# Optimization, implementation, and performance of TMS coils with maximum focality and various stimulation depths

## TL;DR

This paper introduces a new type of TMS coil that can create a more focused electric field in the brain, improving targeting precision for brain stimulation.

## Contribution

The paper presents a novel design and implementation of focal-deep TMS coils with optimized energy efficiency and improved spatial targeting.

## Key findings

- Prototype fdTMS coils produced a more compact electric field in brain models compared to conventional coils.
- fdTMS coils showed improved focality in human motor mapping but had increased energy loss and heating.
- The curved coil design improved placement flexibility but introduced positioning constraints.

## Abstract

Objective. Conventional transcranial magnetic stimulation (TMS) coils generate a diffuse and shallow electric field (E-field) in the brain, resulting in limited spatial targeting precision (focality). Previously, we developed a methodology for designing theoretical TMS coils to achieve maximal focality for a given E-field penetration depth and minimize the required energy. This paper presents the practical design, implementation, and characterization of such focal-deep TMS (fdTMS) coils. Approach. We considered how the coil’s shape affects energy requirements and designed a curved ‘hat’ former that enables a wide range of coil placements while improving energy efficiency compared to flat formers. To improve energy efficiency, we introduced optimized-coverage partial-multi-layer windings of the coil. Through simulations with a spherical head model, we benchmarked 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 implemented two fdTMS coil designs with copper wire wound inside a 3d-printed plastic former. Main results. The E-field of the prototype fdTMS coils and conventional figure-8 counterparts were simulated in spherical and realistic head models and measured with a robotic probe, confirming a more compact fdTMS E-field. The fdTMS coils were also compared to two commercial coils with motor mapping in nine human subjects, which confirmed improved focality of fdTMS at the cost of greater E-field spread in the scalp, increased energy loss and heating from the smaller wire diameter and additional windings, and positioning constraints of the curved coil surface. Significance. The study findings inform TMS coil implementation for precise mapping and targeting applications, and the design framework can be leveraged for future coil optimizations.