Medical imaging offers insights into the body’s internal state, aiding diagnosis and monitoring. Different technologies provide unique views, from detailed anatomical structures to the functional activity of tissues. The imaging system combining tomography with radionuclide tracers to visualize these functional processes is Positron Emission Tomography (PET).
Positron Emission Tomography (PET)
PET is a nuclear medicine imaging technique that provides a three-dimensional map of functional processes within the body. It differs from structural imaging methods like X-rays or CT scans by revealing how organs and tissues are working, rather than just their physical appearance. This capability stems from its reliance on radioactive tracers and the principle of tomography. By measuring body functions such as blood flow, oxygen use, and glucose metabolism, PET scans help medical professionals assess organ and tissue function.
The Science Behind PET Scans
The process begins with a radionuclide tracer introduced into the body through intravenous injection. These tracers are radioactive isotopes attached to biologically active molecules, such as fluorodeoxyglucose (FDG), a glucose analog often used for its uptake by cells with high metabolic rates, like cancer cells. Once inside the body, the tracer distributes to target tissues, and as the radioisotope decays, it emits a positron, a positively charged subatomic particle.
This emitted positron travels a short distance before encountering an electron within the body’s tissues. When a positron and an electron collide, they undergo a process called annihilation, converting their mass into energy. This annihilation event results in the emission of two gamma rays that travel in precisely opposite directions.
A PET scanner, which consists of a ring of detectors surrounding the patient, identifies these gamma rays. The scanner registers pairs of gamma rays that arrive at the detectors simultaneously and in opposite directions. A computer then processes this collected data, using algorithms to reconstruct detailed 3D images of the tracer’s distribution and concentration. The brightness of areas on the image corresponds to the amount of tracer accumulated, indicating the level of metabolic activity or blood flow in those tissues.
Where PET Scans Are Used
PET scans are used across various medical specialties for insights into metabolic and biochemical functions. In oncology, PET is used for detecting cancer, determining if the cancer has spread (staging), evaluating how well treatments are working, and identifying any recurrence. Cancer cells often exhibit a higher metabolic rate than healthy cells, leading to increased uptake of glucose-based tracers like FDG, which appear as bright spots on the scan.
Applications extend to neurology, where PET scans help diagnose various brain disorders. They assist in identifying conditions such as Alzheimer’s disease, Parkinson’s disease, and epilepsy by revealing changes in brain metabolism or blood flow. For example, FDG PET can measure regional glucose use in the brain, aiding diagnosis.
In cardiology, PET scans are used to assess heart muscle viability and blood flow, identifying areas of decreased blood flow. This information guides decisions regarding treatments like angioplasty or coronary artery bypass surgery. Beyond these major areas, PET imaging also detects and monitors infections and inflammation throughout the body, as inflamed or infected tissues often show increased metabolic activity.