GPU-accelerated iterative 3D CT reconstruction using exact ray-tracing method for both projection and backprojection

The model-based iterative reconstruction methods have recently found its popularity in X-ray CT reconstruction due to its ability in providing improved image quality (than analytical methods) especially in low-dose condition where it requires less radiation dose delivering through the patient body. However, the application of iterative methods in practice is still limited due to its expensive computation time. In particular, the iterative methods require time-consuming calculations for repeated projection and backprojection operations. This requirement is more severe in reconstruction from low-dose scan where more iterations are required to regularize the high noise due to low detected counts. While the computational speed of projection and backprojection has been dramatically increased by using GPUs (graphics processing units), efforts to improve the accuracy of modeling a projector-backprojector pair have been hindered by the needs for approximations to maximize the efficiency of the GPU. The unmatched projector-backprojector pairs often used for GPU-accelerated methods also cause additional errors in iterative reconstruction. For low-dose CT reconstruction, the degradation due to these errors becomes more significant as the number of iterations is increased. Despite the appearance of many recent advanced methods to perform projection and backprojection, the ray-tracing method (RTM) is still popular due to its accurate representation of the physics of the Beer´s law as well as the ease of use. In this work, we propose a GPU-accelerated RTM. Unlike the previous works that used the RTM in forward projection and the pixel-driven method in backprojection, this work develops a new GPU-accelerated method for a RTM projector-backprojector pair which does not use any approximations for parallelizing the projection and backprojection. Since our method is exact, the results are as accurate as those obtained from the non-accelerated method.