Coil design considerations for deep-brain transcranial magnetic stimulation (dTMS).

Deep-brain transcranial magnetic stimulation (dTMS) could provide new, non-invasive therapeutic options for various psychiatric and neurological disorders. Figures of merit (FoMs) are proposed to evaluate and compare dTMS coil designs. These FoMs characterize the depth of electric field penetration, scalp stimulation, focality, and energy. Two coil configurations potentially suitable for dTMS are analyzed: circular crown coil and C-core coil. These coils have significantly less attenuation of the electric field strength in depth, compared to conventional TMS coils. In the limiting case as the coil dimensions become large relative to the head, the electric field decay in depth becomes linear, which indicates that, at best, the electric field attenuation is directly proportional to the depth of the target. The charge density and heating induced in the brain are at safe levels, but the risk of unintended neuromodulation and seizures with dTMS has to be evaluated further. Preliminary simulation results suggest that the crown coil has the best overall performance for dTMS. Finally, synchronous firing of all TMS coil elements appears more effective at stimulating deep neurons than is sequential firing.