Feasibility of real-time motion tracking using cine MRI during MR-guided radiation therapy for abdominal targets.

Abstract

Real-time high soft-tissue contrast magnetic resonance imaging (MRI) from the MR-Linac offers the best opportunity for accurate motion tracking during radiation therapy delivery via high-frequency two-dimensional (2D) cine imaging. This work investigates the efficacy of real-time organ motion tracking based on the registration of MRI acquired on MR-Linac. Algorithms based on image intensity were developed to determine the three-dimensional (3D) translation of abdominal targets. 2D and 3D abdominal MRIs were acquired for 10 healthy volunteers using a high-field MR-Linac. For each volunteer, 3D respiration-gated T2 and 2D T2/T1-weighted cine in sagittal, coronal, and axial planes with a planar temporal resolution of 0.6 for 60s was captured. Datasets were also collected on MR-compatible physical and virtual four-dimensional (4D) motion phantoms. Target contours for the liver and pancreas from the 3D T2 were populated to the cine and assumed as the ground-truth motion. We performed image registration using a research software to track the target 3D motion. Standard deviations of the error (SDE) between the ground-truth and tracking were analyzed. Algorithms using a research software were demonstrated to be capable of tracking arbitrary targets in the abdomen at 5Hz with an overall accuracy of 0.6mm in phantom studies and 2.1mm in volunteers. However, this value is subject to patient-specific considerations, namely motion amplitude. Calculation times of<50ms provide a pathway of real-time motion tracking integration. A major challenge in using 2D cine MRI to track the target is handling the full 3D motion of the target. Feasibility to track organ motion using intensity-based registration of MRIs was demonstrated for abdominal targets. Tracking accuracy of about 2mm was achieved for the motion of the liver and pancreatic head for typical patient motion. Further development is ongoing to improve the tracking algorithm for large and complex motions.