A fast CPU/GPU ray projector for fully 3d list-mode PET reconstruction

List mode based image reconstruction is continuously gaining ground in PET imaging. The centerpiece of this reconstruction is the ray projector which unfortunately also represents the most time consuming part of the reconstruction process. Graphics processing units (GPU) represent today a cheap alternative to getting high computation power in medical imaging. However, only few GPU adapted methods have been proposed within the PET image reconstruction context, especially concerning the ray projector. In this work we propose and evaluate a new projector called DDA-ELL, which has the advantage of being applicable to both CPU and GPU architectures. The proposed projector was incorporated and evaluated within the 3D-OPLEM reconstruction algorithm implemented on both CPU and GPU architectures. Quantitative and qualitative performance evaluation was performed using simulated phantom and clinical datasets. We have shown that the proposed ray projector reconstructed images lead to improved noise and contrast characteristics compared to the widely used iterative Siddon approach. The speedup reached by the CPU version of our method compared to the iterative Siddon method was 11, while the GPU version was nearly 140 times faster.