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Investigating the Minimum Scan Parameters Required to Generate Free-Breathing Fast-Helical CT Scans Without Motion-Artifacts

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D Thomas

D Thomas1*, J Tan2 , J Neylon1 , T Dou1 , S Jani1 , J Lamb1 , D Low1 , (1) University of California, Los Angeles, Los Angeles, CA (2) Washington University in St. Louis, St. Louis, MO.


TH-C-18A-11 Thursday 10:15AM - 12:15PM Room: 18A

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 16x1mm slice scan geometry. No significant increase in doubling artifacts was observed for 64x1mm slice scan geometry up to 1.0s rotation time although blurring artifacts were observed >0.6s. Using a 16x1mm 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 64x1mm 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.

Funding Support, Disclosures, and Conflict of Interest: NIH R01CA096679

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