Elastography Basic Principles

Abstract

Elastography displays the biomechanical properties of the tissue whereas B-mode ultrasound displays the acoustic properties. These properties are not related to each other. Elastography can be considered a virtual palpation: under a stress, stiffer tissues show less longitudinal displacement and higher speed of transverse displacement. Elastography techniques are based or on the assessment of the longitudinal displacement of the tissue induced by a stress, as in strain elastography and acoustic radiation force impulse (ARFI) imaging, or on the assessment of the speed of the transverse displacement of the tissue as in shear wave elastography. In strain elastography a part of the body is compressed axially by pressing across its surface and the internal tissue displacement is converted to a strain image (percentage of deformation), because the percentage of deformation will be constant whereas displacement decays with depth. In the ARFI imaging, the acoustic radiation force generates a localized displacement of a few microns in the ultrasound axial direction. ARFI generates also shear wave displacement at one point (point shear wave elastography) or along several lines (2D-shear wave elastography). Shear waves can be generated by an external mechanical push as well, as in vibration controlled transient elastography (VCTE), magnetic resonance elastography (MRE) or vibro-elastography.