Recording characteristics of electrical impedance myography needle electrodes

Objective: Neurologists and physiatrists need improved tools for the evaluation of skeletal muscle condition. Here we evaluate needle electrical impedance myography (EIM), a new minimally invasive approach to determine muscle status that could ultimately become a bedside tool for the assessment of neuromuscular disorders. Approach: We design and study the recording characteristics of tetrapolar EIM needle electrodes combining theory and finite-element model simulations. We then use these results to build and pilot in vivo an EIM needle electrode in the rat gastrocnemius muscle (n = 2). The dielectric properties of muscle are reported (mean +/- standard deviation). Results: The numerical simulations show that the contribution of subcutaneous fat and muscle tissues to needle EIM data is <3% and >97%, respectively, and the sensed volume is 0.70 cm(3). Apparent resistivity 277 +/- 30 Omega cm and relative permittivity (1 +/- 0.4) x 10(5) (dimensionless) measured at 10 kHz are in good agreement with in vivo dielectric properties reported in the literature. Significance: The results presented show the feasibility of measuring muscle impedivity in vivo using a needle electrode from 10 kHz to 1 MHz. The development of needle EIM technology can open up a new field of study in electrodiagnostic medicine, with potential applications to both disease diagnosis and biomarker assessment of therapy.