Multiple coils in a conducting liquid for deep and whole-brain transcranial magnetic stimulation. I. Single-frequency excitation

We present a system comprising multiple coils excited at a single frequency and immersed in a conducting liquid allowing for unprecedented deep, whole-brain transcranial magnetic stimulation (TMS). Finite-element methods were applied onto a spherical head model complemented by an ellipsoidal torso. The head model comprises skin, skull, cerebral spinal fluid, and brain tissue. Results show deep-brain induced currents reaching 85% at 10-cm penetration (brain center) in respect to surface (cortex) maximum. For comparison, state-of-the-art published data reach 47% relative induction at 8-cm depth only. This system counterparts well-known limiting effects occurring due to the enhancement of current densities at the brain/surface interface by immersing the stimulating coils (and partially the head of the patient) into a conducting liquid such as an electrolyte solution or a liquid metal. In addition, several system asymmetries are exploited in order to optimize deep-brain stimulation down to the center of the brain. Although current densities induced in the retinas and in the torso are estimated to reach sub-optimum levels in regard to patient safety, these first positive results show that overcoming the brain/surface induction barrier is feasible.