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In the last few years, one of the main research programs for PET systems has been the improvement of spatial resolution using Depth Of Interaction (DOI) information. In the context of developing a new PET system dedicated to small animals with axially oriented LYSO crystals and DOI capability, we have investigated the influence of the crystal coating and its length on the DOI resolution. The proposed PET system is composed of four detection modules arranged around the animal. Each module consists in 768 LYSO crystals read at both ends by multichannel plate photodetectors. The particular geometry combined with an inner diameter of 61.2 mm, lead to high detection efficiency close to the system solid angle. The LYSO crystal is chosen for its light yield of 33 ph/keV and its attenuation length of 11.2 mm at 511 keV. To obtain a transverse spatial resolution of 1 mm, the section of the crystal was fixed to 1.5 mm. To achieve a DOI resolution close to 1 mm, measurements have been performed on different LYSO crystal coatings with a length ranging from 25 mm to 35 mm. Each crystal is positioned on an xy translation stage and read out at both ends by H3164-10 Hamamatsu PMTs. The DOI information is then derived every 0.5 mm along the crystal extent. The use of an electronic collimation leads to a 22Na source beam size of (1.58 ± 0.04) mm reaching the crystal. The optimized coating in terms of packing fraction and DOI resolution is found to be a mixture made with 30% TiO2 powder in a PMMA binder. With this appropriate coating, an average DOI resolution of (0.82 ± 0.13) mm can be achieved with a 25 mm crystal length using a 20% photopeak energy window. The resolution degrades to (1.39 ± 0.16) mm when a wide-open energy window is used. Those values are corrected for the source beam size. Using this experimental proposal, a matrix of LYSO crystals has been built reaching a packing fraction of 93%. In this study, we demonstrate that us- - ing a 1.5 × 1.5 × 25 mm3 LYSO crystal, a DOI resolution of less than a millimeter can be achieved while keeping a high packing fraction for a system detection efficiency close to 15%.