Key result
In pigs, the average dB/dt threshold for cardiac magnetostimulation was 1.66 ± 0.23 kT/s, which is 11-fold greater than the IEC limit, indicating current safety limits are conservative.
Why the study?
Powerful MRI gradient systems can exceed IEC cardiac stimulation limits, but magnetically induced thresholds have only been measured in vivo in few canine studies, complicating extrapolation to humans.
In vivo measurements of cardiac magnetostimulation thresholds in pigs indicate that current IEC cardiac safety limits for MRI gradient systems are highly conservative for the investigated stimulus duration.
Should not yet relax IEC cardiac stimulation limits; first porcine magnetostimulation data leave open human translation and other pulse durations.
Purpose Powerful MRI gradient systems can surpass the International Electrotechnical Commission (IEC) 60601‐2‐33 limit for cardiac stimulation (CS), which was determined by simple electromagnetic simulations and electrode stimulation experiments. Only a few canine studies measured magnetically induced CS thresholds in vivo and extrapolating them to human safety limits can be challenging. Methods We measured cardiac magnetostimulation thresholds in 10 healthy, anesthetized pigs using capacitors discharged into a flat spiral coil to produce damped sinusoidal waveforms with effective stimulus duration t s,eff = 0.45 ms. Electrocardiography (ECG), blood pressure, and peripheral oximetry signals were recorded to determine threshold coil currents yielding cardiac capture. Dixon and CINE MR volumes from each animal were segmented to generate porcine‐specific electromagnetic models to calculate dB/dt and E‐field values in the porcine heart at threshold. For comparison, we also simulated maximum dB/dt and E‐field values created by three MRI gradient systems in the heart of a human body model. Results The average dB/dt threshold estimated in the porcine heart was 1.66 ± 0.23 kT/s, which is 11‐fold greater than the IEC dB/dt limit at t s,eff = 0.45 ms, and 31‐fold greater than the maximum value created by the investigated MRI gradients in the human heart. The average E‐field threshold estimated in the porcine heart was 92.9 ± 13.5 V/m, which is 6‐fold greater than the IEC E‐field limit at t s,eff = 0.45 ms and 37‐fold greater than the maximum gradient‐induced E‐field in the human heart. Conclusion This first measurement of cardiac magnetostimulation thresholds in pigs indicates that the IEC cardiac safety limit is conservative for the investigated stimulus duration ( t s,eff = 0.45 ms).
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Klein et al. (2022) studied Healthy (n=10). Magnetostimulation vs. IEC 60601-2-33 limit was evaluated on Cardiac magnetostimulation threshold (dB/dt and E-field). In pigs, the average dB/dt threshold for cardiac magnetostimulation was 1.66 ± 0.23 kT/s, which is 11-fold greater than the IEC limit, indicating current safety limits are conservative.
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