In clinical applications, a very small amount of labelled compound (called radiopharmaceutical or radiotracer) is introduced into the patient usually by intravenous injection and after an appropriate uptake period, the concentration of tracer in tissue is measured by the scanner. During its decay process, the radionuclide emits a positron which, after travelling a short distance (3-5 mm), encounters an electron from the surrounding environment. The two particles combine and "annihilate" each other resulting in the emission in opposite directions of two gamma rays of 511 keV each (see animation below).
The image acquisition is based on the external detection in coincidence of the emitted  -rays, and a valid annihilation event requires a coincidence within 12 nanoseconds between two detectors on opposite sides of the scanner. For accepted coincidences, lines of response connecting the coincidence detectors are drawn through the object and used in the image reconstruction. Any scanner requires that the radioisotope, in the field of view, does not redistribute during the scan. A tissue attenuation correction is performed by recording a short transmission scan using -rays from three radioactive (Germanium-68/Gallium-68) rotating rod sources.
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