Khokhar, F., Wilson, M. T., & Steyn-Ross, D. A. (2019). Designing, measuring and modelling a small-scale coil and stimulation circuit for Transcranial Magnetic Stimulation (TMS). In New Zealand Institute of Physics Conference 2019. Christchurch.
Permanent Research Commons link: https://hdl.handle.net/10289/12542
In TMS rapid electromagnetic fields (EM) are applied to the brain. The fundamental effects of TMS are poorly understood so we need smaller coils to perform invasive measurements on mice. Based on established physics principles we designed and built cylindrical TMS coils consisting of 25 and 50 turns of 0.2 mm diameter copper wire around 5 mm diameter powdered iron cores and discharged a capacitor through these coils. We measured the magnetic flux densities (B-field) of 25 and 50 turn TMS coils with a Hall probe as 907 mT, 750 mT and the induced electric fields (E-field) with a wire loop as 10 – 27 V/m, 10 – 22 V/m at 50 V respectively. We have also modelled the coils with COMSOL Multiphysics which gave similar B-fields for 25 and 50 turn of around 900 mT, 700 mT and E- fields as 20 – 25 V/m, 15 – 20 V/m respectively. The temperature for 25 and 50 turn TMS coils increase by 52 °C and 71 °C respectively after 1200 pulses at 5 Hz. We tried to modulate the activity of a mouse brain slice by stimulating with the 50 turn TMS coil 3 mm above the mouse brain slice with 50 V using a B-field of 500 mT, 600 pulses at 5 Hz. There was no hint that the TMS has changed the activity. Although these coils perform better than previously constructed mouse coils, the EM fields are still considerably lower than human TMS coils of 2 T and 250 V/m.
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