High-helical-pitch, cone-beam computed tomography

The object of a helical, cone-beam computed tomography, HCBCT, system is faster scan times for the volume of interest for a patient. The maximum helical-pitch that still produces good quality images limits the reduction in scan time. Although this is a three-dimensional problem, an extension of the completeness condition for a two-dimensional fan-beam reconstruction method serves as a guide to estimate the maximum helical-pitch. In HCBCT, each pixel in an arbitrary image slice is irradiated over a different range of gantry rotation angles. This leads to the idea of a different, or inconsistent, range of backprojection angles for each pixel in order to increase the helical-pitch while satisfying the completeness condition. Using inconsistent backprojection, the normalized helical-pitch ratio increases from 0.90 to 1.40 (for a source radius of 600 mm and field of view of 500 mm), where the normalized helical-pitch ratio is the linear advance of the patient couch per gantry revolution to the full axial height of the area detector (as projected at the isocentre).