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The design of a new scanner for use in small animal PET imaging is described. The goal is to achieve 1 mm FWHM resolution in each of three orthogonal directions throughout a volume suitable for whole body mouse imaging, roughly 40 mm diameter × 80 mm long. Simultaneously, the design should achieve a sensitivity of greater than 5% of all decays from a point source located at the center of the scanner. The scanner uses 12, plane detector banks mounted in a 160 mm diameter ring on a rotating gantry. Each detector bank consists of a 48 × 108 array of 20 mm long LSO crystals with an array pitch of 0.87 mm. Each bank uses two Hamamatsu H8500 large-area, multi-anode photomultiplier tubes for fluorescence detection. The detector banks are divided into two sets with the respective lines of response offset by one quarter of the array pitch to give increased sampling density. Tests using a prototype crystal array demonstrate that individual crystals can be resolved. Simulations have been performed to evaluate the performance expected in the complete scanner. With F-18 point sources, the FWHM resolutions in the radial, tangential, and axial directions are less than 1 mm for source positions throughout the desired field of view (FOV). Simultaneously, the detector sensitivity is greater than 7% of all decays for a point source located at the center of the FOV. Results are also presented for simulations using different PET isotopes to investigate the effect of positron range, and for a phantom containing hot spots added to a uniform background to evaluate the scanner performance for an extended object.