The strength and spread of the electric field induced by transcranial rotating permanent magnet stimulation in comparison with conventional transcranial magnetic stimulation

Background: Weak or low intensity transcranial stimulation of the brain, such as low field magnetic stimulation and electrical stimulation, can produce significant functional and therapeutic neuromodulatory effects. New method: We have recently developed a portable wearable multifocal brain stimulator called transcranial rotating permanent magnet stimulator (TRPMS) that uses rapidly spinning high field strength permanent magnets attached to a cap. It produces oscillatory stimuli of different frequencies and patterns. Here we compared the strengths and spatial profiles of the changing magnetic fields of a figure-of-eight transcranial magnetic stimulator (TMS) coil, a TRPMS prototype, and a scaled-up version of TRPMS. We measured field strengths and directions of voltages induced in a magnetic field sensor oriented along all three orthogonal axes. Results and comparison with existing methods: The spatial spread of the TRPMS-induced electric field is more restricted, and its shape and strength vary less with the orientation of the inductance than TMS. The maximum voltage induced by the current prototype is similar to 7% of the maximal TMS output at depths corresponding to the human cerebral cortex from the scalp surface. This field strength can be scaled up by a factor similar to 8 with a larger diametrically magnetized magnet. These comparative data allow us to estimate that intracortical effects of TRPMS could be stronger than other low intensity stimulation methods. Conclusions: TRPMS might enable greater uniformity, consistency and focality in stimulation of targeted cortical areas subject to significant anatomical variability. Multiple TRPMS microstimulators can also be combined to produce patterned multifocal spatiotemporal stimulation.