TH‐C‐18A‐11: Investigating the Minimum Scan Parameters Required to Generate Free‐Breathing Fast‐Helical CT Scans Without Motion‐Artifacts

Abstract

Purpose:A recently proposed 4D‐CT protocol uses deformable registration of free‐breathing fast‐helical CT scans to generate a breathing motion model. In order to allow accurate registration, free‐breathing images are required to be free of doubling‐artifacts, which arise when tissue motion is greater than scan speed. This work identifies the minimum scanner parameters required to successfully generate free‐breathing fast‐helical scans without doubling‐artifacts.Methods:10 patients were imaged under free breathing conditions 25 times in alternating directions with a 64‐slice CT scanner using a low dose fast helical protocol. A high temporal resolution (0.1s) 4D‐CT was generated using a patient specific motion model and patient breathing waveforms, and used as the input for a scanner simulation. Forward projections were calculated using helical cone‐beam geometry (800 projections per rotation) and a GPU accelerated reconstruction algorithm was implemented. Various CT scanner detector widths and rotation times were simulated, and verified using a motion phantom. Doubling‐artifacts were quantified in patient images using structural similarity maps to determine the similarity between axial slices.Results:Increasing amounts of doubling‐artifacts were observed with increasing rotation times > 0.2s for 16×1mm slice scan geometry. No significant increase in doubling artifacts was observed for 64×1mm slice scan geometry up to 1.0s rotation time although blurring artifacts were observed >0.6s. Using a 16×1mm slice scan geometry, a rotation time of less than 0.3s (53mm/s scan speed) would be required to produce images of similar quality to a 64×1mm slice scan geometry.Conclusion:The current generation of 16 slice CT scanners, which are present in most Radiation Oncology departments, are not capable of generating free‐breathing sorting‐artifact‐free images in the majority of patients. The next generation of CT scanners should be capable of at least 53mm/s scan speed in order to use a fast‐helical 4D‐CT protocol to generate a motion‐artifact free 4D‐CT.NIH R01CA096679