Measuring the 3D resolution of a micro-focus X-ray CT setup

One of the most important specifications of an industrial X-ray computed-tomography (CT) setup used for inspection is the 3-dimensional (3D) resolution that can be achieved in the volumetric image. Sometimes CT setups are evaluated in this regard using a 2D object such as the JIMA micro resolution chart; however, this approach measures the 2D resolution in a projection image and characterizes the performance of X-ray source and detector, not that of the complete CT setup. Depending on the CT acquisition trajectory, component alignment, and reconstruction artifacts, the 3D resolution might be considerably worse than the 2D resolution. It is therefore desirable to directly measure the 3D resolution. A micro-structured silicon phantom suitable for high-magnification X-ray CT with a microfocus source, and containing structure widths down to one micrometer, has recently been made available by QRM GmbH. The phantom can be used to determine the axial and in-plane 3D resolution in several ways: "by eye" using bar and hole patterns as well as a "Siemens star", using a slanted-edge MTF as per ISO 12233, and by evaluation of the image contrast as a function of structure width. We report on experimental results with the new phantom and on the influence of CT correction methods on the 3D resolution.