Shear waves and compressional waves are the two main modes of propagation of acoustic energy in solids. With shear waves, also called transverse waves, the particles of the medium oscillate at a right angle to the direction of propagation. In compressional waves the oscillations occur in the longitudinal direction or the direction of wave propagation. The wave speeds of these different kinds of waves are governed by two different types of moduli. The compressional wave speed is related to the bulk elasticity modulus of the medium while the shear wave speed is related to the shear elasticity modulus. In soft biological tissues the bulk modulus varies no more than 10% while variation of the shear elasticity modulus may be several orders of magnitude depending on the structure and state of tissue.
Shear Waves in Elastography
The bulk modulus is defined by short range molecular interaction forces and depends mainly on molecular composition of tissue which is typically 75% water with little variation. In contrast to that, the shear modulus is defined by long range interactions and is highly sensitive to structural changes. The wide range of variability makes the shear elasticity modulus and, respectively, the shear wave speed, highly sensitive to physiological and pathological structural changes of tissue. For this reason, the use of shear waves in new diagnostic methods and devices has been extensively investigated over the last two decades. Numerous new methods were developed most notable of which are Shear Wave Elasticity Imaging (SWEI), Magnetic Resonance Elastography (MRE), Supersonic Shear Imaging (SSI), Shearwave Dispersion Ultrasound Vibrometry (SDUV), Harmonic Motion Imaging (HMI), Comb-push Ultrasound Shear Elastography (CUSE), and Spatially Modulated Ultrasound Radiation Force (SMURF).
Modes of Shear Wave Generation
In elastographic applications different means to generate and measure the propagation of shear waves in tissue are used. One of the early methods for generating shear waves in tissue was to use external mechanical actuators. Starting in the mid-1990s, another method was introduced to create shear waves which involved the use of focused ultrasound to produce acoustic radiation force.
Various parameters of tissue characterizing its structure and state such as anisotropy, viscosity, and nonlinearity can be assessed using shear waves. In contrast to compressional waves, shear waves are polarized, which makes them sensitive to tissue anisotropy, an important structural anatomical characteristic that can have diagnostic value. By directing shear waves in different directions it is possible to characterize tissue anisotropy. The large frequency range of the shear wave that can be generated in tissue provides potential for using this high bandwidth for tissue viscoelastic properties estimation.
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