Assessment of spatial resolution in CT

To quantify spatial resolution in CT one typically performs separate measurements for the lateral and the longitudinal point spread function (PSF). Many procedures further require reconstructions with very small voxel sizes, e.g. when wire phantoms are scanned. This, however, may already change the shape of the PSF. For example, this is the case when Moiré filters are applied or when iterative image reconstruction algorithms are used. Aiming at assessing the point spread function (PSF) and the modulation transfer function (MTF) of a CT scanner using a single measurement we propose to measure a sphere, perform a standard image reconstruction and evaluate profiles through the sphere surface. The radial symmetry of CT scanners allows to reduce the dimensionality of the PSF and the MTF from three to two by radial averaging. It is shown that the resulting two-dimensional profiles can be decomposed into a radial and into a longitudinal component by two-dimensional parallel-beam filtered backprojection. Our method was assessed using simulated and measured data of a homogeneous sphere. The measurements were performed with the capable in-vivo cone-beam micro-CT scanner VAMP TomoScope 30s. The longitudinal and radial PSFs, and the corresponding MTFs, highly agree with those obtained with conventional methods, for both the simulations and the measurements. Figures of merit extracted from the curves, such as the full width at half maximum of the PSF or the 10% value of the MTF, differ by less than 5% between the new method and the conventional approaches. Therewith it gives a technique which requires only one, easy to handle, measurement of a sphere to calculate radial and longitudinal PSF and therefrom obtain the corresponding MTFs. Furthermore it does not require a dedicated reconstruction with very small voxels. Therefore it appears superior to existing methods.