Study of anisotropy by transient elastography

Abstract

From the low-frequency (150 Hz) shear-waves’ velocity, transient elastography can precisely measure the Youngs modulus in isotropic soft tissues. Using polarized shear waves, the character of an anisotropic media like fiber muscle is clearly visible. The polarization of elastic shear waves is obtained by the use of a rod (3×80 mm) as a low-frequency source. Indeed, the displacement field induced within a medium when an acoustic pulse (150 Hz) is applied at the surface by a rod is no more axisymmetric: the component of the displacement parallel to the rod is smaller than the component perpendicular to the rod. These polarized shear waves are well predicted by a numerical simulation of theoretical Greens functions. It is shown that the shear-wave velocities in a beef muscle (biceps femoris) strongly depend on the position of the rod as regard to the fibers. These results show that the anisotropic model for this muscle is the hexagonal system. Results in vivo on man biceps also show the existence of a low and a fast shear wave. We have started on this subject a collaboration with the myology institute at the hospital La Piti Salptrire in Paris.