Sampling and aliasing consequences of quarter-detector offset use in helical CT

In this work we analyze the sampling anti abasing consequences of employing a quarter-detector offset (QDO) in helical CT. QDO is used in conventional CT to reduce in-plane aliasing by eliminating data redundancies to improve radial sampling. In helical CT, these same redundancies are exploited to improve longitudinal sampling and so it might seem ill-advised to employ QDO. In truth, we find that there is no compelling reason to avoid using QDO in helical CT. It leads to a similar or identical amount of aliasing as not using QDO. We reached this conclusion by conducting a multidimensional sampling analysis of projection-space sampling as well as a Fourier crosstalk analysis of image-space crosstalk among the object´s Fourier basis components. We analyzed both a standard fanbeam helical CT geometry and a hypothetical parallel-beam CT geometry, which helps illuminate the more complicated fanbeam results. Using the sampling analysis, we find that the use of QDO leads to very different spectral filing than arises when not using QDO. However, due to the shape or the essential support of the projection data spectra, we find that both configurations lead to very similar or identical amounts of spectral overlap. This perspective also predicts the spatially variant longitudinal aliasing that has been observed in helical CT. The crosstalk results were consistent with those of the multidimensional sampling analysis.