The Computational Model Of Deep Transcranial Magnetic Stimulation And Induced Electric Field Optimization

Transcranial magnetic stimulation(TMS)is a non-invasive brain modulation technique with a time-varying magnetic field.It has shown great potential in the treatment of neuropsychiatric disorders and the research of brain science.In addition,depth and rhythm play an important role,and deep transcranial magnetic stimulation(DTMS)is a new form to stimulate deep brain regions.However,the mechanism of neuromodulation by TMS has not been fully understood,which brings great difficulties to formulate the TMS application strategy and safety dose.The distribution of electric field induced by TMS is the key to this problem.In this paper,to optimize the effect and improve the scheme of TMS,the electric field induced by TMS in brain was investigated with the perspective of TMS coil and brain tissue.A computational model and a unified evaluation system for analyzing the TMS effect were established.Firstly,the traditional TMS coils were optimized based on the spherical brain model and finite element method.Then the coils that have different focality were obtained.To calculate the effects of stimulation,three kinds of coil attributes have been proposed,namely the loop offset on 2-D plane,the eccentric angle of 3-D space,and the number of independent loops of coil array.Secondly,in order to clarify the effects of TMS on different brain regions,a real head model was built based on magnetic resonance imaging.Different DTMS coils and head model were combined to describe the distributions of electric field under different TMS schemes.The results show that all TMS coils follow a trade-off between penetration depth and focality,but the ratios of two quotas are nonlinear.Among the coil attributes,the coil array with numbers of loops performed obvious advantages which can stimulate deeper brain with little loss of focality.Coils with the eccentric angle of 3-D space achieved significant characteristic in focality.In terms of brain model,the results show that the spherical brain model has limitation in measuring actual electric field intensity compared with the real brain model,but it is better to describe the attenuation trend of electric field.Due to the differences in physiological characteristics of brain tissues,the effects of TMS in different brain regions are diverse.If the whole brain needs to stimulate,the global DTMS may be a better choice than combining multiple local TMS.