Reconstruction of dose distribution in in-beam PET for carbon therapy

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. In case of truncation error, we proposed to get prior knowledge of the location where annihilations occur. Similar to time-of-flight PET reconstruction, we applied a Gaussian distribution to system matrix, through the width of hardron beams in our back-projection routine. Then expectation maximization (EM) updates were performed iteratively. In case of the latter, to fill the gaps, we used the iterative discrete-cosine transform (DCT) domain method proposed by lJygar Tuna, Sari Peltonen, and lJlla Ruotsalainen. The results show that the proposed method can compensate to some extent the error caused by insufficient angle coverage and we can see the path of hardron beam by proposed method. However, we found other artifacts induced beyond the Bragg peak positions the number of iterations increased. We will improve the proposed algorithm to get more accurate dose distribution.