Simulation Analysis For Transcranial Magnetic Stimulation Based On Realistic Craniocerebral Structural Modeling

Transcranial Magnetic Stimulation(TMS)is a painless,noninvasive and relatively safe technology of neural modulation.Time-varying magnetic field induced by the pulse current flowing in the TMS coils could directly cause electrophysiological effects on brain nerves,permeating through outer tissues in craniocerebral structures.In these years,TMS has been getting broadly utilized in many kinds of psychiatric disorders and neuropsychological diseases,as both clinical therapeutical and diagnostic method.TMS is also very famous in exploratory researches for brain and cognitive sciences.The key to perform TMS is stimulating coils,by whose shapes and sizes the spatial distribution of induced electrical field is determined,which hence affects the stimulation.There are mainly two ways to study the effects of coils:one is aiming to the electromagnetic properties of coils themselves,which could be depicted through EMF detection and simulation on simple models,and the other is about the realistic stimulating effects on brain,which is usually realized via indirect techniques,such as MEP testing,and simulation on models more complicated,since the effects are very hard to be directly detected in craniocerebral structures in vivo.Besides,animal experiment is also one of the very important methods in TMS researches.In TMS modeling and simulation studies,craniocerebral models including structures like Scalp,Skull,CSF,Gray matter and White matter are now very popular,while deep brain tissues are rarely reconstructed.Generally,during simulation,structural models of Figure-Eight coils are approximately established,hence the results like induced EMF distribution have notable difference with real coils.When the complicated craniocerebral tissues are exposed to TMS,the stimulating performance of coil is usually evaluated through Half Width Area(Focal Area),yet Area of Validity has not been studied or discussed so much.Besides,simulation studies on TMS animal experiments are also infrequent published.For improving the authenticity of TMS simulation and enhancing its guiding significance further,a systematical method of modeling and simulation has been proposed in this paper,as well as the method to evaluate the Area of Validity during TMS.This method could be used to analyze the distribution of EMF induced by TMS during human experiment,animal experiment and clinical utility,and estimating the stimulating effect.Results of such simulation studies could provide a theoretical basis for TMS applications.In this work,realistic Figure-Eight coil models and craniocerebral structures of a specific subject have been reconstructed in MIMICS,including deep brain areas like Thalamus,Ventricles and Limbic System,and FEM simulation has been performed in ANSYS.During simulation study in this paper,Saline Tank testing system has been established and employed to determine the preferences,making sure the simulation comparable with real utility of TMS.EMF induced by TMS and Direct Electrical Stimulation(DES)has been contrasted based on craniocerebral model established,and the estimating method has been proposed to figure out the Area of Validity of TMS and DES,as well as the relationship between them,which could methodlogically and theoretically help TMS to perform an accurate mapping.Further,method in this paper has also been utilized in reconstructing craniocerebral model of WISTAR rat and animal Figure-Eight coil,and analyzing the pattern in which Area of Validity changes with TMS intensity.As a result,Area of Validity is about 14%of Focal Area estimated during TMS human experiment,and is about 63%during rat experiment.The method laid the foundation for the precise localization in clinical use and animal experiments of TMS.