Reconstruction-plane-dependent weighted FDK algorithm for cone beam volumetric CT

摘要

The original FDK algorithm has been extensively employed in medical and industrial imaging applications. With anincreased cone angle, cone beam (CB) artifacts in images reconstructed by the original FDK algorithm deteriorate, sincethe circular trajectory does not satisfy the so-called data sufficiency condition (DSC). A few “circular plus” trajectorieshave been proposed in the past to reduce CB artifacts by meeting the DSC. However, the circular trajectory has distinctadvantages over other scanning trajectories in practical CT imaging, such as cardiac, vascular and perfusion applications.In addition to looking into the DSC, another insight into the CB artifacts of the original FDK algorithm is theinconsistency between conjugate rays that are 180° apart in view angle. The inconsistence between conjugate rays ispixel dependent, i.e., it varies dramatically over pixels within the image plane to be reconstructed. However, the originalFDK algorithm treats all conjugate rays equally, resulting in CB artifacts that can be avoided if appropriate viewweighting strategy is exercised. In this paper, a modified FDK algorithm is proposed, along with an experimentalevaluation and verification, in which the helical body phantom and a humanoid head phantom scanned by a volumetricCT (64×0.625 mm) are utilized. Without extra trajectories supplemental to the circular trajectory, the modified FDKalgorithm applies reconstruction-plane-dependent view weighting on projection data before 3D backprojection, whichreduces the inconsistency between conjugate rays by suppressing the contribution of one of the conjugate rays with alarger cone angle. Both computer-simulated and real phantom studies show that, up to a moderate cone angle, the CBartifacts can be substantially suppressed by the modified FDK algorithm, while advantages of the original FDKalgorithm, such as the filtered backprojection algorithm structure, 1D ramp filtering, and data manipulation efficiency,can be maintained.