Maccabee P J, Eberle L, Amassian V E, Cracco R Q, Rudell A, Jayachandra M
Department of Neurology, State University of New York Health Science Center, Brooklyn 11203.
Electroencephalogr Clin Neurophysiol. 1990 Aug;76(2):131-41. doi: 10.1016/0013-4694(90)90211-2.
The electric fields induced in finite homogeneous volume conductors by a round and a figure '8' magnetic coil (MC) were measured and related to MC stimulation of the median nerve. The volume conductors, filled with isotonic saline, consisted of a large rectangular trough ('unrestricted') and a smaller trough, whose dimensions approximated human forearm ('restricted'). Various MC orientations were applied to the volume conductor. Bipolar recordings were obtained with a coaxial electrode, which measured the voltage gradient between the exposed edge of the cable shield and the central wire at its tip, 1 cm distant (a linear probe). The probe was moved in 3 dimensions, allowing computer reconstruction of the electric field as a function of the 3 spatial axes. When the probe was parallel to the plane of the round MC and tangential to the direction of current in its windings, the induced electric field was maximal; it tended towards zero when the probe was over the center of the MC, or when the probe, remaining parallel to the plane of the MC, was radial (i.e., perpendicular) to the direction of the current in the windings. For a variety of MC orientations, the electric field was consistently increased when the probe was adjacent and parallel to the edge of the trough, indicating the important effect of boundaries. The electric field was greatly increased focally when the round MC was applied orthogonally to the volume conductor, or when the figure '8' MC was applied tangentially (i.e., flat) to the volume conductor. With the figure '8' MC, a sharp central peak parallel to the long axis was bounded on each side by smaller (less than half amplitude) peaks. The findings from physical modeling led to correct predictions as to the most effective orientations of round and figure '8' MCs for eliciting sensory nerve action potentials (SNAPs) from the median nerve.
测量了圆形和“8”字形磁线圈（MC）在有限均匀体积导体中感应产生的电场，并将其与正中神经的MC刺激联系起来。填充等渗盐水的体积导体由一个大的矩形槽（“无限制”）和一个较小的槽组成，后者的尺寸近似于人类前臂（“受限”）。将各种MC方向应用于体积导体。使用同轴电极进行双极记录，该电极测量电缆屏蔽层暴露边缘与距其尖端1 cm处的中心导线之间的电压梯度（线性探头）。探头在三维空间中移动，从而可以根据三个空间轴对电场进行计算机重建。当探头平行于圆形MC的平面并与其绕组中的电流方向相切时，感应电场最大；当探头位于MC的中心上方时，或者当探头保持平行于MC的平面并与绕组中的电流方向径向（即垂直）时，电场趋于零。对于各种MC方向，当探头靠近并平行于槽的边缘时，电场会持续增加，这表明了边界的重要影响。当圆形MC垂直于体积导体应用，或者“8”字形MC切向（即平放）于体积导体应用时，电场会在局部大大增加。使用“8”字形MC时，与长轴平行的尖锐中心峰值在两侧由较小（小于半幅值）的峰值界定。物理建模的结果对圆形和“8”字形MC从正中神经引出感觉神经动作电位（SNAP）的最有效方向做出了正确预测。
