Designing, measuring and modelling a small-scale coil and stimulation circuit for transcranial magnetic stimulation

Abstract

In Transcranial Magnetic Stimulation (TMS) rapid electromagnetic (EM) fields are applied to the brain, via an external current-carrying coil. This technique has been tried for many neurological disorders such as stroke, Parkinson’s disease and major depression. The fundamental effects of TMS are poorly understood so there is a need to carry out invasive measurements on mice to gain deep understanding about the underlying principles of TMS. However, we require smaller coils than used for a human, equivalent to the size of the mouse brain. Based on established physics principles we designed and built a cylindrical coil consisting of 50 turns of 0.2 mm diameter copper wire around a 4 mm diameter soft ferrite core. We built a simple electronic circuit to discharge a capacitor through this coil. With an applied voltage of 45 V, we measured the magnetic flux density (B-field) with a Hall probe as 338 mT and induced electric field with a wire loop as 10 – 15 V/m. The temperature increased by 31°C after 1200 pulses at 5 Hz. We modelled the coil using MATLAB which gave similar B-field and E field results of around 500 mT and 8 V/m respectively. Although this coil performs better than previously constructed mouse coils, the EM fields are still considerably lower than those of typical human coil of 2 T and 250 V/m. This now allows us to stimulate mouse brains with higher B-fields and E-fields than in previous experiments.