Quantitative assessment of the focality of a double-D coil in the human brain

Transcranial magnetic stimulation (TMS) is widely studied for achieving a stimulus effect using coil shapes. Our group focuses on a coil that can stimulate a specific brain region when a coil navigation system does not provide sufficient accuracy. A patient may have several difficulties in visiting a doctor on a daily basis, and therefore, home-based TMS systems are desirable. However, a conventional coil navigation system is too expensive and large for patients to accommodate it in their homes. Hence, a smaller system with a 5-mm error margin was developed. However, this system cannot achieve accurate results for conventional figure-8 coils when stimulating a specific region of the brain, owing to the limited focal spot. Therefore, we developed a new coil, a double-D coil, which induces a wide electric field in the brain to enable stimulation when the coil is not placed accurately. In this study, five healthy subjects participated voluntarily, and the double-D coil was assessed experimentally. For comparison, a butterfly coil was also used. Motor evoked potential was recorded at the first dorsal interosseous and its amplitude was mapped on the coordinate axes, which intersect at the point of optimal excitability. The distributions of these data were arranged with a Gaussian function to assess focality. The half-value width of the mediolateral (ML) direction improved with a P value less than 0.02, whereas that of the anteroposterior (AP) direction increased with a P value less than 0.07.