Wilson, M. T., Waas, J., Tang, A., Iyer, K., & Rodger, J. (2017). An illustration of interdisciplinary science: Small-scale coils for transcranial magnetic stimulation. In New Zealand Institute of Physics Conference 2017. Dunedin, New Zealand.
Permanent Research Commons link: https://hdl.handle.net/10289/11210
Transcranial Magnetic Stimulation (TMS) is a commonly used medical technique for influencing the strengths of neural connections in the brain. It is used, for example, for treating major depression. In TMS, an electric coil is placed on the surface of the scalp, and a rapidly changing current passed through it. This current-loop creates a changing magnetic field which in turn creates an electric field in the brain through electromagnetic induction. The underlying science beyond this point is less understood. The electric field somehow causes changes in the way brain cells (neurons) behave. This can lead to lasting changes in strengths of connections between neurons. To investigate the basic underlying science of TMS and inform clinical application, it would be advantageous to carry out experiments with rodents and complement these with numerical models. Unfortunately, we run into problems here. The scaling of physical effects is unhelpful – to create the same electric field over a smaller area (e.g. a mouse motor cortex as opposed to a human motor cortex) we generate more heat per unit volume and create larger forces in the coils, meaning it is difficult to apply equivalent situation to mice as to humans. We propose ways of optimising the coil geometry and currents. To increase the electric field strengths we require coils with high permeability cores to provide high field strengths, but at the same time ensuring eddy current heating is minimized. Paradoxically, coils with few turns are favourable because their low inductance allows for a rapid change in current. Numerical models (e.g. SIMNIBS, www.simnibs.de) are widely used for calculating electric field strengths in the human brain under TMS. However, they hide underlying assumptions that are only applicable to humans. We have tried with difficulty to model the equivalent case for the mouse. We have had more success in writing our own finite-area code for evaluating these fields. Example results for the induced electric field are shown in the figure. We find that the simple, unfolded structure of the mouse brain is advantageous in calculating the electric field intensities making the mouse particularly suitable for investigating coil design experimentally. Overall, this work illustrates the value of cross-disciplinary approaches to research – physicists, computer scientists, animal biologists, engineers and medical professionals have all made valuable contributions.