Longitudinal sampling and aliasing in spiral CT

Abstract

Although analyses of in-plane aliasing have been done for conventional computed tomography (CT) images, longitudinal aliasing in spiral CT has not been properly investigated. We propose a mathematical model of the three-dimensional (3-D) sampling scheme in spiral CT and analyze its effects on longitudinal 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 verified, that for a radially uniform object at the isocenter, the spiral sampling scheme results in spatially varying cancellation of the aliased spectral islands 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 insight into the aliasing conditions at off-isocenter locations with respect to the pitch, interpolation algorithm, and reconstruction interval. We conclude that longitudinal aliasing at off-isocenter locations can be significant, and that its magnitude and effects cannot be predicted by measurements made only at the scanner isocenter.

Anti-aliasing weighting functions for single-slice helical CT

“…This work was supported in part by National Institutes of Health (NIH) under Grant R01EB00225. The Associate Editor responsible for coordinating the review of this paper and recommending its publication was C. Crawford a second uniform set of longitudinal samples [1]. The explanation for this apparent paradox lies in the fact that the sampling positions of the second set of samples do not, in general, lie midway between those of the first and, thus, do not automatically produce the aliasing cancellation that would be expected if the sampling interval were truly halved [1].…”

“…These measurements can be viewed as samples of a 3-D function in a space , where is as before and where (1) Here, is the table-translation distance per revolution of the source. The ratio of the table-translation distance per revolution of the source to the longitudinal collimation is known as the pitch, .…”

“…I N RECENT papers [1], [2], Yen et al have demonstrated that spatially variant longitudinal aliasing plagues volumes reconstructed from single-slice helical computed tomography (HCT) data, and that its presence can degrade resolution and distort image structures. The aliasing arises because single-slice HCT data is undersampled in the longitudinal direction by a factor of approximately two at pitch 1 and by proportionally larger factors at higher pitches [1].…”

“…The aliasing arises because single-slice HCT data is undersampled in the longitudinal direction by a factor of approximately two at pitch 1 and by proportionally larger factors at higher pitches [1]. Yen et al also showed that longitudinal aliasing occurs to approximately the same absolute degree when making use of the standard 360LI and 180LI longitudinal interpolation algorithms, despite the latter's exploitation of the redundancy of fanbeam projection data to generate Manuscript received August 21, 2001; revised April 19, 2002.…”

“…The Associate Editor responsible for coordinating the review of this paper and recommending its publication was C. Crawford a second uniform set of longitudinal samples [1]. The explanation for this apparent paradox lies in the fact that the sampling positions of the second set of samples do not, in general, lie midway between those of the first and, thus, do not automatically produce the aliasing cancellation that would be expected if the sampling interval were truly halved [1]. We have shown, however, that under certain conditions the second set of longitudinal samples provided by the fanbeam redundancy does contain sufficient information to eliminate longitudinal aliasing in single-slice HCT if exploited correctly [3].…”