Transcranial magnetic stimulation (TMS) is a neuromodulation technique that has been approved by the U.S. Food and Drug Administration for several neuropsychiatric disorders, including major depression and obsessive-compulsive disorder. However, the therapeutic efficacy of TMS treatment has been modest, despite decades of research. While there are many potential reasons as to why, one of the most obvious is the limitations of current technologies. One prominent example is the penetration depth–focality tradeoff of existing TMS coils. The most widely used figure-of-8 coils stimulate brain regions just superficially under the coil, missing deep brain regions known to be critically involved in psychiatric disorders; while ring-type coils can stimulate deep into the brain, but stimulate a large brain volume (lack of focality). A new coil design strategy is proposed: magnetic materials encompassing the human head are optimized to shape the electromagnetic field generated by the primary coil. Specifically, a mathematical model was developed to describe the physical problem; the magnetic materials were discretized into unit blocks; Newton’s gradient descent method was applied to iteratively optimize the spatial distribution of the unit blocks to achieve a desired electric field distribution inside a head model. Results reveal that the proposed design achieves a coil penetration depth equal to or better than state-of-the art commercial coils, while improving the depth–focality tradeoff by a factor of 2.2 to 2.7. As a proof-of-concept, a prototype coil and a spherical head model were constructed; the spatial distribution of the induced electric field inside the head model was mapped. Results validated the proposed coil design. TMS coils based on this novel design strategy could potentially lead to better therapeutic outcome.

中文翻译：

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用磁性材料增强经颅磁刺激线圈：一种优化方法

经颅磁刺激 (TMS) 是一种神经调节技术，已被美国食品和药物管理局批准用于治疗多种神经精神疾病，包括重度抑郁症和强迫症。然而，尽管进行了数十年的研究，TMS 治疗的治疗效果仍然有限。虽然有很多潜在的原因，但最明显的原因之一是当前技术的局限性。一个突出的例子是现有 TMS 线圈的穿透深度 - 焦点权衡。最广泛使用的 8 字形线圈仅在线圈下方刺激大脑区域，缺少已知与精神疾病密切相关的深部大脑区域；而环形线圈可以刺激到大脑深处，但会刺激较大的脑容量（缺乏焦点）。提出了一种新的线圈设计策略：优化包含人头的磁性材料以塑造初级线圈产生的电磁场。具体来说，开发了一个数学模型来描述物理问题；将磁性材料离散为单元块；牛顿梯度下降法用于迭代优化单元块的空间分布，以在头部模型内实现所需的电场分布。结果表明，所提出的设计实现的线圈穿透深度等于或优于最先进的商用线圈，同时将深度-焦点权衡提高了 2.2 到 2.7 倍。作为概念验证，构建了原型线圈和球形头模型；绘制了头部模型内感应电场的空间分布。结果验证了所提出的线圈设计。基于这种新颖设计策略的 TMS 线圈可能会带来更好的治疗效果。