A complete model for the evaluation of the magnetic stimulation of peripheral nerves

In this paper, a numerical procedure for the analysis of peripheral nerve excitation through magnetic stimulation is presented and used to investigate the physical parameters influencing stimulation. The finite difference technique is used to evaluate the electric field distribution induced inside an arm by the current flowing through a coil, and a nonlinear cable model is used to describe the response of the nerve fiber to the induced electric field. The comparison among several forearm structures has evidenced that the heterogeneous non dispersive forearm model is a good reference condition. With this model, the lowest charging voltage on the stimulator capacitance, able to produce the nerve stimulation, is achieved when the coil is shifted, with respect to the nerve, of a quantity slightly lower than the coil radius but it is also possible to excite the nerve fiber by applying a shift equal to zero. The charging voltage increases when the coil radius is increased and when a three-dimensional coil geometry is considered. Moreover, this voltage is strongly dependent on the nerve position inside the forearm and on the kind of tissue surrounding the nerve.