Introduction to tissue shear wave elastography

Ultrasonic elastography is a technique allowing imaging of the elastic properties of tissue. There are two basic techniques of elastographic imaging; compressional - displaying the evaluation of tissue deformation under the external stress; and dynamic, tracking the propagation velocity of the shear wave generated by the acoustic radiation force. Soft tissue bulk modulus varies, from a few to several GPa, whereas the shear modulus is significantly smaller, not exceeding a few hundred Pa for adipose tissue, breast or liver, up to several hundred kPa for “hard” tissue. Forces generated in the tissue due to the external, axial piston-like stresses depend mainly on the shear modulus. In Shear Wave Elastography, long, several tens of microseconds, ultrasonic pulses successively focused at several depths are sent: generating a conical wave front moving with the supersonic velocity, depending on the tissue stiffness. Velocity of propagation of shear wave depends on the shear modulus μ and the modulus of elasticity E of the examined tissue is equal to E=3μ.