Applications of Supersonic Shear Imaging in the Musculoskeletal System

Abstract

Dear Editor,The skeletal muscle is an anisotropic, viscoelastic, andcomplex passive and active tissue. Therefore, the in vivoevaluation of the biomechanical properties of the skeletalmuscle is a complex issue. A new ultrasound-based tech-nique, supersonic shear imaging (SSI), can be used toquantify soft tissue stiffness [1]. Supersonic shear imagingis based on the conventional ultrasound probe, which in-duces an ultrasonic radiation force deep within the muscle.Propagation of the resulting shear waves is then imagedwith the same probe at an ultra-fast frame rate. The shearelasticity of a tissue can be mapped quantitatively from thispropagation movie. This approach may provide a completeset of quantitative and in vivo parameters describingbiomechanical properties of the skeletal muscle [2].Recent studies have shown excellent intra- and inter-observer reliability of the muscle shear elastic modulusmeasured by SSI [3,4]. Several studies also imply that SSI isa promising tool for evaluating muscle conditions because itmay provide an indirect estimation of passive muscle force[5]. It may also provide a more accurate estimation of in-dividual muscle force, compared to surface electromyog-raphy [6]. Different pathologies of the skeletal muscle(e.g., muscle fibrosis, muscular dystrophy, and spasticity inupper motor neuron diseases) may change the muscle shearelastic modulus. Thus, SSI may contribute to the improveddiagnosis and management of neuromuscular and orthope-dic diseases. However, a few considerations should beaddressed.First, all current studies have investigated healthy par-ticipants. The diagnostic value of SSI in patients should befurther studied. Second, few studies have focused on thetendon in which pathological changes may interfere withmuscle function. The tendon has a much higher elasticmodulus and smaller volume in comparison to the muscle,which makes SSI challenging for examining tendinopathy.Third, because skeletal muscle is compressible, variationsof the probe pressure on the muscle may cause differentshear elastic modulus. The higher pressure on the muscle,the higher is shear elastic modulus. A very light contactbetween the probe and the skin is recommended whenexamining muscle elasticity. Fourth, the region of interest(ROI) in SSI for obtaining shear elastic modulus is circular.Therefore, its representation of an entire muscle is ques-tionable. A standardized surface landmark and the depth ofROI should be clearly described. The average of the datafrom multiple ROIs may be calculated to minimize mea-surement errors.