A Finite Element Analysis and Circuit Modelling Methodology for Studying Electrical Impedance Myography of Human Limbs

Objective: Electrical impedance myography (EIM) measures bioimpedance over muscles. This paper proposes a circuit-based modelling methodology originated from finite element analysis (FEA), to emulate tissues and effects from anthropometric variations, and electrode placements, on EIM measurements. The proposed methodology is demonstrated on the upper arms and lower legs. Methods: FEA evaluates impedance spectra (Z-parameters), sensitivity, and volume impedance density for variations of subcutaneous fat thickness (t$_{f}$), muscle thickness (t$_{m}$), and inter-electrode distance (IED), on limb models over 1Hz-1 MHz frequency range. The limbs’ models are based on simplified anatomical data and dielectric properties from published sources. Contributions of tissues to the total impedance are computed from impedance sensitivity and density. FEA Z-parameters are imported into a circuit design environment, and used to develop a three Cole dispersion circuit-based model. FEA and circuit model simulation results are compared with measurements on ten human subjects. Results: Muscle contributions are maximized at 31.25 kHz and 62.5 kHz for the upper arm and lower leg, respectively, at 4 cm IED. The circuit model emulates variations in t$_{f}$ and t$_{m}$, and simulates up to 89 times faster than FEA. The circuit model matches subjects measurements with RMS errors $\mathbf {< \;}36.43\boldsymbol{\Omega }$ and $\mathbf {< \;}17.28\mathbf {^{\circ }}$, while FEA does with $\mathbf {< \;}36.59\boldsymbol{\Omega }$ and $\mathbf {< \;}4.36\mathbf {^{\circ }}$. Conclusions: We demonstrate that FEA is able to estimate the optimal frequencies and electrode placements, and circuit-based modelling can accurately emulate the limbs’ bioimpedance. Significance: The proposed methodology facilitates studying the impact of biophysical principles on EIM, enabling the development of future EIM acquisition systems.