Simulation Study On Multiphysical Field Coupling Of Transcranial Magnetic Stimulation

Transcranial Magnetic Stimulation(TMS)is a neuromodulation technique proposed in the1980s to affect in brain function.In the clinical application of TMS,the pulsed current in the TMS coil causes a rapid increase in coil surface temperature,and the distance between the coil and the human scalp is about 10 mm,and the coil may cause thermal damage to the human head surface tissue.To further investigate TMS technology,researchers have combined TMS with Magnetic Resonance Imaging(MRI)technology.TMS technology and MRI technology both generate large magnetic fields when working alone,and the forces on the coil can vary significantly in the combined TMS-MRI technology,which can cause damage to the coil and,in severe cases,pose risks to the user.TMS coils can introduce force and thermal instabilities during use,so it is necessary to study the changes in magnetic,force and temperature fields induced by TMS technology.To investigate the problems that may be brought by TMS operation,different types of TMS coil models,human head models and MRI static magnetic field models are constructed in this paper according to the actual situation.Through the magnetic field module of COMSOL Multiphysics finite element simulation software,the electromagnetic field distribution was calculated for TMS working alone and TMS working with MRI simultaneously,and the force field distribution and temperature field distribution were calculated through the coupling of magnetic field with structural mechanics module and the coupling of magnetic field with heat transfer module,respectively.The TMS coil models constructed in this paper include circular coil,figure-of-eight coil and deformed figure-of-eight coil,and the magnetic field and force field distributions induced by different types of TMS coils are investigated.In addition,the TMS stimulation frequency is modified in this paper,and the temperature field changes triggered by TMS coils with different stimulation frequencies in the same time are analyzed.Simulation results show that when a current with an amplitude of 5000 A and a frequency of 2500 Hz flows through the TMS coil,the magnetic flux density and Lorentz force density on the surface of the deformed figure-of-8 coil are greater than those of the undeformed figure-of-8 coil,but the peak stress of the deformed figure-of-8 coil is still less than the copper yield strength of 7×10~7 Pa,and deformation is less likely to occur.Current pulses can cause the coil surface temperature to rise rapidly,and coil cooling techniques must be used to ensure patient safety while the coil is operating.For human head biological tissue,the maximum temperature rise on the brain surface does not exceed 0.16℃after applying 1200 consecutive current pulses in 480 s.When the stimulation pattern is changed and the same current pulses are applied intermittently,the temperature rise on the brain surface is reduced to 0.12℃.When TMS and MRI work together,the brain tissue temperature rises about 0.16℃,and the temperature difference between combined work and separate work is less than 0.01℃.The static magnetic field of MRI does not cause significant thermal effects,and both TMS technology and TMS-MRI combined technology are less likely to cause thermal damage to the brain during normal work.The simulation results in this paper can provide the necessary reference for the future development direction of transcranial magnetic stimulation and related guidelines.