Real-Time Spinal Cord Motion Tracking for MR-guided Spine SBRT (A Pilot Study)

Abstract

The aim of this study was to assess the feasibility of real-time motion tracking of the spinal cord using 2D-cine MRI on the MR-Linac system. Spinal SBRT delivers high-precision doses to targets near the spinal cord but with the dosimetric cord tolerance sometimes defined using a spinal cord PRV and the treatment alignment visualizing a surrogate measure (spinal canal) with cone beam CT guidance. MR-guided spine SBRT enables clear visualization of the true spinal cord and real-time monitoring of the cord during treatment, which may allow for increased cord dose constraints and offer new treatment options for patients with epidural disease. A Motion Monitoring Research Package (MMRP) that has the similar tracking functionality as the future motion management system for a 1.5 T MR-Linac system was used to track real-time target motion on 2D orthogonal T2-weighted bFFE cine MRI using a reference 3D T2 MRI scan to locate tracking object initially. The spinal cord was contoured from the T2 MRI scan as the tracking object, and the cine images were obtained in sagittal and coronal planes at the center of the contoured spinal cord with a temporal resolution of 1.2 seconds. The motion tracking data was collected for 20 min and retrospectively analyzed using standard deviation of motion was collected from two volunteers immobilized with the Klarity SBRT system on the MR-Linac couch during MRI scans of their cervical, thoracic, and lumbar spines. In the thoracic spinal cord, the was 0.07mm (left-right excursion range: ±0.15mm), 0.15mm (anterior-posterior: ±0.26mm), and 0.27mm (superior-inferior: ±0.53mm). In the lumbar spinal cord, the standard deviation was 0.06mm (±0.12mm), 0.1mm (±0.14mm), and 0.1mm (±0.27mm). In the cervical spine, since the tracking accuracy in Sup/Inf direction was compromised by tilted spine orientation in the orthogonal 2D image, a single entire vertebra including the spinal cord was tracked with a standard deviation of 0.19mm (±0.31mm), 0.27mm (±0.62mm), and 0.25mm (±0.4mm). This study demonstrated the possibility of real-time tracking of the spinal cord using 2D-cine MRI on an MR-Linac system. Future work will aim to increase sample size and evaluate the reproducibility of the tracking results over time. The excursion of the thoracic/lumbar spinal cord was found to be less than 0.5mm, while the anterior/posterior or superior-inferior excursion of the cervical spine was greater than 1.0mm during the 20-minute scan.