Background

Transcutaneous auricular Vagus Nerve Stimulation (taVNS) applies low-intensity electrical current to the ear with the intention of activating the auricular branch of the Vagus nerve. The sensitivity and selectivity of stimulation applied to the ear depends on current flow pattern produced by a given electrode montage (size and placement).

Objective

We compare different electrodes designs for taVNS considering both the predicted peak electric fields (sensitivity) and their spatial distribution (selectivity).

Methods

Based on optimized high-resolution (0.47 mm) T1 and T2 weighted MRI, we developed an anatomical model of the left ear and the surrounding head tissues including brain, CSF/meninges, skull, muscle, blood vessels, fat, cartilage, and skin. The ear was further segmented into 6 regions of interest (ROI) based on various nerve densities: cavum concha, cymba concha, crus of helix, tragus, antitragus, and earlobe. A range of taVNS electrode montages were reproduced spanning varied electrodes sizes and placements over the tragus, cymba concha, earlobe, cavum concha, and crus of helix. Electric field across the ear (from superficial skin to cartilage) for each montage at 1 mA or 2 mA taVNS, assuming an activation threshold of 6.15 V/m, 12.3 V/m or 24.6 V/m was predicted using a Finite element method (FEM). Finally, considering every ROI, we calculated the sensitivity and selectivity of each montage.

Results

Current flow patterns through the ear were highly specific to the electrode montage. Electric field was maximal at the ear regions directly under the electrodes, and for a given total current, increases with decreasing electrode size. Depending on the applied current and nerves threshold, activation may also occur in the regions between multiple anterior surface electrodes. Each considered montage was selective for one or two regions of interest. For example, electrodes across the tragus restricted significant electric field to the tragus. Stimulation across the earlobe restricted significant electric field to the earlobe and the antitragus. Because of this relative selectivity, use of control ear montages in experimental studies, support testing of targeting. Relative targeting was robust across assumptions of activation threshold and tissue properties.

Discussion

Computational models provide additional insight on how details in electrode shape and placement impact sensitivity (how much current is needed) and selectivity (spatial distribution), thereby supporting analysis of existing approaches and optimization of new devices. Our result suggest taVNS current patterns and relative target are robust across individuals, though (variance in) axon morphology was not represented.

中文翻译：

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经皮耳廓迷走神经刺激 (taVNS) 期间外耳电流的高分辨率计算模型

 背景

经皮耳廓迷走神经刺激 (taVNS) 将低强度电流施加到耳朵，旨在激活迷走神经的耳支。施加到耳朵的刺激的灵敏度和选择性取决于给定电极蒙太奇（尺寸和位置）产生的电流模式。

 客观的

我们比较了 taVNS 的不同电极设计，同时考虑了预测的峰值电场（灵敏度）及其空间分布（选择性）。

 方法

基于优化的高分辨率（0.47毫米）T1和T2加权MRI，我们开发了左耳和周围头部组织的解剖模型，包括大脑、脑脊液/脑膜、颅骨、肌肉、血管、脂肪、软骨和皮肤。根据不同的神经密度，将耳朵进一步分割为 6 个感兴趣区域 (ROI)：外耳腔、外耳廓、耳轮脚、耳屏、对耳屏和耳垂。复制了一系列 taVNS 电极蒙太奇，涵盖耳屏、耳甲、耳垂、耳甲腔和耳轮小腿上不同的电极尺寸和位置。假设激活阈值为 6.15 V/m、12.3 V/m 或 24.6 V/m，则使用有限元方法预测每个蒙太奇在 1 mA 或 2 mA taVNS 下的整个耳朵（从浅层皮肤到软骨）的电场（有限元）。最后，考虑到每个投资回报率，我们计算了每个蒙太奇的灵敏度和选择性。

 结果

通过耳朵的电流流动模式与电极蒙太奇高度相关。电场在电极正下方的耳朵区域最大，并且对于给定的总电流，电场随着电极尺寸的减小而增大。根据所施加的电流和神经阈值，激活也可能发生在多个前表面电极之间的区域中。每个考虑的蒙太奇都是针对一个或两个感兴趣的区域进行选择性的。例如，穿过耳屏的电极限制了耳屏的显着电场。穿过耳垂的刺激限制了耳垂和对耳屏的显着电场。由于这种相对选择性，在实验研究中使用对照耳朵蒙太奇可以支持目标测试。在激活阈值和组织特性的假设下，相对靶向是稳健的。

 讨论

计算模型提供了关于电极形状和放置细节如何影响灵敏度（需要多少电流）和选择性（空间分布）的额外见解，从而支持对现有方法的分析和新设备的优化。我们的结果表明 taVNS 当前模式和相对目标在个体之间是稳健的，尽管轴突形态（差异）并未得到体现。