Evaluation of electrical conductivity and anisotropy in muscle tissues using conductivity tensor imaging (CTI)

Low-frequency conductivity of an anisotropic tissue is associated with its cellular structure. Imaging of the conductivity tensor inside the human body could prove invaluable to analyses of interactions between electromagnetic fields and biological systems, such as predictions of current pathways during electrical stimulation. Since the muscle is the most abundant anisotropic tissue in the human body, in vitro and in vivo muscle conductivity tensor assessment has been attempted. In this study, we conducted phantom imaging of biological tissues using conductivity tensor imaging (CTI) to validate its in vivo usefulness. We constructed phantoms using bovine and/or porcine muscles and performed a CTI experiment using MRI. High-frequency conductivity was first obtained using B1 mapping with a multi-echo spin-echo pulse sequence. Information about the cellular space was obtained with a multi-b diffusion tensor imaging sequence. We combined the data from these separate scans to reconstruct the conductivity tensor images of the phantoms. The low-frequency and high-frequency conductivities of the muscle tissues in the phantoms were compared with the corresponding values measured by an impedance analyzer. The anisotropy of each muscle tissue was quantified as an anisotropy ratio (AR), defined as the ratio of the eigenvalues of a conductivity tensor along the longitudinal direction to those along the transversal directions. The isotropic conductivity and conductivity tensor in bovine muscles were less than those of porcine muscles. However, the anisotropy was stronger in bovine muscles based on the AR values by fiber directions. Current CTI is a promising noninvasive tool for evaluation of the muscle microstructure.