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We present a preliminary study on the design of a high sensitivity small animal DOI-PET scanner: jPET-RD (for Rodents with DOI detectors), which is under development at the National Institute of Radiological Sciences in Japan. The scanner composed of two rings of six detector blocks arranged in a hexagonal shape with an inner radius of 88 mm. Each detector consists of a four-layer array of 32×32 scintillator crystals each measuring 1.44×1.44×4.5 mm3 and a 256ch flat panel position sensitive photomultiplier tube (FP-PS-PMT). The FP-PS-PMT has a large effective area of 49×49 mm2 and multi anodes of 16×16ch, which can be read out in parallel. We simulated the scanner for various parameters of the number of DOI channels and the length of crystal. Simulated data were reconstructed using the maximum likelihood expectation maximization with accurate system modeling. The trade-off results between background noise and spatial resolution shows that only shortening the length of crystal does not improve the trade-off at all, and that four-layer DOI information improves uniformity of spatial resolution in the whole FOV. Monte Carlo simulations were also performed using the EGS4 code to measure absolute sensitivity and count rate capability with a small cylindrical phantom. We also evaluated the impact of critical design parameters on the noise equivalent count rate (NECR) and LOR errors caused by inter-crystal Compton scatters. Simulation results show that this scanner can achieve high sensitivity and high NECR, which can be further improved by using an appropriate anode segmentation of the FP-PS-PMT.