Characterizing magnetic field interactions between an in-room MRI-on-rails and a radiotherapy linac: A comprehensive simulation and experimental study

Background: In recent years, magnetic resonance imaging (MRI)-guided radiotherapy (RT) has experienced a notable increase in utilization due to technological advancements that leverage MRI's superior soft-tissue contrast and its non-invasive, non-ionizing imaging mechanism. Integrating MRI scanners with linac systems comes with several technical challenges, including the complex interactions between the linac components and the magnetic field of the MRI scanner. Purpose: This study presents a comprehensive in silico finite element method (FEM)-based model for a proximity-type MRI-guided RT system consisting of an MRI-on-rail and a C-arm linac. The methodology enables the precise characterization of the MRI magnet's fringe field both in free space and when interacting with the ferromagnetic structure of the linac system. Methods: A comprehensive in silico FEM-based model was developed to simulate the MRI magnet and linac configuration. The magnet coil configuration was generated using linear programming based on the manufacturer's specifications for the 5 G line. The linac structure was modeled from technical schematics and coupled with the simulated magnet to construct the simulation environment. Fringe field measurements were conducted in a controlled environment to validate the simulation results. The measurements were taken at various distances from the MRI isocenter and in different directions to assess the spatial distribution of the fringe field. Results: Simulations showed good agreement with experimental measurements, with a maximum difference of 1 G observed between simulated and measured fringe fields within the 3 to 5 m range from the MR isocenter, consistent with the 1 G Hall probe measurement tolerance. The linac's ferromagnetic structure significantly perturbed the magnetic fringe field, locally increasing field values to 40 G from an initial range of 0–23 G, and inducing local field differences of up to 30 G at its closest proximity to the magnet. Conversely, a local decrease of up to 2 G was observed near the linac isocenter. Furthermore, the room environment influenced the fringe field's spatial distribution, evidenced by deviations of approximately 6 and 4.2 G from the Espree reference field at 2.5 and 3 m, respectively. Despite these environmental effects, the overall agreement between simulations and experimental values, including measurements at the linac head (maximum difference less than 2 G), was highly satisfactory, confirming axial field symmetry and minimal impact from room layout variations. Conclusions: This study developed and validated a comprehensive FEM-based simulation methodology to accurately characterize magnetic field interactions in a proximity-type MRI-guided RT system. The methodology mapped the MRI magnet's fringe field in both free space and as perturbed by the linac's ferromagnetic structure, with experimental data also supporting these findings. This robust framework offers a reliable tool for guiding engineering activities and defining safety bounds for system design.