Influence of coil geometry on localization of the induced electric field in magnetic (eddy-current) stimulation of excitable tissue

Magnetic stimulation is the process of stimulating excitable tissue with an electric field which is induced by a time-varying intense magnetic field. Calculations were performed to investigate the effect of coil geometry on localization of the induced electric field. The calculations considered a pair of coplanar coils parallel to a planar conducting medium that represents the stimulated tissue and a coil on edge. These two coil geometries give similar localization in a plane that is parallel to the interface, and the best localization is obtained for a coil radius that is small compared to the target depth. A drawback of the small coil is that the field at the target site is much less than the field at the surface of the conductor. A good three-dimensional localization is obtained with a pair of coplanar coils having radii approximately twice the target depth