Introduction to the Physics of Ultrasound

From an acoustical point of view, bone is a complex medium as it is heterogeneous, anisotropic and viscoelastic. This chapter reviews the basic notions of physical acoustics which are necessary to tackle the problem of the ultrasonic propagation in bone, in the perspective of the application of quantitative ultrasound (QUS) techniques to bone characterization. The first section introduces the basic phenomena related to the field of medical ultrasound. Basic description of wave propagation is introduced. Mechanical bases are necessary to understand the elastodynamic nature of the interaction between bone and ultrasound. The physical determinants of the speed of sound of the different types of waves corresponding to the propagation in a liquid and in a solid are considered. The effects of boundary conditions (guided waves) are also detailed. The second section describes the physical interaction between an ultrasonic wave and bone tissue, by introducing reflection/refraction, attenuation and scattering phenomena.