MR Simulation for Radiation Therapy
Role of MR-Sim
MR simulation provides superior soft-tissue contrast for target and OAR delineation, especially brain, prostate, head/neck, and pelvis. Understand the workflow: patient setup in treatment position, immobilization compatible with MR (non-ferromagnetic masks/cradles), indexing to the couch, and acquisition of planning-grade images.
Geometry and positioning
Critical: the MR coordinate system must map to the treatment couch. Use flat table inserts, couch indexing, and fiducial markers when needed. Ensure the isocenter is set and documented. Patient position (supine, arms up) must match treatment.
Sequences for RT
Acquire a large-FOV, distortion-minimized dataset (often T2 or balanced SSFP) plus optional contrast and diffusion. Multiplanar or 3D acquisitions support delineation; use fat suppression for some sites. Consider respiratory motion management (gating, breath-hold) for thorax/abdomen.
Traps
Questions may confuse diagnostic MR (patient comfort, surface coils) with MR-Sim (geometric fidelity, RT positioning). Immobilization must be MR-compatible; a standard thermoplastic mask may contain ferromagnetic components. Distortion correction is essential for accurate dose calculation and registration.
Rule of thumb
Treat every MR-Sim image as a potential primary planning dataset: verify geometry with a phantom and fiducials, and document couch/isocenter coordinates.
Worked example
For prostate MR-Sim, use a flat couch insert, indexed immobilization, T2-weighted 3D acquisition with large FOV, and confirm isocenter alignment before exporting to the planning system.