Search

Purpose: Modern high‐amplitude gradient systems can be limited by the International Electrotechnical Commission 60601‐2‐33 cardiac stimulation (CS) limit, which was set in a conservative manner based on electrode experiments and E‐field simulations in uniform ellipsoidal body models. Here, we show that coupled electromagnetic‐electrophysiological modeling in detailed body and heart models can predict CS thresholds, suggesting that such modeling might lead to more detailed threshold estimates in humans. Specifically, we compare measured and predicted CS thresholds in eight pigs. Methods: We created individualized porcine body models using MRI (Dixon for the whole body, CINE for the heart) that replicate the anatomy and posture of the animals used in our previous experimental CS study. We model the electric fields induced along cardiac Purkinje and ventricular muscle fibers and predict the electrophysiological response of these fibers, yielding CS threshold predictions in absolute units for each animal. Additionally, we assess the total modeling uncertainty through a variability analysis of the 25 main model parameters. Results: Predicted and experimental CS thresholds agree within 19% on average (normalized RMS error), which is smaller than the 27% modeling uncertainty. No significant difference was found between the modeling predictions and experiments (p < 0.05, paired t‐test). Conclusion: Predicted thresholds matched the experimental data within the modeling uncertainty, supporting the model validity. We believe that our modeling approach can be applied to study CS thresholds in humans for various gradient coils, body shapes/postures, and waveforms, which is difficult to do experimentally. [ABSTRACT FROM AUTHOR]

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 ts,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 ts,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 ts,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 (ts,eff = 0.45 ms). [ABSTRACT FROM AUTHOR]