positron emission tomography
Definition: creation of tomographic images revealing certain biochemical properties of tissue by computer analysis of positrons emitted when radioactively tagged substances are incorporated into the tissue. Radiotracers used in PET are analogues of physiologic or pharmaceutical agents into which positron-emitting isotopes with short half-lives (2–110 minutes) have been incorporated. Radioisotopes are produced artificially by bombarding stable isotopes with a proton beam generated by a cyclotron. The uptake and metabolism of these positron emitters mimic, at least in part, those of the radiostable natural substances to which they are analogous. Concentrated in particular organs or tissues and incorporated into metabolic processes, they can reflect biochemical function or dysfunction. The glucose analogue 2-(fluorine-18)fluoro-2-deoxy-d-glucose (FDG) is widely used to locate zones of heightened energy metabolism. When a positron emitted by a radiotracer collides with an electron, the particles annihilate each other and two gamma rays are discharged in opposite directions (at 180°). After intravenous administration of the radiotracer, the subject is positioned within a scanner consisting of a ring of scintillation crystals that convert gamma rays into flashes of visible light. These flashes are detected and recorded electronically, and a computer program assembles the data into a 3-dimensional image, color-coded to reflect concentration density.
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