Cohen D, Cuffin B N
Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge 02139.
J Clin Neurophysiol. 1991 Jan;8(1):102-11. doi: 10.1097/00004691-199101000-00013.
Some general properties of currents induced by magnetic stimulators in volume conductors of any shape are first reviewed. Then, the property of focality (concentration at some internal point) of the current induced in a spherical model of the head is discussed, due to coils of various orientations and configurations. It is shown that one important property for focality is the complete absence of the radial component of current, regardless of the coil used. The theoretically computed current distributions in the sphere produced by three different coils are illustrated. These are (1) a circular coil parallel to the head (very nonfocal). (2) a circular coil erect on the head (more focal), and (3) a figure-eight coil parallel to the head (most focal). A computational search for yet more focal coil configurations shows that the figure-eight focality can only be altered somewhat, but basically not improved. A plot of focality of the figure-eight coil versus the diameter of its coils shows an improvement in focality with decreasing diameter down to one cm. At this diameter, the induced current at a level 2 cm below the coil is concentrated in a band 1.5 cm wide (half-maximum points). Finally, it is noted that as the diameter of the figure-eight coil is decreased, the current in the coil necessary for stimulation increases rapidly, with increasing engineering problems.
首先回顾一下磁刺激器在任意形状的体导体中所感应电流的一些一般特性。然后，讨论在头部球形模型中，由于各种取向和配置的线圈所感应电流的聚焦特性（集中在某个内部点）。结果表明，对于聚焦而言，一个重要特性是无论使用何种线圈，电流的径向分量完全不存在。文中展示了由三种不同线圈在球体中理论计算出的电流分布。它们分别是：（1）与头部平行的圆形线圈（非常不聚焦）；（2）直立在头部的圆形线圈（更聚焦）；（3）与头部平行的8字形线圈（最聚焦）。对更聚焦的线圈配置进行计算搜索表明，8字形的聚焦性只能有所改变，但基本上无法得到改善。8字形线圈的聚焦性与其线圈直径的关系图显示，随着直径减小到1厘米，聚焦性有所提高。在此直径下，线圈下方2厘米处的感应电流集中在一个1.5厘米宽的带内（半最大值点）。最后需要注意的是，随着8字形线圈直径的减小，刺激所需的线圈电流会迅速增加，工程问题也会随之增加。
