Simulated radiation levels and patterns of MRI without a Faraday shielded room.

Abstract

Purpose: We characterize electromagnetic (EM) radiation patterns and levels in conventional MRI systems as a function of field strength and load symmetry, providing a framework for mitigation strategies allowing operation without a shielded room.

Methods: We simulated the far-field radiation pattern and fields at a 10 m radius (|E|_10m and |B|_10m) for a solenoidal superconducting MRI with a body birdcage coil operated between 0.25T and 6.5T. Five load configurations probed the impact of load-symmetry, ranging from a sphere to a body load (least-symmetric). We also assessed simple layered EM absorbers at the bore-ends.

Results: All configurations exceeded regulatory limits for realistic transmit levels. At 1.5T, a 300 V_rms RF-pulse is 2700-fold the |E|_10m limit. Field strength and load symmetry strongly modulate radiation patterns and levels. The radiated power increased by more than four orders of magnitude from 0.25T to 6.5T. Spherical load radiation transitioned from a peak gain at the bore-ends (0.25-0.5T) to a donut-shaped pattern, suggesting current loops around the bore (1 T-1.5T), back to bore-axis-directed gain, suggesting propagating waves along the bore (2T-6.5T). Transition patterns were seen between these regimes; uniform radiation at 0.75T and a combined donut/bore-directed pattern at 1.75T. Load asymmetry increased both strength and pattern asymmetry, with the body load having the highest and least symmetric radiation with the legs facilitating wave propagation at high-fields. A simple optimized layered absorber at scanner's service-end reduced 3T peak radiation by 11 dB.

Conclusion: Radiation from unshielded scanners far exceeds regulatory limits, particularly at high-field. Mitigation strategies must address load-symmetry, field strength, and wave effects.