Maccabee P J, Amassian V E, Eberle L P, Rudell A P, Cracco R Q, Lai K S, Somasundarum M
Department of Neurology, SUNY Health Science Center, Brooklyn 11203.
Electroencephalogr Clin Neurophysiol. 1991 Jun;81(3):224-37. doi: 10.1016/0168-5597(91)90076-a.
We measured the electric fields induced by round and figure "8" magnetic coils (MCs) in homogeneous and inhomogeneous volume conductors. In homogeneous media, the round MC held tangential (i.e., flat) to the volume conductor induced an annular electric field. When the round MC was held on-edge (i.e., orthogonal) to the volume conductor, the induced electric field consisted of two loops mainly parallel to the surface of the volume conductor and which approximated each other directly under the contacting edge of the MC. The tangentially oriented figure "8" MC similarly induced two electric field loops which approximated one another maximally under the region of the junction in its long axis. In a complex inhomogeneous volume conductor, such as a segment of human cervical-thoracic vertebral spine located eccentrically within a large cylindrical tank and submerged in isotonic saline, the direction of electric fields within the spinal canal and across the intervertebral neuroforamina was similar to that observed in the homogeneous volume conductor. However, in and near a single neuroforamen, the electric field and especially its first spatial derivative were markedly elevated compared to that recorded within the long central axis of the vertebral canal. Motor unit and compound muscle action potentials elicited in limb muscles by MC stimulation of human cervical spine confirmed predictions derived from the physical model. The predictions included: (1) absence of spinal cord stimulation compared to relative ease of nerve root stimulation by current that is most likely concentrated at the neuroforamina. When stimulating current is directed towards the periphery, the most likely low threshold site of stimulation is inferred to be just distal to the neuroforamina. It is emphasized that with supramaximal stimulation, more distal sites of excitation may occur; (2) invariant latency shifts at threshold intensities when moving the MC along the rostrocaudal axis of the cervical vertebral column; (3) significant effect (on motor unit activation thresholds) of the direction of induced current flow across the neuroforamina; (4) reduced stimulation when the targeted nerve roots are close to the null point of the electric field, i.e., between locations of high electric field intensity, of opposite polarity; and (5) relatively focal nerve root stimulation by the junction of a transversely orientated figure "8" MC, i.e., parallel to the nerve roots.
我们测量了圆形和数字“8”形磁线圈（MCs）在均匀和非均匀体积导体中感应产生的电场。在均匀介质中，与体积导体相切（即扁平放置）的圆形MC会感应出环形电场。当圆形MC与体积导体边缘相切（即正交）时，感应电场由两个主要平行于体积导体表面的环组成，且在MC接触边缘的正下方彼此近似。切向取向的数字“8”形MC同样会感应出两个电场环，它们在其长轴的交界处区域相互之间的近似程度最大。在复杂的非均匀体积导体中，例如一段偏心位于大圆柱形水箱内并浸没在等渗盐水中的人颈胸椎脊柱，椎管内和椎间神经孔处的电场方向与在均匀体积导体中观察到的相似。然而，在单个神经孔及其附近，与在椎管长中心轴处记录的电场相比，电场尤其是其第一空间导数明显升高。通过对人颈椎进行MC刺激在肢体肌肉中引发的运动单位和复合肌肉动作电位证实了从物理模型得出的预测。这些预测包括：（1）与神经根刺激相对容易相比，脊髓刺激不存在，电流最可能集中在神经孔处。当刺激电流指向周边时，最可能的低阈值刺激部位被推断为恰好在神经孔远端。需要强调的是，在超强刺激下，可能会出现更远端的兴奋部位；（2）当MC沿颈椎的头尾轴移动时，在阈值强度下潜伏期不变；（3）感应电流流过神经孔的方向对（运动单位激活阈值）有显著影响；（4）当目标神经根靠近电场的零点，即在相反极性的高电场强度位置之间时，刺激减弱；以及（5）横向取向的数字“8”形MC的交界处对神经根进行相对局部的刺激，即与神经根平行。
