Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA.
Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA.
Brain Stimul. 2024 Sep-Oct;17(5):1034-1044. doi: 10.1016/j.brs.2024.08.003. Epub 2024 Aug 12.
Although transcranial magnetic stimulation (TMS) has become a valuable method for non-invasive brain stimulation, the cellular basis of TMS activation of neurons is still not fully understood. In vitro preparations have been used to understand the biophysical mechanisms of TMS, but in many cases these studies have encountered substantial difficulties in activating neurons.
OBJECTIVE/HYPOTHESIS: The hypothesis of this work is that conductivity boundaries can have large effects on the electric field in commonly used in vitro preparations. Our goal was to analyze the resulting difficulties in in vitro TMS using a simulation study, using a charge-based boundary element model.
We decomposed the total electric field into the sum of the primary electric field, which only depends on coil geometry and current, and the secondary electric field arising from conductivity boundaries, which strongly depends on tissue and chamber geometry. We investigated the effect of the conductivity boundaries on the electric field strength for a variety of in vitro experimental settings to determine the sources of difficulty.
We showed that conductivity boundaries can have large effects on the electric field in in vitro preparations. Depending on the geometry of the air-saline and the saline-tissue interfaces, the secondary electric field can significantly enhance, or attenuate the primary electric field, resulting in a much stronger or weaker total electric field inside the tissue; we showed this using a realistic preparation. Submerged chambers are generally much more efficient than interface chambers since the secondary field due to the thin film of saline covering the tissue in the interface chamber opposes the primary field and significantly reduces the total field in the tissue placed in the interface chamber. The relative dimensions of the chamber and the TMS coil critically determine the total field; the popular setup with a large coil and a small chamber is particularly sub-optimal because the secondary field due to the air-chamber boundary opposes the primary field, thereby attenuating the total field. The form factor (length vs width) of the tissue in the direction of the induced field can be important since a relatively narrow tissue enhances the total field at the saline-tissue boundary.
Overall, we found that the total electric field in the tissue is higher in submerged chambers, higher if the chamber size is larger than the coil and if the shorter tissue dimension is in the direction of the electric field. Decomposing the total field into the primary and secondary fields is useful for designing in vitro experiments and interpreting the results.
尽管经颅磁刺激（TMS）已成为一种有价值的非侵入性脑刺激方法，但 TMS 激活神经元的细胞基础仍未完全了解。体外制剂已被用于理解 TMS 的生物物理机制，但在许多情况下，这些研究在激活神经元方面遇到了很大的困难。
目的/假设：这项工作的假设是，在常用的体外制剂中，电导率边界会对电场产生很大的影响。我们的目标是使用基于电荷的边界元模型，通过模拟研究来分析体外 TMS 中由此产生的困难。
我们将总电场分解为仅取决于线圈几何形状和电流的初级电场和由电导率边界引起的次级电场之和，这强烈取决于组织和腔室几何形状。我们研究了各种体外实验设置中电导率边界对电场强度的影响，以确定困难的来源。
我们表明，电导率边界会对体外制剂中的电场产生很大的影响。根据空气-盐水和盐水-组织界面的几何形状，次级电场可能会显著增强或减弱初级电场，从而导致组织内的总电场更强或更弱；我们使用真实的制剂展示了这一点。与界面腔室相比，浸没腔室通常效率更高，因为界面腔室中覆盖组织的薄盐水膜中的次级场与初级场相反，从而显著降低了界面腔室中组织内的总场。腔室和 TMS 线圈的相对尺寸对总场至关重要；大线圈和小腔室的流行设置特别不理想，因为空气-腔室边界的次级场与初级场相反，从而削弱了总场。在感应场方向上组织的形式因子（长度与宽度之比）可能很重要，因为相对较窄的组织增强了盐水-组织边界处的总场。
总的来说，我们发现组织中的总电场在浸没腔室中更高，如果腔室尺寸大于线圈，并且较短的组织尺寸在电场方向上，则总电场更高。将总电场分解为初级和次级电场有助于设计体外实验并解释结果。
