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Positron emission tomography (PET) scanners designed to image animals the size of rats and mice should possess simultaneously high and uniform spatial resolution and high sensitivity. ATLAS (Advanced Technology Laboratory Animal Scanner), a 6.0 cm diameter effective transverse field-of-view (FOV), 2 cm axial FOV ring-type research scanner seeks these goals by surrounding the animal with eighteen 15 mm deep, LGSO (7 mm)/GSO (8 mm) phoswich detector modules. A Monte Carlo simulation was used to compare the variation of resolution across the FOV and the absolute central point source sensitivity (ACS) of ATLAS to similar systems comprised only of LSO arrays of different depths with no depth-of-interaction (DOI) capability. For ATLAS radial spatial resolution deteriorated by 27% from the center to 3 cm off-axis. Scanners comprised of 15 mm deep, 10 mm deep and 7 mm deep LSO crystals deteriorated by 100%, 51%, and 20%, respectively, over the same distance. Simulated ACS (absorbed energies ≥250 keV) for ATLAS was 2.0% and for the 15 mm, 10 mm deep and 7 mm deep LSO scanners 2.4%, 1.5%, and 0.9%, respectively. Radial resolution loss 3 cm off-axis and ACS measured for the actual ATLAS scanner were similar to the values obtained by simulation (27% resolution loss, 1.8% ACS). The phoswich design thus achieves good resolution uniformity over a 6 cm FOV while preserving sensitivity compared to equivalent non-DOI LSO scanners with a range of crystal depths.