emission tomography scanners

Figure 6.3) are designed specifically for radionuclides that decay by positron emission. Scintillator crystals with high stopping power are utilized, examples are bismuth germanate (BGO) and lutetium orthosilicate (LSO). These crystals, coupled to photomultiplier tubes, are positioned in rings around a gantry. Electronic circuitry, designed to register if two crystals detect a photon within a ‘coincidence time’, usually a few nanoseconds, allow ‘true’ events to be detected. Many ‘single’ events will be detected. The size of the crystal elements depends on the make and model, but is generally 3 to 6 mm. The elements are arranged in 360° rings, with typically 15 to 30 rings consisting of the order of 10 000 individual crystal elements. The crystal size is one of the main factors in limiting the spatial resolution, commonly 4 mm (FWHM) [7]. Although systems designed for special purposes, such as brain scanning, can achieve 2 mm. The computer system is able to reconstruct the line connecting two detected events in opposite crystals as a line of response (LOR). Millions of LORs are acquired over a period of time and reconstructed to form cross-sectional images using appropriate image reconstruction software.