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

We present a system comprising multiple coils immersed in a conducting liquid allowing for unprecedented deep, whole-brain transcranial magnetic stimulation (TMS). The system exploits a configuration of large-coils with dedicated spatial asymmetries in order to spare current induction in the retinas and in the human trunk, optimizing deep-brain stimulation down to the center of the brain. The resulting axially-induced currents of this system are prone to magnetic steering by means of dipole and quadrupole-focusing electromagnet (DC) assemblies, with its viability also analyzed in this work. 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, not yet comprising the magnetic-steering capability of the system, show deep-brain induced currents reaching 53% 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. These results may potentially enhance clinical applicability of TMS in a number of pathologies such as Alzheimer´s disease, parkinsonism, depression, sleep disorders, pain management, stroke rehabilitation, tinnitus, trigeminal neuralgia, brain palsy,and resolution of epileptic seizures, among others.