Coil optimisation for transcranial magnetic stimulation in realistic head geometry

• Main Authors: Lari M. Koponen, Jaakko O. Nieminen, Tuomas P. Mutanen, Matti Stenroos, Risto J. Ilmoniemi
• Format: Article
• Language: English
• Published: Elsevier 2017-07-01
• Series: Brain Stimulation
• Subjects: Transcranial magnetic stimulation; Coil design; Optimization; Boundary element method; Induced electric field
• Online Access: http://www.sciencedirect.com/science/article/pii/S1935861X17306599

Background: Transcranial magnetic stimulation (TMS) allows focal, non-invasive stimulation of the cortex. A TMS pulse is inherently weakly coupled to the cortex; thus, magnetic stimulation requires both high current and high voltage to reach sufficient intensity. These requirements limit, for example, the maximum repetition rate and the maximum number of consecutive pulses with the same coil due to the rise of its temperature. Objective: To develop methods to optimise, design, and manufacture energy-efficient TMS coils in realistic head geometry with an arbitrary overall coil shape. Methods: We derive a semi-analytical integration scheme for computing the magnetic field energy of an arbitrary surface current distribution, compute the electric field induced by this distribution with a boundary element method, and optimise a TMS coil for focal stimulation. Additionally, we introduce a method for manufacturing such a coil by using Litz wire and a coil former machined from polyvinyl chloride. Results: We designed, manufactured, and validated an optimised TMS coil and applied it to brain stimulation. Our simulations indicate that this coil requires less than half the power of a commercial figure-of-eight coil, with a 41% reduction due to the optimised winding geometry and a partial contribution due to our thinner coil former and reduced conductor height. With the optimised coil, the resting motor threshold of abductor pollicis brevis was reached with the capacitor voltage below 600 V and peak current below 3000 A. Conclusion: The described method allows designing practical TMS coils that have considerably higher efficiency than conventional figure-of-eight coils.