Magnetic stimulation of excitable tissue: Prediction of the eddy-current pathway via a three-dimensional finite-element model
Abstract
Magnetic stimulation of excitable tissue is a consequence of Faraday's law of induction. A time-varying magnetic field near the conducting tissue causes an eddy-current to flow in the tissue. If the eddy-current is of sufficient amplitude and duration, it will stimulate the excitable tissue. The induced current pathways generated by this technique are very different than those generated by directly applied electrodes. Consequently, with magnetic stimulation, it is possible to stimulate deep structures within the body with less sensation than with direct electrodes. Externally-applied pulsed magnetic fields have been shown to be clinically useful in stimulating structures inside the body, although precise current pathways are unknown. Previous investigators have used simplified models to obtain first order approximations to the induced current. Most of these models assume the tissue to be a homogeneous, semi-infinite volume conductor. Such models can not account for the changes in the eddy-current pattern due to the air-tissue interface or tissue conductivity differences. We have developed a three-dimensional numerical technique to solve for the eddy-currents that are induced in magnetic stimulation. This technique accounts for the effects or regions with different conductivities. The technique uses the finite-element method to predict the magnitude and direction of magnetically-induced eddy-currents in a three-dimensional, inhomogeneous medium such as the body. The thorax is a complex structure containing tissues that differ in conductivity by more than a factor of ten. The finite-element technique developed in this research is used, along with experimental measurements, to establish safe limits for exposure to time-varying magnetic fields so that the risk of inducing cardiac arrythmias in patients is minimized.
Degree
Ph.D.
Advisors
Geddes, Purdue University.
Subject Area
Electrical engineering|Biomedical research
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.