Design and demonstration of a mouse-specific Transcranial Magnetic Stimulation coil in-vitro

Abstract

In Transcranial Magnetic Stimulation, 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, I have designed, built and modelled four mouse-specific TMS coils. A capacitor is discharged through these coils at 50 V power supply. The magnetic flux densities of these coils were measured with a Hall probe at the base of the coils and 2 mm above the Hall sensor. The induced electric field strengths of these coils were measured with a wire loop at 50 V power supply. The heating in coils were measured at 50 V supply with thermocouple probe. I aimed to design and demonstrate a mouse-specific TMS coil that can generate high and focused induced electric field. I have designed, measured and modelled 50-turn tapered powdered iron core-coil that generated magnetic field strengths of around 700 mT at 0 mm above the coil and 340 mT at 2 mm above the coil. This shows that the coil is close to the B-field value of human TMS coils. These B-fields are larger than the previously designed coils by other researchers. However, while the induced E-field was focused at 10 V/m, it was still significantly lower in strength than for human TMS coils. Finally, I have applied 1200 pulses of continuous theta burst stimulation (cTBS) and intermittent theta burst stimulation (iTBS) to mouse brain slices and analysed the change in spontaneous electrical activity and evoked potential response in mouse brain with the 50-turn tapered powdered iron-core coil. For SLE experiments, we see a significant change in SLE frequencies. For cTBS, the frequency decreases over 0–20 minutes after stimulation and for iTBS the frequency increases over 0–20 minutes after stimulation. For evoked potential experiments, we see no significant change in either peak-to-peak amplitudes or gradient responses. In this four-year thesis project, a tapered mouse-specific TMS coil has been designed and demonstrated in vitro. The electromagnetic field strengths measured for this coil are sufficient to change the spontaneous behaviour of mouse brain slice with theta burst stimulation.