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A Monte Carlo simulation of the gamma ray transport within a single-slice positron emission tomograph has been generated to study the effects of system parameters on performance. Included in the simulation are the radioactive source distribution, collimators, and detectors with intercrystal septa. Data are first presented to show the coincidence and singles sensitivities as a function of ring radius. Then, for a fixed radius of 26 cm, the variation of sensitivities are shown as a function of the following variables: slice thickness, patient port size, intercrystal septum dimensions, lower energy discriminator level, and coincidence fan angle. Simulation-generated sensitivity data are compared with experimental values for several tomographs andgood agreement is obtained. Discrepancies between two definitions used in experimentally determining scatter fractions are discussed. The Monte Carlo simulation shows that small radii rings have an effective count rate (quality factor) that is more than 90 percent of that for larger rings at low and moderate activity levels (?? 0.25 ??Ci??cm-3), contrary to what is predicted from analytical calculations. It is concluded that small radius rings are better suited for low dose-rate static studies, while larger radius rings are preferred for high dose-rate dynamic studies.