Transcranial magnetic stimulation: U-shaped coil design for improved intracranial induced electrical field

Transcranial magnetic stimulation (TMS) has been widely used in scientific research and clinical application. In TMS, the stimulation coil generates a focusing induced electrical field in the intracranial target area to achieve neuromodulation. Since different disorders correspond to their specific target areas, in order to avoid non-target tissue being stimulated, the figure-of-eight (FOE) coil with good focusing performance is still the most commonly used magnetic coil. In this paper, the U-shaped coil with multi-parameters is introduced and optimized. The spatial distributions of the intracranial induced electrical field produced by the U-shaped coil are obtained using the finite-element method. Compared to the traditional FOE coil, the U-shaped coil not only has a great advantage in improving focalization but also is beneficial to obtain other improved characteristics. The mathematical relationship between the coil geometric parameters and the intracranial induced electrical field characteristics is analyzed under the constraint of fixed coil power loss. To further improve coil efficiency, the particle swarm optimization (PSO) is adopted to optimize the coil parameters. A real human head modeled as homogeneous and isotropic is occupied in this paper to verify our method, and a traditional FOE coil is used as the reference coil. We also checked the maximum induced charge density on the targeted plane generated by the optimized U-shaped coil to make sure that it will not cause induced neurologic damage.