What is Finite-Amplitude Acoustic Wave

“Elastography” or “elasticity imaging” can be defined as the science and methodology of estimating the mechanical properties of a medium (including soft tissue). In this chapter, an overview of elastography and its relation to tissue pathology will be presented. The basic principles of the static and dynamic methods will be described with special emphasis on the dynamic methods that rely on the acoustic radiation force of ultrasound. Of interest are the low-frequency narrowband shear waves that can be generated by a modulated radiation force produced by the interference of two continuous-wave (CW) ultrasound beams of slightly different frequencies. The advantages of using narrowband shear waves to estimate the viscoelastic properties of tissue will be discussed. Furthermore, an implementation of the inverse-problem approach will be presented and it will be shown how harmonic maps of the local shear modulus and viscosity can be reconstructed based on both the fundamental and higher-harmonic components of the propagated narrowband shear waves.