Abstract
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. 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. 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 and , while FEA does with and . We demonstrate that FEA is able to estimate the optimal frequencies and electrode placements, and circuit-based modelling can accurately emulate the limbs' bioimpedance. The proposed methodology facilitates studying the impact of biophysical principles on EIM, enabling the development of future EIM acquisition systems.
Highlights
E LECTRICAL Impedance Myography (EIM) is a technique that consists in measuring the electrical impedance over a single muscle or group of muscles [1]
Volume impedance density, and individual tissues’ contributions were calculated for the entire frequency range of study (1 Hz – 1 MHz), only the 15.625 kHz – 250 kHz range is shown to facilitate the analysis, given that in this range muscle contributions are maximized as compared to the rest of tissues
We proposed a methodology with the aim of studying EIM in terms of its biophysical mechanism through finite element analysis (FEA), and developing a simple circuit-based model which accurately emulates the tissues and effects of parametric changes of subcutaneous fat and muscle thickness
Summary
E LECTRICAL Impedance Myography (EIM) is a technique that consists in measuring the electrical impedance over a single muscle or group of muscles [1]. The outer electrodes apply a sine-wave current stimulus (AC) in the kHz to MHz frequency range (α and β dispersions). EIM has been suggested as a method for reliably detecting muscle contractions [1], [3], [4]. Both single-frequency and multi-frequency EIM have been used to grade the severity of muscular disorders. Multi-frequency EIM provides more information of muscle condition
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