Scharfetter Hermann, Casañas Roberto, Rosell Javier
Institute for Biomedical Engineering, Graz University of Technology, Inffeldgasse 18, A-8010 Graz, Austria.
IEEE Trans Biomed Eng. 2003 Jul;50(7):870-80. doi: 10.1109/TBME.2003.813533.
Magnetic induction spectroscopy (MIS) aims at the contactless measurement of the passive electrical properties (PEP) sigma, epsilon, and mu of biological tissues via magnetic fields at multiple frequencies. Whereas previous publications focus on either the conductive or the magnetic aspect of inductive measurements, this article provides a synthesis of both concepts by discussing two different applications with the same measurement system: 1) monitoring of brain edema and 2) the estimation of hepatic iron stores in certain pathologies. We derived the equations to estimate the sensitivity of MIS as a function of the PEP of biological objects. The system requirements and possible systematic errors are analyzed for a MIS-channel using a planar gradiometer (PGRAD) as detector. We studied 4 important error sources: 1) moving conductors near the PGRAD; 2) thermal drifts of the PGRAD-parameters; 3) lateral displacements of the PGRAD; and 4) phase drifts in the receiver. All errors were compared with the desirable resolution. All errors affect the detected imaginary part (mainly related to sigma) of the measured complex field much less than the real part (mainly related to epsilon and mu). Hence, the presented technique renders possible the resolution of (patho-) physiological changes of the electrical conductivity when applying highly resolving hardware and elaborate signal processing. Changes of the magnetic permeability and permittivity in biological tissues are more complicated to deal with and may require chopping techniques, e.g., periodic movement of the object.
磁感应光谱学（MIS）旨在通过多个频率的磁场对生物组织的无源电学特性（PEP）电导率（σ）、介电常数（ε）和磁导率（μ）进行非接触式测量。以往的出版物要么侧重于感应测量的导电方面，要么侧重于磁方面，而本文通过讨论同一测量系统的两种不同应用，对这两个概念进行了综合：1）脑水肿监测和2）某些病理情况下肝脏铁储备的估计。我们推导了估算MIS灵敏度的方程，该灵敏度是生物物体PEP的函数。针对使用平面梯度仪（PGRAD）作为探测器的MIS通道，分析了系统要求和可能的系统误差。我们研究了4个重要的误差源：1）PGRAD附近的移动导体；2）PGRAD参数的热漂移；3）PGRAD的横向位移；4）接收器中的相位漂移。将所有误差与所需分辨率进行了比较。所有误差对测量复场的检测虚部（主要与σ有关）的影响远小于实部（主要与ε和μ有关）。因此，当应用高分辨率硬件和精细的信号处理时，所提出的技术使得分辨电导率的（病理）生理变化成为可能。生物组织中磁导率和介电常数的变化更难处理，可能需要采用斩波技术，例如物体的周期性移动。
