Modulation transfer function of digitally reconstructed radiographs using helical computed tomography

Abstract

We have measured the modulation transfer function (MTF), at a distance from CT isocenter, in digitally reconstructed radiographs (DRR) using a bar pattern phantom for axial and helical data acquisitions. Spatial resolution in DRR increases for thinner slice thicknesses (43% at 1.6 lp/cm for 2 mm versus 16% for 5 mm). For the three slice thicknesses studied, the axial scanning mode provided better spatial resolution in DRR than helical scans performed at pitches > or = 1.5 (41% at 1.6 lp/cm for axial, 3 mm slice versus 18% for pitch = 2), but is similar to helical scans with pitch of 1. The reconstruction of overlapping slices from helical acquisitions of pitch = 1.5 results in spatial resolution of the DRR that is similar to that resulting from axial scans with contiguous reconstruction, but also results in the fine streaks known as "zebra" artifacts in the DRR.