Tofts P S
Institute of Neurology, London, UK.
Phys Med Biol. 1990 Aug;35(8):1119-28. doi: 10.1088/0031-9155/35/8/008.
Magnetic stimulation of the nervous system is being used as an alternative to electrical stimulation, principally because it is painless. The spatial distribution of induced currents from the stimulating coil is calculated from a computer model with graphical output. Two configurations of a plane circular coil are considered: parallel to the tissue surface and perpendicular to the surface. The surface is assumed planar and infinite in extent. The tissue is modelled as a uniform, isotropic volume conductor. A quasi-static approximation is made in calculating the electric field. Maps of current density, J, as a function of position, including depth, are shown. In both configurations, J is always parallel to the surface, and is maximum at the surface. There is no perpendicular (vertical) current. For a one-turn 10 cm diameter coil, spaced 1 cm from conducting tissue and parallel to it, with rate of change of current 10(8) A s(-1), Jmax = 6.8 A m(-2) (assuming conductivity 0.2 omega -1 m(-1)). In the perpendicular configuration Jmax = 4.1 A m(-2). These results suggest that nerve fibres running parallel to the skin surface are more likely to be stimulated than those running obliquely; and that it is extremely difficult to stimulate nerve fibres running perpendicularly. This model can be used to characterise the performance of other shapes of stimulating coils and the dependence on fibre orientation.
对神经系统进行磁刺激正被用作电刺激的替代方法，主要是因为它无痛。通过具有图形输出的计算机模型来计算来自刺激线圈的感应电流的空间分布。考虑了平面圆形线圈的两种配置：与组织表面平行和垂直于表面。假定表面是平面且范围无限。将组织建模为均匀、各向同性的体积导体。在计算电场时采用准静态近似。给出了电流密度J作为位置（包括深度）函数的分布图。在两种配置中，J始终与表面平行，且在表面处最大。不存在垂直（纵向）电流。对于一个直径为10 cm的单匝线圈，与导电组织相距1 cm并与之平行，电流变化率为10(8) A s(-1)，Jmax = 6.8 A m(-2)（假设电导率为0.2 omega -1 m(-1)）。在垂直配置中，Jmax = 4.1 A m(-2)。这些结果表明，与皮肤表面平行的神经纤维比倾斜的神经纤维更有可能被刺激；并且刺激垂直的神经纤维极其困难。该模型可用于表征其他形状刺激线圈的性能以及对纤维取向的依赖性。
