Computational Study Toward Deep Transcranial Magnetic Stimulation Using Coaxial Circular Coils

Objective: To investigate the possibility for stimulating deeper brain regions while decreasing the electrical field in superficial cortical regions by employing coaxial circular coils. Methods: The Halo coil, Halo-circular assembly coil (HCA coil) and Halo coil working with two circular coils (HTC coil) were applied over a 36-tissue anatomically based head model. Three-dimensional distributions of magnetic flux density, induced electric field in head tissues were obtained by 3-D impedance method. Results: For the case of HCA coil with current flowing in the same direction in each of two coils, the field penetration depth by the conventional circular coil can be effectively increased at the expense of reduced focality. For the case of the HTC coil with currents flowing in opposite direction in the neighboring coils, overthreshold electric fields can be produced in deep brain regions, while the subthreshold fields were produced in superficial cortical areas. Conclusion: The HTC coil with varied coil parameters and different injected currents provides a flexible way for deep brain stimulation with better ratio of deep region field relative to field at the shallow areas. Significance: The HTC coil is promising for deep transcranial magnetic stimulation, which may offer a new tool with potential for both research and clinical applications for psychiatric and neurological disorders associated with dysfunctions of deep brain regions.