A multichannel magnetic stimulation system using submillimeter-sized coils: system development and experimental application to rodent brain in vivo

Objective. Single coil-based systems for magnetic stimulation are widely used for neurostimulation in neuroscience research and clinical treatment of neurological diseases. However, parallelization of magnetic stimulation with multiple coils may generate far greater potential than a single coil, and could thus expand the scope of brain area stimulation. Therefore, we examined whether a multiple coil-based system could improve the effectiveness and focality of conventional single coil-based magnetic stimulation. Approach. We designed and tested a micromagnetic stimulation (mu MS) device with multiple submillimeter-sized coils as a possible substitute for one large coil. Our design concept is spatially-distributed stimulation strategy involving the small number of coils to be able to mimic desired electric field profiles. To this end, the cost function of the error between the desired and coil-induced electric fields was firstly calculated, and coil currents were repetitively estimated to achieve the smaller number of coils under a certain criterion: a minimum error with spatial sparsity. Using these approaches, we evaluated the capability of our multi-channel mu MS via numerical simulations and demonstrated responsive results in animal experiments. Main results. Our approach can enhance control of neural excitation and improve the concentration of the excitation field induced by magnetic stimulation with reduced power consumption. Furthermore, in vivo electrophysiological recordings of mouse brain performed to evaluate our proposed approach for brain stimulation demonstrated experimentally that our multi-channel mu MS device can yield more effective stimulation than the single-channel device. In addition, our device permitted electronic spatial adjustment of the stimulus shape and location without moving the coils. Significance. The development of new multichannel mu MS-based therapeutic approaches may be useful because the mu MS affects only a restricted brain area. Indeed, the small size of micro-coils and their finer focality with multichannel contribution might be suitable for chronic use, which is difficult using conventional large transcranial magnetic stimulation (TMS) with simple round or figure-eight coils. Thus, our findings support new opportunities to explore magnetic stimulation as a therapeutic approach for neurological disorders.