Dear Editor,
The skeletal muscle is an anisotropic, viscoelastic, and complex
passive and active tissue. Therefore, the in vivo evaluation of the
biomechanical properties of the skeletal muscle is a complex issue. A new
ultrasound-based technique, supersonic shear imaging (SSI), can be used to quantify
soft tissue stiffness [1].
Supersonic shear imaging is based on the conventional ultrasound probe, which induces
an ultrasonic radiation force deep within the muscle. Propagation of the resulting
shear waves is then imaged with the same probe at an ultra-fast frame rate. The
shear elasticity of a tissue can be mapped quantitatively from this propagation
movie. This approach may provide a complete set of quantitative and in vivo
parameters describing biomechanical properties of the skeletal muscle [2]. Recent
studies have shown excellent intra- and interobserver reliability of the muscle
shear elastic modulus measured by SSI [3,4]. Several studies also imply that
SSI is a promising tool for evaluating muscle conditions because it may provide
an indirect estimation of passive muscle force [5]. It may also provide a more accurate estimation of individual
muscle force, compared to surface electromyography [6]. Different pathologies
of the skeletal muscle (e.g., muscle fibrosis, muscular dystrophy, and
spasticity in upper motor neuron diseases) may change the muscle shear elastic
modulus. Thus, SSI may contribute to the improved diagnosis and management of
neuromuscular and orthopedic diseases. However, a few considerations should be addressed.
First, all current studies have investigated healthy participants.
The diagnostic value of SSI in patients should be further studied.
Second, few studies have focused on the tendon in which
pathological changes may interfere with muscle function. The tendon has a much
higher elastic modulus and smaller volume in comparison to the muscle, which
makes SSI challenging for examining tendinopathy.
Third, because skeletal muscle is compressible, variations of
the probe pressure on the muscle may cause different shear elastic modulus. The
higher pressure on the muscle, the higher is shear elastic modulus. A very
light contact between the probe and the skin is recommended when examining
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 questionable. A standardized surface landmark and the depth of ROI
should be clearly described. The average of the data from multiple ROIs may be
calculated to minimize measurement errors.
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