Imaging the Viscoelastic Properties of Tissue

Abstract

Elasticity and viscosity of soft tissues are often related to pathology. These parameters, along with other mechanical parameters, determine the dynamic response of tissue to a force. Tissue mechanical response, therefore, may be used for diagnosis. Measuring and imaging of the mechanical properties of tissues is the aim of a class of techniques generally called elasticity imaging or elastography. The general approach is to measure tissue motion caused by a force or displacement and use it to reconstruct the elastic parameters of the tissue. The excitation stress can be either static or dynamic (vibration). Dynamic excitation is of particular interest because it provides more comprehensive information about tissue properties in a spectrum of frequencies. In one approach an external stress field must pass through the superficial portion of the object before reaching the region of interest within the interior. An alternative strategy is to apply a localized stress directly in the region of interest. One way to accomplish this task is to use the radiation force of ultrasound. This approach offers several benefits, including: (a) safety—acoustic energy is a noninvasive means of exerting force; (b) adaptability — existing ultrasound technology and devices can be readily modified for this purpose; (c) remoteness — radiation force can be generated remotely inside tissue without disturbing its superficial layers; (d) localization — the radiation stress field can be highly localized, thus allowing for precise positioning of the excitation point; and (e) a wide frequency spectrum. Several methods have been developed for tissue probing using the dynamic radiation force of ultrasound, including: (a) transient methods which are based on impulsive radiation force; (b) shear-wave methods which are based on generation of shear-waves; and (c) vibro-acoustography, recently developed by the authors, where a localized oscillating radiation force is applied to the tissue and the acoustic response of the tissue is detected by a hydrophone. Here, we focus on vibro-acoustography and present a detailed description of the theory and the experimental results. We conclude with the capabilities and limitations of these radiation-force methods.