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, q uantitative description of its resolution. In computed tomography (CT) it c an be obtained conveniently and directly from the image of a thin metal wir e. However, the shape of the observed PSF is affected by arbitrary sub-pixe l shifts in alignment between the wire object and the imaging raster, causi ng partial volume averaging. We investigated the loss of symmetry of the PS F due to this misregistration, using a thin wire at a small angle to the ax is of the CT scanner. We were able to identify the optimal registration, an d 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 st ructures of sub-pixel size, such as small blood vessels or trabeculae in sp ongy bone. The method is particularly useful for modalities with coarse ras ters such as magnetic resonance imaging and nuclear medicine. (C) 2001 IPEM . Published by Elsevier Science Ltd. All rights reserved.