Estimation of Induced Eddy Current in the Brain and Characteristics of Nerve Excitation by Magnetic Stimulation

近年来，非接触、非侵入性磁刺激在临床应用中被用于神经功能的诊断。在磁刺激中，对生物体内感应涡流的分布进行估计，对于明确神经兴奋的位置具有重要意义。然而，在人脑这样的非均匀介质中，很难预测感应涡流的分布。为了研究颅骨中感应涡流的特性，用一根细长的微型同轴电缆制成探头，测量了充满盐水的简易颅骨模型中感应涡流的分布。此外，利用牛蛙坐骨神经的神经肌肉模型对神经刺激点进行了估计，目的是了解在充满生理盐水的颅脑模型中，当模型暴露在脉冲磁场中时，产生的感应涡流的分布情况。我们讨论了感应涡流的焦点与由于线圈取向而在头部的激发位置之间的关系。因此，感应涡流的分布取决于颅骨或狭窄空间的边界结构。实验表明，当电流在枕大孔后缘附近流动时，感应涡流的密度最高。枕大孔处的感应涡流强度因水流方向的不同而略有不同。从磁刺激内听道神经肌肉模型的结果可以看出，在感应涡流密度变化很大或神经在有限介质中弯曲的地方，例如手臂水箱模型，神经容易受到刺激。

In recent years, the contactless and non-invasive magnetic stimulation is used to diagnose the function of nerve in the clinical application. In magnetic stimulation, it is important for clarifying the location of nerve excitation to estimate the distributions of induces eddy current in a living body. It is, however, difficult to predict distributions of induced eddy currents in an inhomogeneous medium like as the humanbrain. In order to investigate the characteristics of induced eddy currents in the cranium, the distributions of induced eddy current in the simple cranium model which is filled with the saline solution was measured by the probe made of a thin flexible mini coaxial cable. Moreover, the estimation of the stimulated point in the nerve was performed by using the nerve muscle model of a bull frog's sciatic nerve.The purpose of this study is to find out the distributions of induced eddy current generated in the cranial model filled with the saline solution, when the model was exposed to pulsed magnetic fields. We discussed the relationship between a focality of the induced eddy currents and an excited location in the head due to the orientation of the coil. As a result, the distribution of induced eddy current depends on the structure of boundary of the skull or narrow space. This experiment showed that the highest density of induced eddy currents provided a result when the current flowed in the vicinity of the rear edge of foramen occipitale magnum. The intensity of induced eddy currents at the foramen magnum differs a little due to the direction of current flow. From the results of the nerve muscle model in the internal acoustic meatus by the magnetic stimulation, it is showed that the nerve is easy to be stimulated at where the change of induced eddy current density is very large or the nerve is bent in the finite medium, for example an arm tank model.