Deep transcranial magnetic stimulation for the addiction treatment : Electric field distribution modeling

Deep transcranial magnetic stimulation (dTMS) is a neurostimulation technique for deep brain structures that has recently been successfully applied in the clinic for the treatment of addiction. In contrast to the conventional magnetic stimulation, which uses planar coils [Figure-of-Eight (FoE)] to target specific superficial regions of the brain, dTMS requires the design of complex 3-D coils in order to induce deeply penetrating fields. Recent clinical studies have focused on the use of H4 coils, which utilizes a left-right symmetric structure for bilateral stimulation of the prefrontal cortex, and demonstrated efficacy for therapy such as smoking cessation. The mechanism of activity, however, remains poorly understood, in part because the affected regions of the brain are not known in detail. To this purpose, computational techniques applied to highly detailed inhomogeneous tissue phantoms provide a powerful tool for testing coil efficacy. In this work, we quantified both electric field E distribution and its penetration depth in the prefrontal cortex, induced by a specific Hesed-coil, H4, designed for the addiction treatment and by the traditional FoE coil for comparison. Results show that H4 coil preferentially targets the insula and cingulate cortex. Moreover, it can induce in the deepest tissues E amplitude ranging between 20% and 40% of the cortical peak, and it can penetrate the cortex up to 4 cm with a E > 50% of the cortical peak, thus noticeably increasing the penetration depth of the traditional TMS systems.