Low-cost monophasic transcranial magnetic stimulator

Abstract

Transcranial magnetic stimulation (TMS) excites neurons noninvasively by electromagnetic induction and is used in neurophysiology research and in approved therapy for depression. Commercial stimulators cost tens of thousands of dollars. Existing open-source designs are either low-energy and unvalidated or rely on expensive switches and laboratory infrastructure. We present a monophasic, fixed-pulse-shape TMS device built at a parts cost of ~USD 700 which, under specific modeling assumptions, can exceed average human motor thresholds. Our design assumes access to basic, off-the-shelf equipment such as a 24 V power supply unit, an oscilloscope, and a few basic tools. The device charges a 230 F film-capacitor bank and discharges it through a self-wound figure-of-eight coil using a thyristor, producing a fixed pulse with a positive lobe lasting approximately 90 s. A Zero-Voltage Switching (ZVS) driver-based charging circuit charges the capacitor bank up to 1460 V from a 24 V bench supply. Three galvanically isolated voltage domains, redundant interlocks, and passive and active discharge paths help mitigate the safety risks involved with handling lethal energy levels. We also present a low-cost way to characterize the device by reconstructing coil di/dt from pickup-coil dB/dt maps to estimate the induced cortical E-fields. At the maximum capacitor voltage, the recovered maximal di/dt is 110.86 A/s, giving estimated 99.9th percentile cortical E-fields of 159 V/m at Oz and 196 V/m at C3 on an example anatomy. Although not yet approved for clinical trials and routine stimulation, the device demonstrated the possibility of a cost-effective TMS unit.