Positron emission tomography (PET) is a nuclear medicine imaging technique which produces a three-dimensional image or map of functional processes in the body.
The system detects pairs of gamma rays emitted indirectly by a positron-emitting radioisotope (tracer), which is introduced into the body on a biologically active molecule. Images of tracer concentration in 3-dimensional space within the body are then reconstructed by computer analysis.
In modern scanners, this reconstruction is often accomplished with the aid of a CT X-ray scan performed on the patient during the same session, in the same machine.
If the biologically active molecule chosen for PET is FDG, a derivative of glucose, the concentrations of tracer imaged then give tissue metabolic activity, in terms of regional glucose uptake.
Although use of this tracer results in the most common type of PET scan, other tracer molecules are used in PET to image the tissue concentration of many other types of molecules of interest.
See Wikipedia, Positron emission tomography.