Effect of sub-pixel misregistration on the determination of the point spread function of a CT imaging system

The point spread function (PSF) of an imaging system provides a complete, quantitative description of its resolution. In computed tomography (CT) it can be obtained conveniently and directly from the image of a thin metal wire. However, the shape of the observed PSF is affected by arbitrary sub-pixel shifts in alignment between the wire object and the imaging raster, causing partial volume averaging. We investigated the loss of symmetry of the PSF due to this misregistration, using a thin wire at a small angle to the axis of the CT scanner. We were able to identify the optimal registration, and hence the best estimate of the PSF, from the minimum skewness of a series of images. Determination of an exact PSF is important in the imaging of structures of sub-pixel size, such as small blood vessels or trabeculae in spongy bone. The method is particularly useful for modalities with coarse rasters such as magnetic resonance imaging and nuclear medicine.