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Transcranial magnetic stimulation (TMS) is one of the most effective non-invasive neuromodulation techniques used to treat neurological and psychiatric disorders. It is also highly popular to be used for studying and understanding brain functions and mechanisms. Multisite simultaneous or sequential stimulation of different brain regions provides a new methodology to study and comprehend the functional dependence and causal relationships among different regions and networks. However, due to the large coil size, which doesn't allow more than two coils being simultaneously utilized, and the non-ideal field focality and penetration depth of existing commercial coils, multisite simulation methods have not been well exploited. In this study, an angle-wrapped and multi-stacked circular coil is proposed to form an array of coils and used for this purpose. The induced electric field hot spot of the single coil and the coil array designs for fixed multisite stimulation were studied with the finite element method. The FEM simulations were then verified with experiments performed on fabricated coils. The performed simulations and experiments indicated the proposed coils' enhanced performance regarding the hot spot size and electric field decay rate compared to conventional circular coils. The successfully demonstrated close proximity and small spot size of stimulation show great potential for future fixed and adjustable multisite brain stimulation.
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