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A gamma-ray imaging system has been developed for acquiring in vivo images of the distribution of radiopharmaceuticals in unrestrained, unanesthetized small animal. This system uses two gamma-ray cameras and an infrared position tracking system. The two gamma-ray cameras and infrared position tracking system are mounted on a rotational gantry with a burrow for the small animal (typically mice) at the center. A system of PC computers is used to control the rotation of the cameras and tracking system to various angles of viewing, collect the gamma-ray data, and track the position of the animal with the infrared system. The gamma-ray data, infrared position tracking data and gantry location information will be postprocessed to reconstruct a single photon emission computer tomography (SPECT) image of the animal that is corrected for motion of the animal. A 24-bit counter is incremented on a 10-millisecond time scale and read by the three computers in the system. This provides a common time base to synchronize the data collected by the separate systems. The gantry controller records the times that the gantry is moving and stationary. The infrared position tracking system records the position of the object up to 15 times per second. The gamma-ray camera records the data in list mode with a separate 1-millisecond clock incorporated into the event data. This higher time resolution in the gamma-ray data will allow for correction of motion by interpolation between the position data.