Development of double-D coils for Transcranial Magnetic Stimulation treatment at home

Transcranial Magnetic Stimulation (TMS) treatments have been employed in hospitals. Conventional figure-8 coils can only stimulate a highly localized area, and it is difficult to stimulate specific points accurately without an infrared camera system that is large and expensive. To ease patients' burden, we suggest home treatment with a smaller driving circuit and a smaller positioning system that has an accuracy of approximately 5 mm; this is critical for figure-8 coils. Thus, a wide focus coil is required. In the present study, we propose a new coil named the double-D coil. The radius of side elements and the number of coil turns were defined based on scalar potential finite difference method simulations. The distribution and intensity of electric field were simulated based on finite element method. We also conducted the simulations using brain models constructed from MR images to evaluate its robustness against the coil positioning error. Finally, we developed the double-D coil by using a 3D printer and measured the magnetic flux density. The results show that the double-D coil induced an electric field in an expanded area. This was enough to be applied to a smaller positioning system and the intensity of the electric field was improved at more than 16 mm from the coil surface in the simulation. In the brain model from the MR image, the mean value of the electric field by the double-D coil was 1.2 times stronger than that of the figure-8 coil. The double-D coil created by a 3D printer induced a similar magnetic field to the simulation. That indicates the efficiency of the double-D coil at more than 20 mm from the coil surface.