A Novel Method to Characterize the MTF in 3D for Computed Mammotomography

摘要

A novel phantom has been developed to measure the modulation transfer function (MTF) in 3D for x-ray computed tomography. The phantom consists of three tungsten wires, positioned nearly orthogonal to each other. Simultaneous measurements of the MTF are taken at various locations along the three orthogonal reconstructed planes. Our computed mammotomography (CmT) system uses a Varian Paxscan 2520 digital x-ray detector which can be positioned anywhere in ～2pi steradian band and can have arbitrary trajectories. With a half-cone beam geometry and with the phantom positioned near the center of rotation, projection images are acquired over 360 degrees. Various 3D orbits are evaluated including vertical axis of rotation and saddle. Reconstructions were performed using an iterative ordered-subsets transmission algorithm on rebinned projection images, using various numbers of iterations. Rotation of reconstructed slices isolated each wire into its own plane. At various locations along the length of each wire, corresponding MTFs were calculated from 1D line spread functions. Through measurement, accuracy of wire method was verified by comparison of the projection MTFs computed from a wire and a standard edge device. Results indicated minor variations in MTF among the three orthogonal planes, which imply a high degree of uniform sampling in the imaged volume. Findings indicate that the phantom can be used to assess the intrinsic image resolution in 3D as well as potential degradative effects of measurements in various media.