SU‐D‐206‐02: Evaluation of Partial Storage of the System Matrix for Cone Beam Computed Tomography Using a GPU Platform

Abstract

Purpose:Iterative reconstruction algorithms in computed tomography (CT) require a fast method for computing the intersections between the photons’ trajectories and the object, also called ray‐tracing or system matrix computation. This work evaluates different ways to store the system matrix, aiming to reconstruct dense image grids in reasonable time.Methods:We propose an optimized implementation of the Siddon's algorithm using graphics processing units (GPUs) with a novel data storage scheme. The algorithm computes a part of the system matrix on demand, typically, for one projection angle. The proposed method was enhanced with accelerating options: storage of larger subsets of the system matrix, systematic reuse of data via geometric symmetries, an arithmetic‐rich parallel code and code configuration via machine learning. It was tested on geometries mimicking a cone beam CT acquisition of a human head. To realistically assess the execution time, the ray‐tracing routines were integrated into a regularized Poisson‐based reconstruction algorithm. The proposed scheme was also compared to a different approach, where the system matrix is fully pre‐computed and loaded at reconstruction time.Results:Fast ray‐tracing of realistic acquisition geometries, which often lack spatial symmetry properties, was enabled via the proposed method. Ray‐tracing interleaved with projection and backprojection operations required significant additional time. In most cases, ray‐tracing was shown to use about 66 % of the total reconstruction time. In absolute terms, tracing times varied from 3.6 s to 7.5 min, depending on the problem size. The presence of geometrical symmetries allowed for non‐negligible ray‐tracing and reconstruction time reduction. Arithmetic‐rich parallel code and machine learning permitted a modest reconstruction time reduction, in the order of 1 %.Conclusion:Partial system matrix storage permitted the reconstruction of higher 3D image grid sizes and larger projection datasets at the cost of additional time, when compared to the fully pre‐computed approach.This work was supported in part by the Fonds de recherche du Quebec ‐ Nature et technologies (FRQ‐NT). The authors acknowledge partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council of Canada (Grant No. 432290).