Motion correction in CBCT imaging using gate-less model-based reconstruction of non-rigid motion and images.

backgroundCone beam computed tomography (CBCT) plays a crucial role in verifying patient positioning during radiotherapy. However, CBCT images are often blurred by motion originating from breathing, bowel motion, and/or repositioning. Conventional methods often employ gating techniques to mitigate motion artifacts by assuming periodicity, which is restricted to respiratory motion. However, this does not resolve a-periodic motion such as irregular breathing. PURPOSE: The purpose of this study is to estimate and correct for both periodic motion and irregular motion. We present a novel method for gate-less reconstruction of motion and images (CBCT-MOTUS), thereby estimating and correcting for all non-rigid 3D motion at high temporal frequency (per-projection temporal resolution of 182 ms).

methodsStarting from a motion-corrupted image, we perform the reconstruction process by alternating between a motion estimation step and an image correction step. Motion estimation is model-based, that is, it is performed directly in projection space by comparing acquired projections to simulated projections that take motion-fields into account. The optimization process benefits from assumptions, including (i) compressing the motion-fields using B-spline parameterization to reduce the number of parameters to estimate, (ii) exploiting spatio-temporal correlation of motion using a low-rank motion model, and (iii) enforcing smoothness using spatial regularization to include a priori knowledge on motion-fields. High temporal resolution (182 ms for this scanner) is achieved as motion-fields are estimated for each acquired gantry angle. The method is validated in silico, on phantoms and on clinical in vivo acquisitions.

resultsThe framework can estimate and correct irregular and periodic motion with high temporal resolution (per-projection temporal resolution of 182 ms). The motion-corrected images show improved image features for all experiments with reduction of motion artifacts, such as deblurring on organ interfaces.

conclusionsWe have developed and validated a gate-less reconstruction method (CBCT-MOTUS) for model-based motion estimation and correction in CBCT imaging for any kind of motion. High temporal resolution (per-projection temporal resolution of 182 ms) is achieved by exploiting the smoothness and compressibility of motion using a low-rank B-spline motion model, enabling the correction for non-rigid irregular and periodic motion. These proof-of-principle results warrant further clinical validation.