In vivo anisotropic mechanical properties of dystrophic skeletal muscles measured by anisotropic MR elastographic imaging: the mdx mouse model of muscular dystrophy.

PURPOSE: To evaluate the utility of mechanical anisotropy (shear storage modulus parallel to fiber/shear storage modulus perpendicular to fiber) measured by combined magnetic resonance (MR) elastography and diffusion-tensor imaging ( DTI diffusion-tensor imaging ) technique (anisotropic MR elastography) to distinguish between healthy and necrotic muscle with different degrees of muscle necrosis in the mdx mouse model of muscular dystrophy. MATERIALS AND METHODS: The experimental protocol was approved by the regional animal ethics committee. Twenty-one mdx and 21 wild-type ( WT wild type ) mice were used in our study. Animals were divided into exercised and sedentary groups. Anisotropic MR elastography was used to obtain mechanical anisotropic shear moduli for the lateral gastrocnemius and plantaris muscles in a 7-T MR imager, from which the mechanical anisotropic ratio was calculated. The animals were imaged before and after 10 weeks of a horizontal treadmill running protocol. Spearman rank correlations were used to compare MR elastographic data with muscle necrotic area percentage from histologic analysis. Mechanical anisotropy in WT wild type and mdx mice muscle were compared by using t test and one-way analysis of variance, and receiver operating characteristic curves were constructed by using statistical software. RESULTS: Anisotropic MR elastography was able to be used to distinguish between the muscles of mdx and WT wild type mice, with an area under the receiver operating characteristic curve of 0.8. Strong negative correlation (rs = -0.701; P < .001) between the mechanical anisotropic ratio and the percentage of muscle necrotic area was found. By comparing mice with no or mild (0%-5% mean necrotic area) and severe (>5% mean necrotic area) muscle necrosis, an area under the receiver operating characteristic curve of 0.964 was achieved. Diffusion parameters alone were unable to distinguish between the WT wild type and mdx mice at any time point. CONCLUSION: The mechanical anisotropic ratio of the shear storage moduli measured by an anisotropic MR elastographic technique can distinguish between healthy muscle and dystrophic muscle.

Combining MR elastography and diffusion tensor imaging for the assessment of anisotropic mechanical properties: a phantom study.

PURPOSE: To investigate the anisotropic elasticity of soft tissues using MR elastography (MRE) combined with diffusion tensor imaging (DTI). MATERIALS AND METHODS: The storage moduli parallel (µ(‖)) and perpendicular (µ(⊥)) to the local fiber orientation were calculated assuming a transversely isotropic model. The local fiber orientation was provided by DTI. The proposed technique was validated against rheometry using anisotropic viscoelastic phantoms with various fiber volume fractions (V(f) = 0%, 15%, and 35%) and bovine skeletal muscle samples. RESULTS: The anisotropic ratio (µ(‖)/µ(⊥)) as measured by MRE correlated well with rheometry for all samples (R(2) = 0.809). The combined MRE/DTI technique was also able to differentiate different levels of mechanical anisotropy with the mechanical anisotropy (µ(‖)/µ(⊥)) of the V(f) = 35% phantoms being significantly higher than the V(f) = 15% and the isotropic (V(f) = 0%) phantoms. The bovine muscle samples showed significantly higher mechanical anisotropy than all phantoms. CONCLUSION: This study has demonstrated the feasibility of the proposed imaging technique for characterizing mechanical anisotropy of anisotropic materials and biological tissues, and validated the mechanical anisotropy results.