a functional imaging test that provides a detailed look at the body’s metabolic processes. This technology uses a small amount of a radioactive tracer, most commonly fluorodeoxyglucose (FDG), a sugar analog, to highlight areas of high biological activity. Once injected, the FDG tracer travels through the bloodstream and accumulates in cells actively consuming glucose for energy. The resulting scan shows “uptake,” or where the tracer has gathered, providing a map of the body’s metabolic activity.
The Meaning of Physiological Uptake
Physiological uptake refers to the expected accumulation of the FDG tracer in healthy organs and tissues. Since FDG acts like glucose, any part of the body that naturally uses glucose as its primary fuel source will absorb the tracer and “light up” on the PET scan. This uptake measures the inherent metabolic rate necessary for regular cellular function, confirming that the tissue is active and healthy.
This normal absorption is not an indication of disease or abnormality; it simply reflects the baseline energy needs of various organs. Recognizing these expected patterns is foundational to reading a PET scan, as it establishes the normal background against which potential disease can be identified.
Common Sites of Normal Metabolic Activity
Several organs consistently exhibit high physiological uptake due to their constant energy demands. The brain, for instance, is the body’s most glucose-avid organ and typically shows the most intense tracer accumulation. This intense uptake is symmetrical and diffuse across the cerebral cortex and basal ganglia, reflecting its continuous need for glucose as a main energy substrate.
The urinary system also displays significant uptake as it processes and clears the tracer from the body. FDG is filtered by the kidneys and is not reabsorbed, leading to visible activity in the kidneys, renal pelvis, and bladder before excretion. The liver generally shows a uniform, low-to-moderate level of uptake.
Other common sites of normal activity include:
- Lymphoid tissues, such as the palatine and lingual tonsils in the neck, which show low to moderate FDG uptake related to immune function.
- The heart muscle, or myocardium, which has variable but often high uptake because it is a constantly working muscle.
- The stomach and parts of the bowel, particularly the cecum and ascending colon, which can exhibit mild-to-moderate uptake due to muscle activity and the presence of bacteria.
Interpreting the Difference: Normal vs. Pathological Uptake
The primary challenge in PET scan interpretation is distinguishing normal physiological uptake from pathological uptake, which indicates disease like cancer, infection, or inflammation. Pathological uptake is characterized by a significantly higher intensity of FDG accumulation than is typical for that location, reflecting the increased glucose consumption of diseased cells. This intensity is quantified using the Standardized Uptake Value (SUV), though a high SUV alone does not confirm malignancy.
A key differentiator is the pattern and location of the uptake. Physiological uptake is usually diffuse, follows known anatomical borders, and is symmetrical in paired organs. Conversely, pathological uptake is frequently focal, appearing as a distinct, bright spot in an unexpected location, often correlating with an abnormal structure seen on the accompanying CT scan.
Radiologists consider the context of the patient’s symptoms and other imaging results to make a distinction. Increased uptake in a healing surgical site or an area of known infection is considered a benign process. Therefore, the final interpretation integrates the metabolic data from the PET scan with the anatomical information from the CT scan and the patient’s clinical history.
How Preparation Affects Normal Readings
Patient preparation is a controlled process designed to minimize variations in physiological uptake that could complicate interpretation. Factors like recent muscle use can lead to high FDG uptake in skeletal muscles, which can mimic disease. Patients are instructed to rest quietly before and during the tracer uptake period to prevent these false-positive readings.
Exposure to cold temperatures can dramatically influence normal readings by activating brown adipose tissue, or brown fat. Brown fat is highly metabolic and absorbs the FDG tracer, creating widespread, symmetrical patches of bright uptake, usually in the neck, chest, and spine areas. Keeping the patient warm before and during the scan helps to suppress this physiological activity.
Dietary status is another factor, particularly concerning the heart and liver. Fasting for several hours before the scan is standard practice to lower blood glucose and suppress normal myocardial FDG uptake. This suppression ensures that any remaining heart uptake is not merely normal function, but potentially an area of inflammation or pathological process.