Positron emission tomography scan, a non-invasive technique of brain imaging that monitors regional cerebral blood flow in the brain by recording the emission of gamma rays when radioactively labelled glucose or some other substance introduced into the bloodstream is metabolized by neurons as they are activated. The radioactive atoms used in a PET scan emit subatomic particles called positrons (positive electrons), which collide with their negatively charged counterparts, namely electrons. The two particles annihilate each other and emit two gamma rays that radiate in opposite directions and can be recorded by a ring of detectors round the person's head and traced back to their point of origin. A PET scan is expensive, requiring an onsite cyclotron because of the short half-life of the radioactive materials produced, but it provides a dynamic record, usually colour-coded, of brain activity and can also be used to study other organs such as the heart and lungs. Compare radioisotope scan, SPECT scan.