How does PET work?

A small amount of a radioactive substance (tracer) is administered to the patient by injection
The radioactive substance travels to a specified location in the body and decays emitting a positron and a neutrino
The positron travels a short distance (~ 1mm in tissue), then it annihilates with an electron: two anti-parallel 511 keV photons are created
The two annihilation photons are detected simultaneously by the ring of detectors surrounding the patient
The positions of interaction of the annihilation pair in the detector ring define a Line Of Response (LOR) along which the annihilation must have occurred
The image of the distribution of the tracer is reconstructed from the LORs