Modeling, measurement and prediction of cardiac magneto-stimulation thresholds

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Project Summary/Abstract In MRI, cardiac stimulation (CS) is the direct stimulation of the heart by rapid switching of the gradient coils, a phenomenon referred to as magneto-stimulation. The Federal Drug Administration (FDA) imposes limits on the speed and amplitude of gradient switching in MRI, however there is an almost complete lack of data on magneto-stimulation, which has resulted in a necessarily restrictive safety limit. Instead, the IEC 60601-2-33 limit for CS in MRI, which is the limit that the FDA relies on, is based on simplified electric field calculations in uniform cylinders that mimic the human torso, and open-chest CS experiments performed in animals using electrodes. Because of the known limitations of this methodology, the IEC built in large safety factors in their choice of the CS limit, thus resulting in a conservative safety recommendation. For example, for the four Siemens Connectome scanners in use, the CS safety limit is more restrictive than the peripheral nerve stimulation limit at long rise times, which is unlikely since conducting cells in the myocardium are harder to stimulate than large peripheral nerves and the heart is deep inside the body, and therefore more shielded from electric fields than shallow nerves. As the IEC MRI cardiac stimulation safety limit becomes more and more restrictive for improving gradient systems, it is essential to study the magneto-stimulation phenomenon in a more detailed manner using direct animal measurements and state-of-the-art simulation methods. To our knowledge, only two groups have published direct measurements of cardiac magneto- stimulation performed, in dogs, in the early 1990s. We propose a multidisciplinary bioengineering research project that aims to measure, for the first time, cardiac magneto-stimulation thresholds in pigs, a widely used animal model for the human cardiovascular system. In addition, we develop a detailed simulation pipeline of cardiac magneto-stimulation using state-of-the-art modeling techniques in order to shed light on the mechanisms of action of CS in MRI and estimate stimulation thresholds in humans for a variety of coils and gradient waveforms. We will work closely with the MT40 sub-committee of the IEC throughout the duration of the project in order to ensure that our work maximally informs the setting of the IEC limit in the next update of the IEC 60601-2-33 standard.