Longitudinal sampling and aliasing in spiral CT

Although analyses of in-plane aliasing have been done for conventional comp uted tomography (CT) images, longitudinal aliasing in spiral CT has not bee n properly investigated. We propose a mathematical model of the three-dimen sional (3-D) sampling scheme in spiral CT and analyze its effects on longit udinal aliasing, We investigated longitudinal aliasing as a function of the helical-interpolation algorithm, pitch, and reconstruction interval using CT simulations and actual phantom scans, Our model predicts, and we verifie d, that for a radially uniform object at the isocenter, the spiral sampling scheme results in spatially varying cancellation of the aliased spectral i slands which, in turn, results in spatially varying longitudinal aliasing. The aliasing is minimal at the scanner isocenter, but,worsens with distance from it and rapidly becomes significant, Our results agree with published results observed at the isocenter of the scanner and further provide new in sight into the aliasing conditions at off-isocenter locations with respect to the pitch, interpolation algorithm, and reconstruction interval, We conc lude that longitudinal aliasing at off-isocenter locations can be significa nt, and that its magnitude and effects cannot be predicted bg measurements made only at the scanner isocenter.