177 - MRI for Spinal Cord Visualization near Surgical Hardware to Replace CT Myelogram for Spine Sarcoma Radiotherapy

S. N. Huh¹, J. Y. Park^1,2, P. Johnson^1,2, E. D. Brooks^1,2, D. J. Indelicato^1,2, and R. Dagan^1,2; ¹Department of Radiation Oncology, University of Florida College of Medicine, Gainesville, FL, ²University of Florida Health Proton Therapy Institute, Jacksonville, FL

Purpose/Objective(s): We developed a novel MRI sequencing approach in spine sarcoma with spinal fusion hardware to eliminate the need for a CT myelogram (CTM) in spinal cord visualization for radiotherapy. Classically, spinal hardware prevents visualization of the cord for sparing due to clover artifacts in MRI. However, the MRI solution can replace an invasive CTM, which carries the risk of infection, cord damage, and contrast reaction. Materials/

Methods: MRI sequences using 3D balanced fast field echo (bFFE) were optimized in a phantom test using titanium screws. The sequence was adapted to clinical cases with lumbar spine hardware, such as eight titanium screws, two stabilizing rods, and an interbody fusion cage made of polyether-ether-ketone. The 3D bFFE was employed to have a relatively steady-state precession with TR/TE = 8.8/4.0 msec, flip angle of 45° to 60°, phase modulation in AP/PA direction, and slice thickness of 2 mm to visualize the spinal cord with minimized clover artifacts. We also acquired T1-weighed high-resolution isotropic volume examination (THRIVE) images for tumor delineation in addition to visualization of spinal cord and neural elements for sparing. The spinal cord in 3D-bFFE was compared with CTM co-registered with MRIs. The developed MRI sequences were applied to other bone sarcomas with metallic hardware.
Results: The optimized MRI sequence demonstrated that the spinal cord agreed with CTM with suppressed hardware artifacts. The image distortion in the 3D bFFE images was minimal and within 1 mm compared to CT-based anatomies, except for clover artifacts shown at a 3 mm distance from the titanium screw heads. The sequence using the low-tesla MRI protocol was powerful in delineating residual and recurrent tumors to create a sharp dose gradient in planning to spare the spinal cord effectively.
Conclusion: The MRI sequence developed can replace the invasive CTM procedure for spine sarcoma radiotherapy planning in patients with spinal hardware. Future implications also include monitoring tumor progression with artifact-free MR surveillance. Further reduction of clover artifacts around screw heads is underway.