Transcranial magnetic stimulation (TMS) has been widely used in the treatment of varied physical and neuropsychiatric disorders, especially in major depression. The intracranial electromagnetic field is generated by the the time-varying current in the stimulation coil to change the potential of targeted neurons during the treatment. Since different mental disorders correspond to specific stimulation targets and broad stimulation range might raise serious side effects, stimulation focalization is very important in TMS. To achieve focalized stimulation, a novel magnetic stimulation coil with the field shaper and the crescent ferromagnetic core (the FSMC coil) is proposed and optimized in this study. The Finite-Element Method (FEM) is adopted to analyze the relationships between the design parameters of the field shaper and crescent ferromagnetic core and the characteristics of the intracranial electromagnetic field. Compared to traditional single circular coil, the focalization of the intracranial electromagnetic field generated by the optimized FSMC coil can be significantly improved both from 2D and 3D levels. To verify 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.
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