A multi-parameter overlapping coil with a ring-shaped shielding for improved transcranial magnetic stimulation

As a non-invasive and painless treatment method for neurological diseases, transcranial magnetic stimulation (TMS) has shown great potential in the diagnosis and treatment of central nervous system diseases and mental disorders. In this paper, a novel design of the multi-parameter overlapping coil (MPO coil) is proposed to improve the stimulation focalization and reduce Joule heat during TMS treatment. The MPO coil consists of two vertically overlapped semi-elliptical coils. The induced electric fields (E-fields) superimpose below their intersection point to enhance the stimulation intensity. The angle between the coils is flexibly adjustable, allowing for tailored optimization of the spatial distribution pattern of the induced E-fields beneath the MPO coil assembly. To further improve the stimulation performance of the MPO coil, the ring-shaped shielding (the R-shielding) made of silicon steel material is placed around the MPO coil. According to the principle of electromagnetic induction, the R-shielding can generate induced eddy currents that counteract the intracranial induced E-fields and effectively enhance the stimulation focalization of the MPO coil. The finite element analysis method is adopted to obtain the stimulation characteristics of the MPO coil. Results show that compared with the traditional figure-of-eight coil, the proposed MPO coil with the R-shielding can effectively narrow down the focusing area and also has a considerable enhancement in reducing the Joule heat loss of the coil while ensuring the stimulation intensity. A real human head model is used in this paper to verify our method.