Elastogram: Physics, clinical applications, and risks

Abstract

The tissue stiffness is always an interesting issue to clinicians. Traditionally, it is assessed by the manual palpation, and this now can be measured by the ultrasound-based elastography. The basic physics is based on Young's modulus through the Hooke's law: E= S/e, where the Young's modulus (E) equals to the stress applied to the object (S) divided by the generated strain (e). With the rapid advancement of technology, the elastography has evolved from quasi-static elastography (ie, strain elastography) to dynamic elastography (i,e, shear wave elastography). The key differentiation of these two categories roots in the stimuli applied, namely mechanical or acoustic radiation force, and the response of the soft tissue. The strain elastography requires the operator to compress and decompress the tissue manually and the motion of the tissue during the stimuli is tracked to calculate the strain to reflect the tissue stiffness. While strain elastography is operator-dependent, shear wave elastography is not. Using shear wave elastography, the tissue is stimulated by the acoustic radiation force which can generate shear wave traveling through the tissue transversely. The shear wave propagation speed (Vs) is related to the shear modulus (μ) of the medium: μ = ρVs2, where ρ is the density of the tissue and assumed to be a constant as 1000 kg/m3. In the incompressible biological tissue, the Young's modulus is approximately three times the shear modulus (E≈3 μ). So the quantitative measurements of the tissue stiffness can be attained by shear wave elastography. The clinical application of elastography and its diagnostic capability has been extended. The knowledge of the basic physics of the various type of elastography facilitates the effective use of elastography. This review presented the clinical application and the risks of different types of elastography.