Predicting Magnetostimulation Thresholds in the Peripheral Nervous System using Realistic Body Models

Mathias Davids,Matthias Malzacher,Bastien Guérin,Lothar R Schad,Lawrence L Wald

doi:10.1038/s41598-017-05493-9

Abstract

Rapid switching of applied magnetic fields in the kilohertz frequency range in the human body induces electric fields powerful enough to cause Peripheral Nerve Stimulation (PNS). PNS has become one of the main constraints on the use of high gradient fields for fast imaging with the latest MRI gradient technology. In recent MRI gradients, the applied fields are powerful enough that PNS limits their application in fast imaging sequences like echo-planar imaging. Application of Magnetic Particle Imaging (MPI) to humans is similarly PNS constrained. Despite its role as a major constraint, PNS considerations are only indirectly incorporated in the coil design process, mainly through using the size of the linear region as a proxy for PNS thresholds or by conducting human experiments after constructing coil prototypes. We present for the first time, a framework to simulate PNS thresholds for realistic coil geometries to directly address PNS in the design process. Our PNS model consists of an accurate body model for electromagnetic field simulations, an atlas of peripheral nerves, and a neurodynamic model to predict the nerve responses to imposed electric fields. With this model, we were able to reproduce measured PNS thresholds of two leg/arm solenoid coils with good agreement.

Highlights

Of electric fields, the magnitude of the local electric field is shaped by the geometry of the conductive tissues
The applied magnetic field is typically known. Based on this magnetic field, many approaches have used analytic or semi-analytic methods in order to describe the induced electric field and its effect on the nerve membrane[21,22,23,24,25], but analytic solutions are restricted to simple geometries and not applicable to complex structures of differing electrical properties such as those found in the body
In order to predict the stimulation thresholds that a person would experience from the magnetic and electric fields created by a specific coil winding pattern, a complete model of the peripheral nervous system is needed together with the map of the time-varying E-fields created by the coil within the conductive body model

Summary

Introduction

Of electric fields, the magnitude of the local electric field is shaped by the geometry of the conductive tissues. A more accurate approach to assess the electric fields in the body is to use heterogeneous tissue models in conjunction with nerve membrane models to investigate the mechanism of magnetostimulation[29,30,31] In these works, the simulations have been performed based on simplified nerve segments which does not allow for PNS threshold prediction unless it is a priori known that the analyzed segment has the lowest threshold in the body. Performing this simulation for different coil designs allows for comparison of the design’s PNS thresholds during the design/optimization stage, prior to coil construction

Methods

Results

Conclusion