A noninvasive imaging technique that can produce three-dimensional pictures of certain biochemical changes within the body, such as areas of increased tissue metabolism. It is used for research in several areas, including neurology and pharmacology, and as a diagnostic technique in medicine, for example in detecting brain tumours. The technique is based on the detection of photons produced by the decay of short-lived radioactive isotopes, such as fluorine-18 in labelled fluorodeoxyglucose (see labelling), injected into the body. The isotopes emit positrons, which almost immediately collide with electrons to produce a pair of photons that travel in opposite directions. Simultaneously arriving photon pairs are detected by the scanner on either side of the body as flashes of light, and a computer calculates the point of their mutual origin within the body. By collating many thousands of such coincident flashes, the computer creates a map showing where the isotope is concentrated, which corresponds to locally increased blood flow or uptake as a consequence of elevated tissue activity. PET scans are often now integrated with images obtained by computerized tomography or magnetic resonance imaging so that the biochemical information can be correlated with anatomy of the site.