| Transcutaneous magnetic stimulation(TMS) is a new method of non-invasive stimulation of centre nerve system. It minimizes the discomfort to the patient compared with conventional electrical stimulation. With the performance of contactless stimulation and being painless, valid, safe and easy to repeat and operate, it open a range of new and exciting applications in examining both the peripheral nerve system and the cortex. The distribution of induced electric fields in the brain is an important part of the research of the TMS.Now, the research of the distribution of induced electric fields in the brain has two problems. One is that the model of brain is a uniform and isotropic medium, it is different with the reality, so the results only have academic sense, the other is that traditional calculation method of induced electric fields is only suitable to calculate the fields distribution at one time point, the result can not reflect the change of induced electric fields when the time is changing. To solve these two problems, a model of spherical brain is constructed, which consists of skull and interior tissue. Bring forward the method that combine traditional calculation method with FDTD to calculate the induced electric fields in brain during magnetic stimulation. The electric fields in the air induced by stimulation coil are analyzed using traditional method. Taking the induced electric field produced by the stimulation coil as the incident plane wave, calculates the induced electric fields in the model of spherical brain using FDTD method. Some progress is made during the research in several aspects.1. A model of spherical brain is constructed, which consists of skull and interior tissue with different constructive parameters. It is more close to the reality. 2. Taking the induced electric field produced by the circular coil in the air as the incident plane wave, calculates the induced electric fields in the brain using FDTD method. The change of induced electric fields in the model is displayed actively. The calculation results are validated through the experiments. The process and mechanism of magnetic stimulation can be understood more deeply.3. During the calculation using FDTD, the Liao's absorbing boundary condition is adopted, the process is compact and it can be realized simply at corner. The model of spherical brain is more close to the reality. The results have some practical sense. The new method can reflect the change of induced electric fields at thepoints in the brain. It is helpful to understand the mechanism of magnetic stimulation. According to the demand, for the coils which have different size and shape, the new method can be used to calculate the induced electric fields at the point that need to be stimulated, then choose the best one which satisfied the need, thus the magnetic stimulator can be optimized.
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