Figure 11

Contour map of predicted change in axon transmembrane potential in response to magnetic stimulation by a single circular coil of radius, R, at the body surface (z = 0). The horizontal axis represents radial distance from the z-axis. The vertical axis represents depth into a semi-infinite volume of tissue. The origin is located at the center of the coil. The width of the map is two coil diameters. The height of the map is one coil diameter. The contour interval is 20 mV. In three dimensions zones of equivalent neural stimulation form nested toroidal rings, having a major radius roughly equal to coil radius and increasing slightly with depth. The zone of effective neural stimulation is roughly one half coil diameter in depth for this standardized test case (dI/dt = 10⁸ A/sec, one turn coil, Lcosθ = 1 cm).