There are two main artifacts in reconstructed images from in-beam positron emission tomography(PET). Unlike generic PET, in-beam PET uses the annihilation photons which occur during heavy ion therapy. Therefore, the geometry of in-beam PET is not a full ring, but a partial ring in order for the hardrons to arrive the tumor without penetration of detector blocks. The partial ring, however, causes truncation in projection data, due to an absence of detector modules in the openings. The other is ring artifact caused by the gaps between detector modules which can be also founded in generic PET. In this study, we aim to investigate the effect of gaps in reconstructed images and propose possible solutions to compensate the artifacts. We acquired the data by GATE v6.1 with initial ion energies 170, 290, 350AMeV of carbon beams. Each detector module consists of a 13 by 13 LYSO crystal array. The dimension of a crystal was 4mm * 4mm * 20 mm and the radius of inner circle of the gantry was 15cm.