Positron emission tomography (PET) is a nuclear medicine medical imaging technique, which produces a three-dimensional image or map of functional processes in the body.

In PET, a short-lived radioactive tracer isotope, which decays by emitting a positron, chemically incorporated into a metabolically active molecule, is injected into the living subject (usually into blood circulation). The short-lived isotope decays, emitting a positron. These are detected when they reach a scintillator material in the scanning device, creating a burst of light, which is detected by photomultiplier tubes.

PET is a valuable technique for some diseases and disorders, because it is possible to target the radio-chemicals used for particular bodily functions.

Oncology: PET scanning with the tracer (18F) fluorodeoxyglucose (FDG, FDG-PET) is widely used in clinical oncology. This tracer is a glucose analog and is taken up by cells, phosphorylated by hexokinase (whose mitochondrial form is greatly elevated in rapidly-growing malignant tumors), and retained by tissues with high metabolic activity, such as the brain, the liver, and most types of malignant tumors. As a result FDG-PET can be used for diagnosis, staging, and monitoring treatment of cancers, particularly in Hodgkin's disease, non-Hodgkin's lymphoma, and lung cancer. However because individual scans are more expensive than conventional imaging with CT and MRI, expansion of FDG-PET in cost-constrained health services will depend on proper Health Technology Assessment. Oncology scans using FDG make up over 90% of all PET scans in current practice.

Neurology: PET neuroimaging is based on an assumption that areas of high radioactivity are associated with brain activity. What is actually measured indirectly is the flow of blood to different parts of the brain, which is generally believed to be correlated, and usually measured using the tracer oxygen (15O). Research continues into the use of radiolabeled F-DOPA and FDDNP as more specific probes.

Cardiology: In clinical cardiology FDG-PET can identify so-called "hibernating myocardium", but its cost-effectiveness in this role versus SPECT is unclear.

Neuropsychology / Cognitive neuroscience: To examine links between specific psychological processes or disorders and brain activity.

Pharmacology: In pre-clinical trials, it is possible to radio-label a new drug and inject it into animals. The uptake of the drug, the tissues in which it concentrates, and its eventual elimination, can be monitored far more quickly and cost effectively than the older technique of killing and dissecting the animals to discover the same information. PET scanners for rats and apes are marketed for this purpose.

PET scanning is invasive, in that radioactive material is injected into the subject. However the total dose of radiation is small, usually around 7 mSv.

Positron Emission Tomography