Principles of ultrasound elastography

Many manufacturers of ultrasound systems offer elastography capabilities. Though the research literature describes techniques involving external actuators, commercial systems have preferred to adopt techniques that may be used with a single transducer. Techniques may be divided into those which measure strain and those which measure shear wave velocity. Strain elastography involves deformation of the tissue followed by imaging of the degree of compression or extension of the tissue. Strain elastography does not estimate tissue stiffness; however, the strain ratio may be used as a surrogate index of stiffness. Shear wave elastography provides true quantitative information on elastic modulus. This involves induction of shear waves, estimation of shear wave velocity c s and conversion to elastic modulus E using the equation E = 3ρ c s 2 where ρ is the density. The description of tissues as being purely elastic is simplistic. In practice they may also exhibit time-dependent viscous behaviour. Recent literature describe methods that have been developed for estimation of the viscoelastic behaviour from the change in strain with time or by estimation of the shear wave dispersion, a technique known as ‘shear wave spectroscopy’. These methods may become commercially available in the medium term, offering a new quantity (tissue viscosity) for diagnostic use.