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

Abstract

Purpose: We characterize electromagnetic (EM) radiation patterns and levelsin conventional MRI systems as a function of field strength and load symmetry,providing a framework for mitigation strategies allowing operation without ashielded room. Methods: We simulated the far-field radiation pattern and fields at a 10 mradius (|E|10m and |B|10m ) for a solenoidal superconducting MRI with abody birdcage coil operated between 0.25T and 6.5T. Five load configura-tions probed the impact of load-symmetry, ranging from a sphere to a bodyload (least-symmetric). We also assessed simple layered EM absorbers at thebore-ends. Results: All configurations exceeded regulatory limits for realistic transmit lev-els. At 1.5T, a 300 V rms RF-pulse is 2700-fold the |E|10m limit. Field strengthand load symmetry strongly modulate radiation patterns and levels. The radi-ated power increased by more than four orders of magnitude from 0.25T to6.5T. 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 alongthe bore (2T–6.5T). Transition patterns were seen between these regimes; uni-form radiation at 0.75T and a combined donut/bore-directed pattern at 1.75T.Load asymmetry increased both strength and pattern asymmetry, with the bodyload having the highest and least symmetric radiation with the legs facilitat-ing wave propagation at high-fields. A simple optimized layered absorber atscanner’s service-end reduced 3T peak radiation by 11 dB. Conclusion: Radiation from unshielded scanners far exceeds regulatory lim-its, particularly at high-field. Mitigation strategies must address load-symmetry,field strength, and wave effects.