The effects of misregistration of the projections on spatial resolution of CT scanners

Misregistration of the projections in 360 degrees computed tomographic (CT) scanners has been found to blur the image without generating artifacts. The effects of this error were investigated by analytical methods and by reconstruction of real and simulated data. The point-spread function which results from shifting each projection by a constant distance epsilon consists of a two-dimensional impulse function surrounding a region of negative density. The locus of the impulse function is a circle for parallel-beam geometry and a sixth-order curve for fanbeam geometry. The anisotropy and position dependence of the point-spread function in fanbeam geometry have been characterized. The line-spread function due to the error in parallel-beam geometry consists of two delta functions located at +/- epsilon. In fanbeam geometry, the line-spread function consists of two delta functions separated by approximately 2 epsilon, with the locations of the impulses dependent on the position and orientation of the line. This error, combined with other sources of blurring, results in a system edge-response function which contains a flat region at one-half the maximum density.