Breathing-Frozen Abdominal Organ at Risk Motion Dynamics Characterized Using Golden-Angle Radially Sampled Magnetic Resonance Imaging and Hierarchical Motion Modeling
Abstract
Purpose
Non-breathing physiological motions of abdominal organs at risk (OARs) are insufficiently characterized to support understanding of their dosimetric impact and possible need for monitoring. The dynamics of such motions were characterized in this investigation.
Methods
Golden-angle radially sampled MRI (170 ms intervals) was acquired for 22 patients (5.7 - 19.8-minute scans). Hierarchical motion modeling removed breathing motion-induced deformations, allowing view-sharing reconstructions of non-breathing motions at high and low (4.8- and 17-second) temporal resolutions without breathing-induced blurring, which were aligned sequentially to calculate displacement velocities of voxels within OARs (small bowel, colon, stomach, and duodenum).
Results
Population-averaged velocities with 17-second reconstructions (1.4±1.0, 1.3±1.0, 1.0±0.6, and 2.3±0.7 mm/min for small bowel, colon, stomach, and duodenum, respectively) were smaller than those from 4.8-second intervals (4.1±3.0, 3.9±3.0, 3.5±2.4, and 3.3±2.2 min/mm). No cyclic patterns were observed in the velocities or cumulative displacements for any of the OARs. Although movement was bidirectional, there was a general trend for the OARs to cumulatively drift from their reference locations over time (final displacements of 3.5±2.2, 3.2±2.0, 2.9±1.4, and 2.6±1.3 mm). Displacements appeared to change faster during the first several minutes of observation and generally slowed for the remainder of the scan times. Maximum displacements of 38.3, 22.4, 20.6, and 17.3 mm were observed in individual patients. Large variation in individual patient dynamics was observed.
Conclusion
Non-breathing abdominal motions are potentially significant in magnitude and velocity and are not readily observed to be cyclic within the time scales of radiation therapy treatment fractions, indicating potential need to further characterize and/or manage such motions during planning and treatment delivery. The change in rate of displacement suggests potential benefit of delaying IGRT until after several minutes of the patient lying down, and inter-subject variation in motions suggests the possible advantages of individual characterization and/or non-breathing motion monitoring for treatment.