Imaging of radiotracer data collection and analysis

PET involves the disintegration of positrons emitted from unstable nuclei such as 11C (Plate.4). Emitted positrons travel a short distance in tissue before annihilation by collision with an electron.(1) On annihilation, two high-energy gamma rays are generated with a separation angle of 180° ( Plate 4). Radiation detectors (e.g. bismuth germanate), 180° apart and linked in electronic coincidence circuits, detect the resulting gamma radiation and therefore localize the source of radiation to a volume between any two detectors (PJate.3). By arranging rings of detectors around the subject's head and using computer-based back-projection techniques, the distribution of radiotracer within tomographic slices of the brain can be obtained. (12.) SPET radioisotopes, in contrast, decay by emitting a single gamma ray and therefore the radiation detectors are not linked in coincidence circuits. State-of-the-art PET and SPET cameras have transaxial spatial resolutions of the order of 4 to

5 mm (full-width half maximum—a standard measure of spatial resolution) and can detect subnanomolar concentrations of receptors. (1,2)

Positron-emitting isotopes can be incorporated into molecules associated with diverse biochemical processes in the brain. For example, the positron emitter (11)C can be incorporated into a molecule WAY 100635, which selectively binds to 5-HT 1A receptors, and injected intravenously in tracer amounts; the resulting gamma radiation is detected with a positron camera. Brain regions will show different profiles of radio-activity accumulation over time as the radiotracer binds in areas with a high density of 5-HT1A receptors (medial temporal cortex) whilst in regions with no or sparse receptors (cerebellum), it will be washed out ( PJate.5). By this means, specific and non-specific binding can be distinguished. With an appropriate model of the radiotracer's history in tissue over time, a quantitative measurement of 5-HT1A receptor number in tomographic slices of the human brain can be obtained.(2) With some radiotracers (e.g. [11C]diprenorphine to label opiate receptors) it may be necessary to undertake radial artery cannulation to obtain an 'input function'(2) that describes the time course of presentation of radiotracer to the brain used in the quantitative modelling of the radiotracer ( Plate.6).

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