Spatial shift variance and anisotropic blurring of cone-beam computed tomography imaging system

Abstract

Cone-beam computed tomography (CBCT) is a three-dimensional (3-D) imaging modality. The spatial blurring or spatial resolution of a CBCT system can be quantitatively characterized by a 3-D point spread function (PSF), which can be experimentally measured through the use of a small ball phantom and an edge-blurring algorithm. A solid ball offers a number of step edge functions along its radial lines, in terms of density profile. The CBCT system PSF is responsible for blurring a step edge into a spread edge, which can be determined by an iterative algorithm. By a spatial separability assumption, a 3-D PSF can be decomposed into 2-D and 1-D PSFs. The spatial shift variance refers to the nonuniform distribution of PSF full width at half maximum (FWHM) over the object domain. The anisotropic blurring is reflected by the blurring differences along three orthogonal directions of a PSF ellipsoid. The experimental results show that our CBCT system varies in the range between 0.43 and 0.82 mm in the object domain of 80×80×120 mm3, and the anisotropy of 3-D PSF is generally insignificant and is bounded by 0.04 mm.