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. (C) 2001 IPEM. Published by Elsevier Science Ltd. All rights reserved.