Simultaneous assessments of local muscle quantity and quality by electrical impedance tomography (EIT) imaging

Objectives. Local muscle quantity and quality in the human leg have been assessed simultaneously across multiple muscle compartments by electrical impedance tomography (EIT) imaging. Approach. Three EIT parameters are defined, which are (1) the spatial-mean conductivity <σ> (2) the spatial-mean phase angle <Φ>, and (3) the mean resistance R ̅. Three parameters are assessed in the calf and thigh of ten subjects by comparing <σ> with the local muscle thickness t by ultrasound, <Φ> with the local fat infiltration ratio FIR by ultrasound, and R ̅ with the ratio of extracellular water to total body water (ECW/TBW) e by bioelectrical impedance analysis (BIA). Moreover, three parameters are compared with isometric strength tests, which assess knee extension power, knee flexion power, ankle plantar flexion power, and ankle flexion power. Main results. As the experimental results, the local muscle quantity and quality in the calf and thigh of ten subjects were imaged using σ and Φ distributions. Moreover, <σ> has a positive correlation with t (correlation coefficient cc = 0.735, p < 0.05), <Φ> has a negative moderate correlation with the local FIR (cc = −0.585, p < 0.05), and R ̅ has a strong positive correlation with e (cc = 0.862, p < 0.001). Furthermore, multiple regression using <σ>, <Φ>, and R ̅ as explanatory variables has the highest correlation for ankle plantar flexion power (cc = 0.823, R^2 = 0.706) among isometric strength tests, which is related to the tibialis anterior muscle. Significance. The reason for the observed associations with local muscle quantity, quality, and strength is that <σ> reflects muscle fiber density characterized by high tissue conductivity, <Φ> reflects muscle quality related to cell membrane capacitance, structural integrity, and fat infiltration, and R ̅ reflects extracellular water and local edema, which are collectively considered to constitute muscle strength.