Positron emission tomography (
PET) is a
nuclear medicine,
functional imaging technique that produces a three-dimensional image of functional processes in the body. The system detects pairs of
gamma rays emitted indirectly by a
positron-emitting
radionuclide (
tracer), which is introduced into the body on a biologically active molecule. Three-dimensional images of tracer concentration within the body are then constructed by computer analysis. In modern
PET-CT scanners, three dimensional imaging is often accomplished with the aid of a
CT X-ray scan performed on the patient during the same session, in the same machine.