Design and Demonstration In Vitro of a Mouse-Specific Transcranial Magnetic Stimulation Coil

Abstract

Transcranial magnetic stimulation (TMS) is a technique used to treat different neurological disorders non-invasively. A pulsed current to a coil generates a magnetic field (<inline-formula> <tex-math notation= LaTeX >$B$ </tex-math></inline-formula>-field) which induces an electric field (<inline-formula> <tex-math notation= LaTeX >$E$ </tex-math></inline-formula>-field). Underlying biophysical effects of TMS are unclear. Therefore, animal experiments are needed; however, making small TMS coils suitable for mice is difficult and their field strengths are typically much lower than for human sized coils. <italic>Objectives/Hypothesis</italic>. We aimed to design and demonstrate a mouse-specific coil that can generate high and focused <inline-formula> <tex-math notation= LaTeX >$E$ </tex-math></inline-formula>-field. <italic>Methods</italic>. We designed a tapered TMS coil of 50 turns of 0.2 mm diameter copper wire around a 5 mm diameter tapered powdered iron core and discharged a <inline-formula> <tex-math notation= LaTeX >$220~\\mu \\text{F}$ </tex-math></inline-formula> capacitor at 50 V through it. We measured <inline-formula> <tex-math notation= LaTeX >$B$ </tex-math></inline-formula>-field with a Hall probe and induced <inline-formula> <tex-math notation= LaTeX >$E$ </tex-math></inline-formula>-field with a wire loop. We measured temperature rise with a thermocouple. We applied 1200 pulses of continuous theta burst stimulation (cTBS) and intermittent theta burst stimulation (iTBS) to mouse brain slices and analyzed how spontaneous electrical activity changed. <italic>Results</italic>. The coil gave maximum <inline-formula> <tex-math notation= LaTeX >$B$ </tex-math></inline-formula>-field of 685 mT at the base of the coil and 340 mT at 2 mm below the coil, and maximum <inline-formula> <tex-math notation= LaTeX >$E$ </tex-math></inline-formula>-field 2 mm below the coil of approximately 10 V/m, at 50 V power supply, with a temperature increase of 20 °C after 1200 pulses of cTBS. We observed no changes in <inline-formula> <tex-math notation= LaTeX >$B$ </tex-math></inline-formula>-field with heating. cTBS reduced frequency of spontaneous population events in mouse brain slices up to 20 min after stimulation and iTBS increased frequency up to 20 min after stimulation. No frequency changes occurred after 20 min. No changes in amplitude of spontaneous events were found. <italic>Conclusion</italic>. The design generated fields strong enough to modulate brain activity <italic>in vitro</italic>.