Deng Zhi-De, Lisanby Sarah H, Peterchev Angel V
Department of Psychiatry and Behavioral Sciences, Duke University, Durham, NC, USA.
Department of Psychiatry and Behavioral Sciences, Duke University, Durham, NC, USA; Department of Psychology and Neuroscience, Duke University, Durham, NC, USA.
Clin Neurophysiol. 2014 Jun;125(6):1202-12. doi: 10.1016/j.clinph.2013.11.038. Epub 2013 Dec 22.
To explore the field characteristics and design tradeoffs of coils for deep transcranial magnetic stimulation (dTMS).
We simulated parametrically two dTMS coil designs on a spherical head model using the finite element method, and compare them with five commercial TMS coils, including two that are FDA approved for the treatment of depression (ferromagnetic-core figure-8 and H1 coil).
Smaller coils have a focality advantage over larger coils; however, this advantage diminishes with increasing target depth. Smaller coils have the disadvantage of producing stronger field in the superficial cortex and requiring more energy. When the coil dimensions are large relative to the head size, the electric field decay in depth becomes linear, indicating that, at best, the electric field attenuation is directly proportional to the depth of the target. Ferromagnetic cores improve electrical efficiency for targeting superficial brain areas; however magnetic saturation reduces the effectiveness of the core for deeper targets, especially for highly focal coils. Distancing winding segments from the head, as in the H1 coil, increases the required stimulation energy.
Among standard commercial coils, the double cone coil offers high energy efficiency and balance between stimulated volume and superficial field strength. Direct TMS of targets at depths of ~4 cm or more results in superficial stimulation strength that exceeds the upper limit in current rTMS safety guidelines. Approaching depths of ~6 cm is almost certainly unsafe considering the excessive superficial stimulation strength and activated brain volume.
Coil design limitations and tradeoffs are important for rational and safe exploration of dTMS.
探讨用于深部经颅磁刺激（dTMS）的线圈的场特性和设计权衡。
我们使用有限元方法在球形头部模型上对两种dTMS线圈设计进行参数模拟，并将它们与五种商用TMS线圈进行比较，其中包括两种已获美国食品药品监督管理局（FDA）批准用于治疗抑郁症的线圈（铁磁芯8字形线圈和H1线圈）。
较小的线圈比较大的线圈具有聚焦优势；然而，随着目标深度的增加，这种优势会减弱。较小的线圈的缺点是在浅表皮质产生更强的场且需要更多能量。当线圈尺寸相对于头部尺寸较大时，电场深度衰减变为线性，这表明，充其量，电场衰减与目标深度成正比。铁磁芯提高了针对浅表脑区的电效率；然而，磁饱和会降低铁芯对较深目标的有效性，特别是对于高度聚焦的线圈。如H1线圈那样将绕组段与头部保持一定距离会增加所需的刺激能量。
在标准商用线圈中，双锥线圈具有高能效，并且在刺激体积和浅表场强之间实现了平衡。对深度约4厘米或更深的目标进行直接TMS会导致浅表刺激强度超过当前重复经颅磁刺激（rTMS）安全指南中的上限。考虑到过度的浅表刺激强度和激活的脑体积，接近约6厘米的深度几乎肯定是不安全的。
线圈设计的局限性和权衡对于合理、安全地探索dTMS很重要。
