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Detector geometry for PET camera is one of the most important factors that will determine the resolution and image quality of the camera. In this work, the point source resolutions and hot-rod phantom images of the PET systems with different detector geometries are studied using Monte Carlo simulations. The PET systems been studied include a human brain PET and an animal PET with the typical detector ring dimensions but different crystal sizes, number of layers and block geometries. The detector ring diameter is 480 mm for the brain PET and 160 mm for the animal PET. The blocks been studied include 1-layer and 2-layer geometries. Two types of 2- layer blocks are studied, one has a half-crystal-offset (HCO) between the top and bottom layers to double the radial and axial samplings, and the other one is the regular (non-HCO) 2-layer block. The blocks in the brain PET is 40times40times20 mm3 and with a crystal matrix of 10times10, 13times13 and 15times15; the block in the animal PET is 20times20times10 mm3 and with a crystal matrix of 10times10, 12times12 and 14times14. Point source resolution curves (radial, tangential and axial) as the function of off-center distance are obtained with an F-18 source. Two Derenzo-like hot-rod phantoms are used for overall image quality test. The results show the significant improvement on the DOI effect with 2-layer block especially in the radial direction. Using the HCO block, the transaxial resolution is a little better than that of non-HCO block in the center region of the FOV but become worse with some distance from the center because of the sampling rates in different radial offset positions are not the same for these two types of blocks. Another important finding is that the axial resolution with HCO block is much better than that from 1- layer or 2-layer non-HCO block. Therefore using HCO block with larger crystal pitch can still achieve better image quality not only in transaxial plane at the outside regi- on of a large FOV compare to the small crystal pitch with 1-layer block, but also better resolution in axial direction without additional cost, which might be a better choice for high-resolution PET camera design.