Longitudinal resolution in volumetric x-ray computerized tomography--analytical comparison between conventional and helical computerized tomography.

The primary advantage of helical computerized tomography (CT) is the capability of scanning a complete anatomical volume in a single breath hold. Due to the table motion and subsequent interpolation process, the slice sensitivity profile (SSP) in helical CT is worse than the response function of the detector array. In this paper, image longitudinal resolution in volumetric x-ray CT is analytically characterized, and a comparison made between conventional and helical CT. First, the SSPs are derived for both conventional and helical CT with the half-scan interpolation method under the condition that the table increment and detector collimation are the same. Then, the corresponding transfer functions are obtained for bandwidth determination, which directly describe the spatial resolution. Both one-tenth-cutoff and mean-square-root measures are used to quantify the bandwidth. Although it appears that broadening the SSP in helical CT could adversely affect longitudinal resolution, it is proved that for a given x-ray dose, helical CT allows substantially better longitudinal resolution than conventional CT due to its inherent retrospective reconstruction capability. To make full use of the potential of helical CT scan data, it is recommended that about five slices be reconstructed per table increment. Helical CT is superior in applications requiring a high longitudinal resolution.