Shielding the magnetic field from a transcranial stimulator using aluminium and iron: Simulation and experimental results

Repetitive transcranial magnetic stimulation (rTMS) is an up-and-coming, noninvasive technique that holds therapeutic promise in a range of neuropsychiatric and neurological diseases. In rTMS, a time-varying magnetic field induces an electric current in the brain. Since its introduction close to 30 years ago, numerous studies have widely recognised it in the research or treatment of several diseases (e.g. epilepsy, Parkinson´s disease, stroke or neuropathic pain). rTMS treatments already occurring in the USA include psychiatric conditions like major depression (approved in 2008), and migraine (approved in 2013). Nevertheless, throughout several years it has been found that the stimulation of subcortical brain structures is inaccessible with standard rTMS equipment. Accessing such deep-brain regions may potentially result in the improvement of a variety of neuropsychiatric and neurological disorders. The design of TMS coils to stimulate deep brain targets is limited by the rapid attenuation of the electric field in depth. This is mainly due to the physical limiting effect arising from the presence of surface discontinuities. To the best of our knowledge the Hesed coil represents the state of the art of clinical deep-brain TMS. Nonetheless, there is no configuration able of producing an effective field at the very center of the brain. We have proposed a TMS system termed orthogonal configuration that is capable of reaching the very center of a spherical brain phantom (at 10-cm depth) with 58% strength in respect to the surface maximum. The high, external magnetic field of this configuration was designed so that it is incapable of inducing heart fibrillation in the patient by four orders of magnitude in respect to its threshold. Nevertheless, Comsol^® AC/DC simulations show that a system operator positioned sideways, 10 cm apart from the orthogonal configuration will experience an induced current density in his heart of 0.7 A/m² (heart fi- rillation threshold is 1 A/m²). Only 3.4 m away from the orthogonal configuration will a heart current density of 0.001 A/m² be achieved. In this work we focus on the shielding aspects necessary to install an orthogonal TMS system providing full safety to patient and any of its operators. For that, we have measured the TMS signal attenuation induced by an iron or aluminium slab of material positioned between a TMS coil and a current density sensor located inside a cylinder container filled with a saline solution (7 S/m, i.e. 5% ^w/v of NaCl in water). Simulations combined with experimental results show that a simple 25-mm-thick slab of aluminium surrounding five walls of the orthogonal TMS system (positioned 40 cm apart from its edges) is enough to achieve a current density in the heart of any operator inferior to 0.001 A/m², i.e. at least three orders of magnitude below fibrillation threshold. This allows us to conclude on the viability of implementing an R&D orthogonal TMS system in the near future.