Krieg Todd D, Salinas Felipe S, Narayana Shalini, Fox Peter T, Mogul David J
Department of Biomedical Engineering, Illinois Institute of Technology, Chicago, IL, USA.
J Neural Eng. 2015 Aug;12(4):046014. doi: 10.1088/1741-2560/12/4/046014. Epub 2015 Jun 8.
Transcranial magnetic stimulation (TMS) represents a powerful technique to noninvasively modulate cortical neurophysiology in the brain. However, the relationship between the magnetic fields created by TMS coils and neuronal activation in the cortex is still not well-understood, making predictable cortical activation by TMS difficult to achieve. Our goal in this study was to investigate the relationship between induced electric fields and cortical activation measured by blood flow response. Particularly, we sought to discover the E-field characteristics that lead to cortical activation.
Subject-specific finite element models (FEMs) of the head and brain were constructed for each of six subjects using magnetic resonance image scans. Positron emission tomography (PET) measured each subject's cortical response to image-guided robotically-positioned TMS to the primary motor cortex. FEM models that employed the given coil position, orientation, and stimulus intensity in experimental applications of TMS were used to calculate the electric field (E-field) vectors within a region of interest for each subject. TMS-induced E-fields were analyzed to better understand what vector components led to regional cerebral blood flow (CBF) responses recorded by PET.
This study found that decomposing the E-field into orthogonal vector components based on the cortical surface geometry (and hence, cortical neuron directions) led to significant differences between the regions of cortex that were active and nonactive. Specifically, active regions had significantly higher E-field components in the normal inward direction (i.e., parallel to pyramidal neurons in the dendrite-to-axon orientation) and in the tangential direction (i.e., parallel to interneurons) at high gradient. In contrast, nonactive regions had higher E-field vectors in the outward normal direction suggesting inhibitory responses.
These results provide critical new understanding of the factors by which TMS induces cortical activation necessary for predictive and repeatable use of this noninvasive stimulation modality.
经颅磁刺激（TMS）是一种用于无创调节大脑皮质神经生理学的强大技术。然而，TMS线圈产生的磁场与皮质中神经元激活之间的关系仍未得到充分理解，这使得通过TMS实现可预测的皮质激活变得困难。本研究的目的是探讨感应电场与通过血流反应测量的皮质激活之间的关系。具体而言，我们试图发现导致皮质激活的电场特征。
使用磁共振图像扫描为六名受试者中的每一位构建头部和大脑的个体特异性有限元模型（FEM）。正电子发射断层扫描（PET）测量了每位受试者对图像引导下机器人定位到初级运动皮质的TMS的皮质反应。在TMS的实验应用中，利用给定的线圈位置、方向和刺激强度的有限元模型来计算每个受试者感兴趣区域内的电场（E场）矢量。分析TMS诱导的电场，以更好地理解哪些矢量分量导致了PET记录的局部脑血流（CBF）反应。
本研究发现，根据皮质表面几何形状（以及皮质神经元方向）将电场分解为正交矢量分量，导致活跃和不活跃皮质区域之间存在显著差异。具体而言，活跃区域在正常向内方向（即平行于树突到轴突方向的锥体神经元）和高梯度切向方向（即平行于中间神经元）具有显著更高的电场分量。相比之下，不活跃区域在向外法线方向具有更高电场矢量，表明存在抑制反应。
这些结果为TMS诱导皮质激活的因素提供了重要的新认识，这对于这种无创刺激方式的可预测和可重复使用至关重要。
