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.
To conduct the scan, a short-lived radioactive tracer isotope which decays by emitting a positron, and which has been chemically incorporated into a metabolically active molecule, is injected into the living subject (usually into blood circulation). There is a waiting period while the metabolically active molecule becomes concentrated in tissues of interest; then the subject is placed in the imaging scanner.
As the short-lived isotope decays, it emits a positron. After travelling up to a few millimeters the positron encounters and annihilates with an electron, producing a pair of annihilation photons moving in nearly opposite directions. These are detected when they reach a scintillator material in the scanning device, creating a burst of light which is detected by photomultiplier tubes.