Feasibility of ultra-low-field MRI in an ion beam treatment room for online MR-guided particle therapy

Abstract

Background and purpose: Online MR-guided particle therapy may improve beam delivery precision and treatment efficacy. Ultra-low-field (ULF) MRI may enable image guidance with minimal modification of existing facilities. The feasibility of operating a ULF magnetic resonance (MR) scanner for online MR-guided particle therapy in an unmodified ion-beam therapy environment was investigated. Material and methods: A portable 49 mT ULF-MRI system (OSI2 ONE MR scanner) was placed at the isocenter of a research room at an ion-beam therapy facility without radio-frequency shielding. Image quality was assessed using a grid phantom under static and dynamic irradiation conditions for protons and carbon ions. Electromagnetic interference (EMI) was characterized and mitigated via grounding and local shielding. Beam deflections were quantified using a scintillation detector and compared to numerical simulations. The impact of ongoing proton and carbon ion irradiation on the imaging process was assessed. Dosimetric measurements were performed using a Semiflex ionization chamber in a PMMA phantom with and without simultaneous MR imaging. Results: Stable image quality was achieved across proton and carbon ion irradiation with no relevant irradiation-induced artifacts. Initially substantial EMI was reduced to near-baseline levels by internal shielding. Proton beam deflections ranged from 0.9–1.7 mm at isocenter, consistent with simulations. Dosimetric deviations were below 0.05% for 2D fields and 0.02% for spread-out Bragg peaks (SOBP). Conclusion: ULF-MRI operation in an ion-beam therapy environment without a dedicated shielded room is feasible, without compromising imaging, beam delivery, or dosimetry. This approach supports future MR-guided particle therapy research with reduced infrastructural requirements.