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A BGO based small animal PET scanner dedicated for imaging small rodent was simulated by GATE. The virtual PET scanner had the same ring diameter, axial field of view (FOV) and crystal arrangement as the LSO based Siemens Inveon PET system, but was simulated with varied crystal lengths. The simulated system sensitivity was 11.6%, 19.3% and 25.5% for 10, 15 and 20 mm BGO at an energy window of 250-750 keV. The spatial resolution was measured at radial offsets of 0, 15 and 28 mm from the center of the FOV for the three crystal thicknesses. The FWHM in the radial and tangential directions was below 2.5 mm and 1.8 mm respectively for all three crystal thicknesses, up to a 30 mm diameter FOV. Scatter fraction and count rate performance were measured using a line source inserted in a water cylinder 70 mm long and 25 mm diameter for the 20 mm BGO system. The maximum NECR was 0.99 Mcps at 24 MBq and the phantom scatter fraction was 4.5% with an energy window of 250-750 keV and a timing window of 12 ns. The BGO based PET scanner was compared with the Inveon and the microPET Focus 220 systems. With the same crystal thickness, the BGO scanner had higher system sensitivity than the LSO and further improvement in sensitivity can be achieved by using thicker crystals without sacrificing much spatial resolution. Both radial and tangential resolutions were comparable to the LSO based systems. At the evaluated energy window of 250-750 keV, the phantom scatter fraction was similar to the Inveon system, while the crystal scatter fraction was about 10% lower. The maximum NECR was lower than Inveon and was achieved at a lower activity level. Simulation of the BGO PET scanner proved the design concept of a high sensitivity small animal PET scanner, with comparable spatial resolution, similar phantom scatter fraction, and acceptable count rate performance.