The distribution of induced currents in magnetic stimulation of the nervous system

Magnetic stimulation of the nervous system is being used as an alternative to electrical stimulation, principally because it is painless. The spatial distribution of induced currents from the stimulating coil is calculated from a computer model with graphical output. Two configurations of a plane circular coil are considered: parallel to the tissue surface and perpendicular to the surface. The surface is assumed planar and infinite in extent. The tissue is modelled as a uniform, isotropic volume conductor. A quasi-static approximation is made in calculating the electric field. Maps of current density, J, as a function of position, including depth, are shown. In both configurations, J is always parallel to the surface, and is maximum at the surface. There is no perpendicular (vertical) current. For a one-turn 10 cm diameter coil, spaced 1 cm from conducting tissue and parallel to it, with rate of change of current 10(8) A s(-1), Jmax = 6.8 A m(-2) (assuming conductivity 0.2 omega -1 m(-1)). In the perpendicular configuration Jmax = 4.1 A m(-2). These results suggest that nerve fibres running parallel to the skin surface are more likely to be stimulated than those running obliquely; and that it is extremely difficult to stimulate nerve fibres running perpendicularly. This model can be used to characterise the performance of other shapes of stimulating coils and the dependence on fibre orientation.