Practical TMS coils with maximum focality and various stimulation depths

Abstract

Conventional transcranial magnetic stimulation (TMS) coils generate a diffuse and shallow electric field (E-field) in the brain. This results in limited spatial targeting precision (focality). Previously, we developed a methodology for designing theoretical TMS coils to achieve optimal trade-off between the depth and focality of the induced E-field, as well as the energy required by the coil. This paper presents methods for the practical design and implementation of such focal-deep TMS (fdTMS) coils. First, we consider how the coils shape affects energy requirements and design a curved "hat" former that enables a wide range of coil placements while improving energy efficiency compared to flat formers. Second, we introduce a hybrid layer winding implementation to improve energy efficiency by using multi-layer windings in some regions of the coil and a single layer in others. Using simulations with a spherical head model, we benchmark the focality of the fdTMS E-field in the brain and the scalp, as well as the required energy, against conventional TMS coils. We then implement two fdTMS coil designs using copper wire wound inside a 3d-printed plastic former. The E-field of the prototype fdTMS coils and two conventional figure-8 counterparts is measured with a robotic probe to validate the designs. The experimental measurements corroborate the simulations, demonstrating that the fdTMS coils produce a more compact E-field relative to standard figure-8 coils. One potential disadvantage of the fdTMS coil prototypes is wider spread of the scalp E-field and increased energy loss due to the additional windings. Nonetheless, the presented fdTMS coils could offer advantages for precise mapping studies, and the design framework could be leveraged for other coil optimizations.