Parallelizing a Matched Pair of Ray-Tracing Projector and Backprojector for Iterative Cone-Beam CT Reconstruction

Iterative reconstruction methods in X-ray CT can provide better image quality than analytical methods but their applications in practice are still limited due to computationally expensive calculations of repeated projection and backprojection operations. In the past decade, GPU-accelerated methods have been successfully used to reduce the computation time for projection and backprojection. However, it has been of a difficult problem to overcome a trade-off between the accuracy of reconstructed images and the efficiency of parallel computations. For example, when the size of the voxel in the reconstructed volume is larger than that of the detector bin, the use of the conventional unmatched projector-backprojector pair can lower the accuracy of reconstructed images due to the error caused by the mismatch between the projector and backprojector. In this paper, we propose a new GPU-accelerated scheme for the most widely used ray-tracing method (RTM) to perform projection and backprojection operations. Unlike the previous works that accelerate the computation of backprojection by using approximations, our method does not use any approximations for parallelizing the projection and backprojection operations. Since our method is exact, the results are as accurate as those obtained from the nonaccelerated method. We apply our method to iterative reconstruction for dental cone-beam CT systems and test its performance using both the simulated data using a 3-D digital phantom and the real data acquired from an offset flat-panel X-ray CT system. Our experimental results show that, the proposed method achieves a substantially high acceleration rate while retaining the accuracy of the RTM for both projection and backprojection.