Physics department, Northeastern University, Boston, MA, USA.
Physiol Meas. 2013 Nov;34(11):1513-29. doi: 10.1088/0967-3334/34/11/1513. Epub 2013 Oct 22.
A major problem confronting application of impedance techniques to studies of disease is the extraction of intrinsic properties of the tissue from the measured impedances, which unavoidably involve geometric factors as well. Amongst the foremost are the sizes and locations of the measuring electrode arrays, and this paper addresses one of these, the location of current injecting electrodes. Tetrapolar impedance measurements on a 17.5 cm segment of the thigh gave R and X values three to four times larger when the current injecting electrodes were placed 2.5 cm from the sensing electrodes than when very distant placement was used. The frequency dependences of R and X were affected as well, though the X versus R plots still showed virtually perfect depressed-center semicircles, as in the Cole model. R(f) and X(f) for the set of contiguous 2.5 cm wide sub-segments show that these behaviors can be explained by a combination of the transverse orientation of current flow lines near the injecting electrodes and the anisotropy of the resistivity associated with the bundled fiber structure of muscle tissue. The measured impedance was found to be a separable function of geometric and intrinsic tissue variables, but far more complicated than is implicit in the usual cylindrical models. The results also suggest that many full and segmental body composition studies in the literature may be prone to substantial errors due to too close placement of the current injecting electrodes.
将阻抗技术应用于疾病研究面临的一个主要问题是从测量阻抗中提取组织的固有特性，这不可避免地涉及到几何因素。其中最重要的是测量电极阵列的大小和位置，本文解决了其中一个问题，即电流注入电极的位置。在大腿的 17.5 厘米段上进行的四极阻抗测量结果表明，当电流注入电极距离感应电极 2.5 厘米时，R 和 X 值比使用非常远的位置时大三到四倍。R 和 X 的频率依赖性也受到了影响，尽管 X 与 R 的关系图仍然显示出几乎完美的中心凹陷半圆形，如科尔模型所示。对于一组连续的 2.5 厘米宽的子段，R(f)和 X(f)表明这些行为可以通过电流流线在注入电极附近的横向取向和与肌肉组织纤维束结构相关的电阻率各向异性的组合来解释。测量到的阻抗被发现是几何和固有组织变量的可分离函数，但比通常的圆柱形模型所隐含的要复杂得多。结果还表明，文献中许多全身和节段性身体成分研究可能由于电流注入电极的放置过于接近而容易出现较大误差。
