positron emission tomography (PET)

Imaging technique used in diagnosis and biomedical research. A chemical compound labeled with a radioactive isotope (see radioactivity) that emits positrons is injected into the body, and detectors measure their activity in the tissues as they combine with electrons and are annihilated. Computers analyze, integrate, and reconstruct the data to produce images of the organs scanned. PET is particularly useful for studying brain and heart functions.

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Subatomic particle having the same mass as an electron but with an electric charge of +1 (an electron has a charge of −1). It constitutes the antiparticle (see antimatter) of an electron. The existence of the positron was a consequence of the electron theory of P.A.M. Dirac (1928), and the particle was discovered in cosmic rays by Carl D. Anderson (1905–1991) in 1932. Though they are stable in a vacuum, positrons react quickly with the electrons of ordinary matter, producing gamma rays by the process of annihilation. They are emitted in positive beta decay of proton-rich radioactive nuclei and are formed in pair production.

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