Angle-tuned Coil: A Focal Spot-size Adjustable Transcranial Magnetic Stimulator

Background: Conventional transcranial magnetic stimulation (TMS) coils are limited by the depth-focality tradeoff rule and the emission field intensity from coils with either small or large apertures will diverge quickly at less than one aperture diameter distance away from the coil. To utilize a better depth-focality tradeoff rule and accomplish deep and focused stimulation, a new approach needs to be employed. Objectives: We report a new TMS coil design that can deliver deep and spot size adjustable stimulation to deep brain regions. Methods: In our design, we introduce a magnetic core at the center of a coil to help confine the magnetic field and prevent leakage. We further tilted the wire wrapping angle of the coil to break its ring symmetry and accomplish tunable focusing by adjusting the tilting angle. Results: By comparing the electric field decay curves of five types of coils, our results concluded the proposed novel method to improve the coil depth-focality profile. Both theoretical calculations and experimental data collectively demonstrated that by using a larger tilting angle, we were able to accomplish a more tightly focused stimulation at any distance away from the coil. Conclusion: Enlarging the tilting angle of the coil wire wrapping and applying magnetic core significantly improved the spatial resolution of the field without inducing considerable effect on field decay speed. Our novel TMS coil design plots a new curve in the depth-focality profile with better performance than the existing conventional coil designs in the tradeoff rule.