ABSTRACT
Ultrasound consists of mechanical sound waves whose frequencies are above the audible range, that is, 20,000 Hz (Hz stands for the number of wave cycles per second). Sound is defined as a mechanical wave that propagates in a medium due to molecular interaction. The mode of propagation of ultrasound is related to successive molecular compressions and rarefactions occurring in that medium (Fig. 1.1). When individual molecular motion is in the same direction as the wave propagation, it forms a longitudinal wave. When molecular motion is perpendicular to wave direction, it is a transverse (or shear) wave. Solids, such as biological tissues, can experience both transverse (or shear) and longitudinal waves. Ultrasound in fluids and gases mostly experiences longitudinal propagation because of the lack of strong coupling between the molecules. Recent research suggests that shear waves may become clinically useful to characterize the viscoelastic properties of biological tissues and be used in sonoelasticity imaging and dynamic elastography.
