| Transcranial magnetic stimulation (TMS) is a non-invasive nerve stimulation technology, which is widely used in a variety of mental illness for diagnosis and treatment. Researchers suggest that disorders like depression, schizophrenia and autism spectrum are related to the deep structure of the brain, however, the current magnetic stimulation apparatus with the coil can only stimulate the shallow cortex effectively. Deep transcranial magnetic stimulation (d-TMS) is presented to improve the depth of the stimulus, and meanwhile, to reduce the stimulating intensity acted on the shallow tissues of the cortex.Firstly, modeling method for the conductivity of human head was proposed by using MRI data. A model of H-coil, which was parallel to the head contour, was created according to the structure of the real head model. After that, we conducted simulation study to analyze the electrical field distribution in the location of scalp and limbic system. Results showed that the distribution of the electrical field produced by H-coil was not focused in scalp but in the limbic system, the so-called in anterior cingutate. To evaluate the stimulating depth of H-coil, ratio between the peak value of the electric field in scalp and limbic system was set as the evaluation parameter. Then, in order to improve the deep stimulation ability, we optimized the wire spacing of H-coil. Result showed that the effect of deep stimulation could be optimized well when number of conductors was 4 in the front of the prefrontal and 7 in the profile of the prefrontal, the wire spacing was 6 mm in upper prefrontal.Next, we compared the effects of different coils acting in deep area, such as Eight coil, H-coil, Halo coil, combined coil of Halo coil and H-coil, Double vertebral coil, etc.. Result shew that H-coil, Halo coil, combined coil performed better in deep area. We also studied the influence of current direction in combined coil for deep stimulation. When the current in Halo coil and H-coil was in the same direction, the increased stimulus intensity in deep area of limbic system is more than that in scalp. Otherwise, the decreased stimulus intensity in limbic system is large than that in scalp. However, the inverse current in circle coil could reduce the intensity of electrical field in the area near the orbit.Finally, this paper use inductance coefficient method and energy method to design inductance of coil. The inductance for regular coil is calculated by analytic function. As for irregular coil, energy method is used to calculate the energy produced by magnetic coil, then the inductance can be obtained from the energy. Based on this method, this paper design a stimulation coil, which is matching to magnetic stimulator. The result show that the real inductance of coil is the same as the theoretical value.In summary, this study proposed a modeling method for real head conductivity used in d-TMS. With the model, an optimization method was developed to improve the performance of H-coil. Hence, it might give a new idea in designing the H-coil and simulation study in d-TMS. In addition, we also presented a calculation method for the inductance of coil, which would give theoretical support for the production of coil. |
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