A Positron Emission Tomography (PET) scan is a medical imaging tool that provides a view into the body’s function, rather than just its structure. Unlike traditional imaging methods, such as X-rays or Computed Tomography (CT) scans, which capture physical anatomy, a PET scan maps out chemical activity and metabolic processes. By observing how the body uses certain compounds, this technology can detect disease at a molecular level, often before structural changes become visible. This ability makes the PET scan a powerful technique for locating and evaluating conditions, including cancer.
The Metabolic Basis of Detection
Cancer detection with a PET scan relies on the altered metabolism common to many tumor cells. These rapidly growing cells require far more energy than most healthy tissues to fuel their uncontrolled proliferation. This accelerated growth drives them to consume glucose, the body’s primary sugar source, at a much higher rate.
To exploit this difference, a patient receives an injection of a specialized radioactive compound called a radiotracer. For cancer imaging, the most common radiotracer is Fluorodeoxyglucose (FDG), a molecule that is chemically very similar to glucose. When injected into the bloodstream, the FDG travels throughout the body and is drawn into cells that are actively consuming glucose.
Cancer cells, with their heightened metabolic appetite, take up a proportionally larger amount of this FDG tracer compared to surrounding normal cells. Once the FDG enters the cell, it is partially metabolized, but unlike true glucose, it becomes chemically trapped inside. This process, known as metabolic trapping, causes the tracer to accumulate significantly within the tumor tissue.
The concentration of trapped FDG acts as a beacon, directly correlating with the metabolic activity of the tissue. Areas with high glucose consumption, which often indicates the presence of a tumor, will hold a much greater amount of the tracer. This differential uptake allows the PET scanner to distinguish cancerous masses from healthy tissue.
Creating the Image From Tracer Emissions
The process of converting the accumulated radiotracer into a visual image involves detecting the energy released as the tracer naturally decays. The \(\text{FDG}\) radiotracer contains a fluorine isotope that is unstable and undergoes radioactive decay by emitting a tiny particle called a positron. This decay happens within the patient’s body where the \(\text{FDG}\) has accumulated.
The emitted positron travels a short distance before it collides with a nearby electron. This collision results in a matter-antimatter reaction known as annihilation, which instantly converts the mass of both particles into pure energy. This energy is released as two high-energy photons, which are a form of gamma rays.
These two gamma rays are emitted simultaneously and travel in exactly opposite directions, 180 degrees apart. The PET scanner uses a ring of detectors surrounding the patient to register these simultaneous emission events, known as “coincidence detection.” By recording the precise time and location where both photons strike the detector ring, the system determines the line along which the annihilation occurred.
A computer algorithm uses data from thousands of these coincidence events to reconstruct a three-dimensional map. This map shows the concentration of the tracer throughout the body, with the areas of highest accumulation rendered as the most intense part of the final image. The resulting image represents the body’s metabolic function based on the distribution of the \(\text{FDG}\) tracer.
Understanding the Scan Results
The reconstructed PET image presents a functional map where areas of high metabolic activity appear brightly colored, often referred to as “hot spots.” In the context of cancer, these bright areas are where the \(\text{FDG}\) accumulated most heavily, indicating a high rate of glucose uptake consistent with tumor growth. The intensity of these spots is quantified by the Standardized Uptake Value, or \(\text{SUV}\).
The \(\text{SUV}\) is a ratio comparing the concentration of the tracer in a specific tissue to the amount injected, normalized for the patient’s body weight. This standardized measure allows physicians to compare metabolic activity between different lesions, track changes over time, and assess tumor aggressiveness. A higher \(\text{SUV}\) signifies greater metabolic activity, which often correlates with a more aggressive or fast-growing cancer.
A high \(\text{SUV}\) or a bright spot does not automatically confirm a cancer diagnosis. Other highly active biological processes, such as inflammation, infection, or recent surgical wounds, also require a large amount of glucose and show a high uptake of \(\text{FDG}\). For example, the brain and the heart naturally use large amounts of glucose and will always appear brightly on a PET scan.
Because of these potential false positives, the final interpretation of a PET scan requires careful correlation with the patient’s medical history and anatomical imaging, such as a co-registered \(\text{CT}\) or \(\text{MRI}\) scan. Certain slow-growing or low-grade cancers may not exhibit significantly increased glucose metabolism, meaning they might not show up clearly on the scan.