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The technique relies on a transient mechanical vibration which is used to induce a shear wave into the tissue. The propagation of the shear wave is tracked using ultrasound in order to assess the shear wave speed from which the Young's modulus is deduced under hypothesis of homogeneity, isotropy and pure elasticity (E=3ρV²).
MR elastography for detection of liver fibrosis in two patients. The left column shows the anatomy, with the liver outlined. The middle row shows images of propagating shear waves in the liver, captured with the MRE technique. The right column shows elastograms computed from the wave images, with tissue stiffness depicted with on color scale.
Shearography uses the test object itself as the known reference; it shears the image so a double image is created. The superposition of the two images, a shear image, represents the surface of the test object at this unloaded state. This makes the method much less sensitive to external vibrations and noise.
Tomoelastography of the abdomen; upper a healthy state, lower with malignancy.. Tomoelastography (from ancient Greek τόμος tomos, “slice” and elastography – imaging of viscoelastic properties) is a medical imaging technique that provides quantitative maps of the mechanical properties of biological soft tissues with high spatial resolution (called elastograms).
The acoustoelastic effect is an effect of finite deformation of non-linear elastic materials. A modern comprehensive account of this can be found in. [1] This book treats the application of the non-linear elasticity theory and the analysis of the mechanical properties of solid materials capable of large elastic deformations.
The shear modulus is one of several quantities for measuring the stiffness of materials. All of them arise in the generalized Hooke's law: . Young's modulus E describes the material's strain response to uniaxial stress in the direction of this stress (like pulling on the ends of a wire or putting a weight on top of a column, with the wire getting longer and the column losing height),
In mechanics, strain is defined as relative deformation, compared to a reference position configuration. Different equivalent choices may be made for the expression of a strain field depending on whether it is defined with respect to the initial or the final configuration of the body and on whether the metric tensor or its dual is considered.
Shear testing is the alternative method to determine the strength a bond can withstand. Various variants of shear testing exist. Like with pull testing, the objective is to recreate the failure mode of interest in the test. If that is not possible, the operator should focus on putting the highest possible load on the bond. [18]