Hyodo Akira, Iramina Keiji, Ueno Shoogo
Graduate School of Systems Life Sciences, Kyushu University and with the Biomedical Instrument Technology Center, Nihon Kohden Corporation, Japan.
Annu Int Conf IEEE Eng Med Biol Soc. 2009;2009:6501-4. doi: 10.1109/IEMBS.2009.5333594.
In this study, we used a computer simulation to investigate the effects of the coil current waveform and direction on the excitation processes of the nerve axon in inhomogeneous and anisotropic conducting media in magnetic stimulation. We assumed that the nerve axon was located in the media with 2 regions having different conductivities or electrical anisotropy that simulate different tissue types. The distribution of induced electric fields was calculated with the finite element method (FEM). The nerve fiber was modeled after equivalent electrical circuits having active nodes of Ranvier. The direction of the coil current at the intersection of a figure-eight coil was assumed to flow perpendicular to the nerve axon. We observed the excitation threshold when the coil current waveform and direction are changed with varying the electrical properties such as tissue electrical conductivity and anisotropy. The simulation results show that the threshold decreases with the increase of conductivity ratio between 2 regions and it also depends on the coil current waveform and direction. Biphasic coil current has lower threshold than monophasic one when the current direction is the same in both waveforms. The results also suggest that the tissue anisotropy strongly affects the excitation threshold. The threshold increases with the increase of tissue anisotropic ratio of longitudinal direction to the transverse one respect to the nerve axon. The results in this study give useful information to explain the experimental results of the magnetic stimulation of human peripheral nervous systems and the theoretical model is applicable to understand the characteristics in magnetic stimulation of both peripheral and central nervous systems.
在本研究中，我们使用计算机模拟来研究线圈电流波形和方向对磁刺激中不均匀和各向异性导电介质中神经轴突兴奋过程的影响。我们假设神经轴突位于具有两个不同电导率或电各向异性区域的介质中，这两个区域模拟不同的组织类型。用有限元法（FEM）计算感应电场的分布。神经纤维是根据具有郎飞氏结活性节点的等效电路建模的。假设在八字形线圈交叉处的线圈电流方向垂直于神经轴突流动。当改变诸如组织电导率和各向异性等电学性质时，我们观察了线圈电流波形和方向变化时的兴奋阈值。模拟结果表明，阈值随着两个区域之间电导率比值的增加而降低，并且它还取决于线圈电流波形和方向。当两种波形中的电流方向相同时，双相线圈电流的阈值低于单相线圈电流。结果还表明，组织各向异性强烈影响兴奋阈值。阈值随着相对于神经轴突的纵向与横向组织各向异性比值的增加而增加。本研究结果为解释人体周围神经系统磁刺激的实验结果提供了有用信息，并且该理论模型适用于理解周围和中枢神经系统磁刺激的特征。
