Khokhar, F. A., Steyn-Ross, D. A., & Wilson, M. T. (2018). Designing, measuring and modelling a small-scale coil and stimulation circuit for transcranial magnetic stimulation. In K. Hillman (Ed.), Proceedings of the 36th Australasian Winter Conference on Brain Research (Vol. 2018). Conference held Queenstown, New Zealand.
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.
