Evaluation of shear properties in muscle tissue via novel superficial elastography technique

Noninvasive measurement of the mechanical properties of soft tissue is of significant clinical interest. Recent work has demonstrated the ability of ultrasound (US) elastography techniques to measure shear modulus and shear viscosity of bicep muscle and liver tissue in vivo using specialized equipment. The objective of this study was to evaluate a new elastography technique in the upper trapezius muscles in patients with acute neck pain. Ultrasound RF data were acquired from both symptomatic and asymptomatic (pain free) subjects. The upper trapezius muscle was externally vibrated at frequencies in the range of 60 - 200 Hz. The phase of the received RF signals were used to compute the shear wave images that represent the phase of the externally-induced vibration as it travels through the tissue. The spatial gradient of the vibration phase yielded the shear wave velocity, which was used to estimate shear modulus and shear viscosity based on the shear Voigt model for viscoelastic materials. Myofascial Trigger Points (MTrPs) in the upper trapezius had significantly higher shear modulus (12.5±4.1 kPa) than normal tissue sites (6.4±2.4 kPa) (P<;0.05). The approach presented here provides a quick and effective method for quantitatively measuring region specific shear properties in both normal and symptomatic muscle tissue, and can be easily translated to a clinical setting using commercially-available equipment.