Numerical Analysis of Microcoil-Induced Electric Fields and Evaluation ofIn vivoMagnetic Stimulation of the Mouse Brain

Magnetic stimulation is widely used as a medical treatment for neurological diseases. A reduction in the size of magnetic stimulation devices is desirable, however, because the coils are relatively large compared with electrode stimulation devices. Furthermore, in transcranial magnetic stimulation, the exact magnetically evoked areas in the brain and the mechanisms of neural activation are largely unknown. This study aimed to develop a new implantable microcoil device that stimulates the brain locally, on the order of several tens of micrometers, as well as to understand the mechanisms of micromagnetic stimulation-induced neural activity. First, to investigate the effects of microcoil shape on neural activation, the induced electric fields of semicircular microcoils with different diameters were calculated numerically in a microcoil model using the standard finite element method. Next, on the basis of the obtained numerical results from the microcoil model with different diameters, we compared the spatial properties of the possible activated areas in neural tissue and examined the most effective microcoil shape for neural activation. Finally, we measured microcoil-evoked responses using autofluorescent flavoprotein imaging of the mouse brainin vivo, evaluated the validity of our proposed microcoil devices, and discussed possible future improvements for a chronic implant. (c) 2020 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.