Method, Mechanism, and Metrology for Measurement of Multi-Dimensional MTF in Medical Imaging

The spatial resolution characteristics of CT and cone-beam CT (CBCT) systems is well characterized by the spatial-frequency-dependent modulation transfer function (MTF). Emerging scanner technologies and reconstruction algorithms challenge conventional methods for MTF assessment, including the degree to which the system / image exhibits 3D resolution characteristics that are isotropic (vary in direction), stationary (vary with location), and linear (vary with contrast). We report an angled-edge test tool and oversampling method to measure the MTF in any direction in 3D image data, with extension to a spherical test tool for measurement in all directions. One particular direction (45° relative to the axial plane) is shown to avoid null space effects in cone-beam geometries and provide useful 1D quantitation of the fully 3D resolution characteristics. This method was tested using a mobile C-arm CBCT system and a high-resolution diagnostic CT scanner and shown to reveal underlying sources of non-isotropic resolution characteristics – for example, asymmetric apodization filters, detector binning modes, and focal spot size. The angled-edge or spherical test tools provide a practical means for quantitative characterization of 3D MTF characteristics for medical imaging systems.