Ultrasound shear wave elastography was developed the past decade, bringing new stiffness biomarker in clinical practice. This biomarker reveals to be of primarily importance for the diagnosis of breast cancer or liver fibrosis. In muscle this biomarker become much more complex due to the nature of the muscle itself: an anisotropic medium. In this manuscript we depict the underlying theory of propagating waves in such anisotropic medium. Then we present the available methods that can consider and quantify this parameter. Advantages and drawbacks are discussed to open the way to imagine new methods that can free this biomarker in a daily clinical practice.

Through the past few years, ultrasound (US) elastography has been widely applied to quantify muscle anisotropy. Generally, it is performed with an acoustic radiation force push beam that generates shear waves followed by US imaging. Recently, Ngo et al. (2021) proposed to use a steering push beam to comprehensively assess the mechanical properties of transverse isotropic skeletal muscle tissue. Here, we integrate the equation of shear vertical wave mode to the steering push beam method which allows to retrieve the mechanical parameters of anisotropic muscle tissue. Ex vivo experiments showed a good agreement between the tensile anisotropy χE found by our method and by the mechanical tensile tests. In vivo experiments were conducted to evaluate the anisotropy ratio measured by steering push beam method during different isometric contraction intensities. We observed a growing trend of this ratio with the contraction intensity in both fusiform (Biceps brachii) and pennate muscles (Medial gastrocnemius) of two healthy volunteers. Despite this trend was different between the two types of muscle architecture across contractions intensities, the overall difference had about the same magnitude for both volunteers.