Optimum Design of the Eccentric Trapezoidal Magnetic Stimulation Coil Considering Stimulation Effect and Heat Dissipation

To improve the magnetic stimulation effect, the eccentric trapezoidal magnetic coil (ETM coil) is designed based on the inspiration of special-shaped magnets. The proposed ETM coil consists of two identical coil units and each unit is in the shape of isosceles-trapezoid when projected onto the stimulated target plane. When two coil units are placed adjacent or overlapped, they are referred to as Type I-ETM coil and Type II-ETM coil, respectively. To limit the induced electromagnetic field within a smaller domain, the ETM coil is bent along the symmetrical axis at specific angles. Considering the overheating problem when repetitive stimulation is required, the ETM coil is wound with hollow copper wire and the central hollow part is used to circulate low-temperature deionized water. The finite element analysis method is adopted to obtain the 3D distributions of the induced electromagnetic field generated by ETM coil within the human brain and the internal cooling parameters design is carried out with coupled multi-physical field analysis. Results show that the ETM coil can produce the same stimulation intensity as the traditional FOE coil with less driving current, and it can also significantly improve the intracranial stimulation focalization and reduce the risk of side effects. Meanwhile, because of the internal cooling design, the temperature of the ETM coil could be maintained at room temperature after 3000 effective pulses were applied, while the temperature of the FOE coil exceeds the safe range. To verified our method, an anatomically realistic human head model with different electrical properties assigned to each tissue of the brain is employed in this paper. We also checked the maximum induced charge density on the targeted plane generated by the optimized coil to make sure that it will not cause any induced neurologic damage.