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

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.