Magnetic nerve stimulation: field focality and depth of penetration

Magnetic nerve stimulation is a non-invasive method of exciting neural tissue. The major limitation of using magnetic stimulation is the lack of a focused field. At sufficiently high magnetic pulses the diffused field not only stimulates the target population of neurons, but also stimulates adjacent structures as well. Further, for deeply penetrating fields, as is the case in transcranial stimulation, excessively high amplitude current pulses are required in the coils because a significant fraction of the field energy is spread throughout the tissue under the coil. In this paper we propose two new coil designs that can be used for magnetic stimulation of the peripheral or central nervous system. The purpose of the design was to increase field focality and depth of penetration. The magnetic fields produced by these coils, when driven by biphasic pulses, were simulated using a finite element technique coupled with a transient solver. The resultant field densities and gradients were compared with those obtained from the commonly used Figure-8 coil. Both the air core and the ferromagnetic core designs have superior results when compared to the Figure-8 coil.