Quantification of anisotropic biophysical properties of lower leg muscles at passive dorsiflexion and plantarflexion using magnetic resonance elastography and diffusion tensor imaging.

Abstract

Determining the biomechanical properties of skeletal muscle in-vivo is challenging due to structural anisotropy. In this study, we developed combined diffusion tensor imaging (DTI) and magnetic resonance elastography (MRE) to quantify direction-dependent biophysical properties of the lower leg muscles and their changes during passive plantarflexion (PF) and dorsiflexion (DF). Thirteen male volunteers were studied using DTI-MRE. Anisotropic shear-wave-speeds parallel (c_∥) and perpendicular (c_⊥) to the fiber orientation were reconstructed by aligning MRE vector wave fields to the principal fiber axis with rotation angles obtained from DTI tractography. Isotropic c_iso was also calculated without rotation for comparison. Fractional anisotropy (FA), radial (RD) and axial diffusivity (AD) were obtained from DTI. c_∥ was higher than c_⊥ in tibialis anterior (TibA), whereas the opposite was observed in posterior soleus (SolP). From PF to DF, c_⊥ and c_∥ changed significantly in all muscles: TibA (-15 ± 11 %, -15 ± 13 %), SolP (8 ± 12 %, 9 ± 11 %), and gastrocnemius medialis (GasM) (11 ± 15 %, 21 ± 14 %), respectively (all p < 0.05). c_iso was only sensitive in TibA (-13 ± 7 %) and GasM (4 ± 11 %), both p < 0.05. For DTI, from PF to DF, FA and RD changed significantly in TibA (-20 ± 12 %, 10 ± 7 %), SolP (26 ± 12 %, -6±6 %), and GasM (19 ± 12 %, -5±7 %), respectively (all p < 0.001). AD only changed in SolP (3 ± 5 %, p < 0.01). In conclusion, anisotropic MRE was more sensitive to ankle positions in lower leg muscles than isotropic MRE and revealed biomechanical differences between muscle types. In the future, DTI-MRE with anisotropic parameter reconstruction could be used for the detection of subtle structural changes in muscle diseases.