Abstract
Several acquisition geometries were investigated to optimize coincidence count rates and coincidence/singles rate ratios for brain imaging on a rotating gamma camera PET system. Detection efficiency and singles count rate limits on these systems dictate that keeping unused singles from hitting the cameras should improve coincidence rates if radiotracer doses can be adjusted to match the singles detection efficiency of the system. Radius of rotation and camera field of view were adjusted within the constraints of patient positioning. Acquisitions were done with transaxial septa on a graded absorber, and with graded absorber alone. Phantoms included a Ge-68 pin source and an extended head-thorax phantom with F-18 solution. Coincidence detection efficiency was best for the absorber only, with a small radius of rotation. Coincidence/singles ratio was best for graded absorber with small radius of rotation and axial camera field limited to 27 cm. The septa yielded a slightly lower coincidence/singles ratio, but their reduction of scatter and restriction of incidence angle of detected photons (leading to higher spatial resolution) makes their use favorable.
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