KEY POINTS
- Investigators placed a portable 49 mT ultra-low-field MRI scanner directly at the treatment isocenter of an unmodified MedAustron ion-beam research room. The system operated without a dedicated RF-shielded room and was tested during both proton and carbon-ion delivery.
- Imaging used a 3D RARE sequence with 2 × 2 × 5 mm³ resolution and an acquisition time of approximately 4 minutes. Static proton beams, dynamically scanned carbon-ion beams and homogeneous proton fields were delivered during MRI acquisition.
- Electromagnetic interference was initially substantial, but grounding and local RF shielding reduced the measured noise factor from 11.96 to 1.09 relative to baseline. Adding internal shielding improved signal-to-noise ratio by approximately 20% compared with grounding alone.
- Simultaneous beam delivery produced no relevant irradiation-induced imaging artifacts. Image geometry, signal-to-noise ratio and peak signal-to-noise ratio remained essentially unchanged during proton and carbon-ion irradiation.
- The MRI magnetic field produced measurable but small proton-beam displacement. Calculated lateral offsets at isocenter were approximately 2.1 mm at 62.4 MeV, 1.3 mm at 148.2 MeV and 0.9 mm at 252.7 MeV, without additional spot deformation.
- Beam characteristics remained highly stable. Field size, penumbra and symmetry agreed within 0.9%, while measured proton doses with and without simultaneous MRI differed by no more than 0.05% for monoenergetic fields and 0.02% for spread-out Bragg peaks.
- This remains a phantom-based proof-of-principle experiment. Bragg-peak shifts were not directly measured, imaging parameters were not optimized, and the prototype had limited SNR. Clinical imaging, treatment-planning integration and patient-level online adaptation remain to be demonstrated.
CLINICAL TAKEAWAY
Ultra-low-field MRI could offer a technically simpler route toward online MR guidance in proton and ion therapy without requiring the infrastructure of a conventional high-field MR system. The engineering results are promising, but this is still an experimental phantom study rather than a clinically deployable MR-guided particle therapy platform.