Anisotropic Point Spread Function of Cone-Beam Computed Tomography

摘要

Cone-beam computed tomography (CBCT) provides a real volume imaging modality, which can reconstruct a digital volume with an isotropic grid resolution. However, the imaging resolution of a CBCT system is not identical among spatial orientations, which manifests an anisotropic three-dimensional point spread function (3D PSF), or a PSF ellipsoid. According to the CBCT imaging procedure, the anisotropic factors include the x-ray cone-beam quality, focal spot size and shape, divergence projection angle, scanning orbit, and reconstruction algorithms. Since x-rays are generated by electron bombardment on a slope target anode, the x-ray source is of finite size and non-circle shape, which can neither be considered as a point nor a planar spot in strict sense. Due to non-uniform penetration depths in the anode target, the wavefronts of the emanating x-rays assume non-spheric distributions, as described by the heel effect, which becomes pronounced in a large cone-angle projection. The spatial blurring thereby is rather complicated. In this paper, we will study the anisotropy of 3D PSF under diverse imaging factors, including the heel effect, the focal spot size and shape, and the cone angle. Both computer simulation and phantom experiment on a flat-panel-detector CBCT system will be reported.