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Spatial variation of section sensitivity profile in spiral computed tomography.

The section sensitivity profile (SSP) is widely used to describe the longitudinal image resolution in computed tomography (CT). Due to asymmetry in spiral CT interpolation, the SSP depends on the transverse position of the associated longitudinal line. Spatial variation in the spiral CT SSP has not been investigated previously. To determine the SSP variation over the scan field, the SSP for the half-scan interpolation method in terms of its transverse position was formulated. Based on the SSP formula, mathematical analysis and numerical simulation were performed. It was shown that the SSP associated with a transverse slice is antisymmetric with respect to the abscissa axis (which connects the slice center and the source location in the slice). Moment analysis was conducted on the SSP up to order three. The area under the SSP is equal to that under the longitudinal detector response function. As a result, with a normalized SSP the total mass of an object approximately remains after reconstruction. The mean of the SSP is equal to zero, although the SSP is asymmetric. The standard deviation and skewness of the SSP were numerically simulated and plotted for typical imaging parameters. It was demonstrated that for a 50 degrees fan angle (which is extended by two extreme rays in a fan beam) the relative change in the slice thickness is no more than 10%, while the skewness of the SSP is within about 0.15. The smaller the fan angle, the less the SSP spatial variation. Therefore, in spiral CT the SSP along the longitudinal axis can be used as a representative of the SSP family.