Simulation of dB/dt-Over-Electric Field Cardiac Magnetostimulation Safety Ratios in 75 Body Models and 18 Gradient Systems

The IEC 60601-2-33 standard provides consensus-based safety provisions for MRI equipment. Protection of patients against cardiac stimulation (CS) is based on limiting the maximum E-field induced by MRI gradient coils. In practice, this is achieved by imposing a conservative dB/dt threshold on any gradient waveform. The dB/dt-over-E-field conversion ratio currently used in IEC 60601-2-33 was derived in a homogeneous ellipsoid exposed to a uniform B-field and is 10 (T/s)*(V/m)^-1. This limit is becoming increasingly restrictive in high performance clinical systems. We therefore evaluate dB/dt-over-E-field ratios in realistic body models and coils using state-of-the-art electromagnetic simulations.

We performed two independent simulation studies in a total of 75 realistic body models and 13 commercial gradient systems and derived dB/dt-over-E-field ratios in the heart. We thresholded the E-field maps to mitigate the impact of staircasing artifacts in boundary voxels between the myocardium and the lungs.

Thresholding the E-field maps at the 99th percentile E-field value (E99) eliminates staircasing artifacts in both simulation studies. Study #1 predicts a larger range of dB/dt-over-E99 ratios (13-53 (T/s)*(V/m)^-1) than study #2 (12-35 (T/s)*(V/m)^-1). Despite differences in EM solvers, body models, coils, mesh resolution, and post-processing, both studies find similar worst-case ratios of dB/dt-over-E99 of 12-13 (T/s)*(V/m)^-1.

Our simulations of dB/dt-over-E-field ratios for cardiac safety in MRI cover a large range of realistic clinical scenarios. An increase of the allowable dB/dt beyond the current CS limit in IEC 60601-2-33 may be feasible.