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Abstract

Magnetic resonance imaging (MRI) employs time-varying magnetic gradient fields, which induce electric fields (E-fields) in the patient that can potentially stimulate the heart. The performance of novel gradient systems is increasingly restricted by regulatory safety limits, thus motivating a deeper understanding of cardiac stimulation (CS) in MRI. This thesis investigates the thresholds and mechanisms underlying CS using an integrative approach of modeling and measurements. First, a numerical modeling framework was developed that combines E-field simulations in computational body models with electrophysiological cardiac fiber models to predict stimulation thresholds and sites in the heart. The CS thresholds predicted for two commercial gradient systems were >10-fold higher than the regulatory limits. Second, cardiac magnetostimulation thresholds were measured in ten healthy pigs using magnetic field pulses created by capacitor discharges into a coil. The average threshold E-field in the porcine heart was 92.9 pm 13.5 V/m. CS thresholds predicted in individualized porcine models derived from MR images reproduced the measurements with deviations of <18%, thus demonstrating the validity of the model for the experimental magnetic field waveform. The numerical and experimental results presented in this thesis inform the derivation of safe operational limits for MRI without unnecessarily restricting gradient performance and thus imaging speed and resolution.