In the mid-1990s a method named Shear Wave Elasticity Imaging (SWEI) for mapping tissue elasticity has been proposed. The method is based on the use of acoustic radiation force of focused ultrasound to create shear waves in soft tissue. By measuring shear wave propagation parameters using ultrasound or MRI the tissue elasticity map can be created. Since the terms "Elasticity Imaging" and "Elastography" are synonyms, the original term SWEI is often changed to SWE, which commonly stands for Shear Wave Elastography. The shear wave speed is governed by the shear modulus of tissue which is highly sensitive to physiological and pathological structural changes of tissue. Variation of the shear modulus may be several orders of magnitude depending on the structure and state of tissue. This variation of the shear wave speed increases in many tissues in the presence of disease, e.g. the cancerous tissues can be significantly stiffer than normal tissue. For this reason, the possibility of using shear waves in new diagnostic methods and devices has been extensively investigated over the last two decades.
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Use in different elastographic methods
Numerous new methods were developed most notable of which are Shear Wave Elasticity Imaging (SWEI), Acoustic Radiation Force Impulse Imaging (ARFI), 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).
These methods use different means to generate and measure the propagation of shear waves in tissue. The first elasticity imaging technologies based on the use of ARF were SWEI and ARFI. Principal difference between these technologies is that SWEI is based on the use of shear waves propagating sideways from the beam axis and creating elasticity map by measuring shear wave propagation parameters whereas ARFI gets elasticity information from the axis of the pushing beam and uses multiple pushes to create a two-dimensional stiffness map. No shear waves are used in ARFI and no axial elasticity assessment is involved in SWEI. Shear wave elasticity imaging has been developed into a clinical imaging modality over the last two decades and the radiation force-based methods are currently implemented in the commercial devices: SuperSonic Imagine Aixplorer, in the Siemens Acuson S2000 and S3000 as Virtual Touch Quantification, and in the General Electric Logiq E9.
Implementation of SWEI in Supersonic Shear Imaging
One of the most advanced modalities of shear wave elasticity imaging is Supersonic Shear Imaging (SSI). There are two principal innovations implemented in SSI. First, by using many near-simultaneous pushes, SSI creates a source of shear waves which is moved through the medium at a supersonic speed. Second, the generated shear wave is visualized by using ultrafast imaging technique. Using inversion algorithms, the shear elasticity of medium is mapped quantitatively from the wave propagation movie. SSI is the first ultrasonic imaging technology able to reach more than 10,000 frames per second of deep seated organs. SSI provides a set of quantitative and in vivo parameters describing the tissue mechanical properties: Young’s modulus, viscosity, anisotropy.
Application for tissue characterization
Various parameters of tissue characterizing its structure and state such as anisotropy, viscosity, and nonlinearity can be assessed using shear 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. The shear wave attenuation is high so the waves do not propagate very far. This is often an advantage because the shear waves induced by acoustic radiation force are less prone to artifacts from reflections and interactions with other tissue boundaries.