Field-of-view dependent helical pitch in multi-slice CT

A goal of multi-slice CT (MSCT) is to decrease the scan time needed to image a volume of the patient. Detectors with multiple rows of sensors enable higher helical pitches and thus shorter scan times. One of the difficulties in devising an efficient reconstruction method with good image quality and good dose utilization is that each image pixel is irradiated by the cone-beam for a different range of gantry orientations. We derive a new half-scan weighting scheme for a helical, cone-beam backprojection algorithm based on the virtual fan angle. The virtual fan angle, in turn, determines the gantry view range such that image quality is maintained by allowing only valid ray-sums while using the available dose. This restricts the virtual fan angle to be at least the true geometric fan angle but less than 180°. The result is a computational efficient and dose efficient reconstruction algorithm with a continuous range of field-of-view dependent helical pitches. A 43% higher helical pitch is possible for the smallest field-of-view compared with the largest field-of-view, using the parameters of a commercial MSCT-scanner. Dose efficiency is compared among the new method and standard half scan and full scan approaches.