Ultrasound waves are mechanical waves with frequency over 20 KHz. In an infinite homogeneous lossless propagating medium, waves are called bulk waves and they can be classified as compressional waves (or irrotational, longitudinal, dilatational, volume or P-waves) when particles displacement is parallel to the wave propagation, or shear waves (or rotational, transversal, distortional or S-waves) when particles displacement is orthogonal to the wave propagation (terminology is often borrowed from seismology).
|Compressional wave||Shear wave|
|(images by Christophe Dang Ngoc Chan)|
In practical terms a medium can be considered infinite when its characteristic dimension is much larger than the involved wavelength.
In homogeneous lossless media, velocity of bulk waves only depends on the medium effective mechanical properties (i.e., effective properties at the wavelength scale). In a homogeneous, elastic and isotropic medium the velocity of compressional (Vc) wave is given by:
In a multi-layered medium, the interface between mediums generates reflections and refractions which can produce new propagation modes (mode conversion). In the particular case of the skin-tendon complex (or the skin-bone complex), tendon propagates at a higher velocitiy than skin, since it is more rigid, so a wave generated in the skin (e.g., at the skin surface) can produce a faster wave propagating through the tendon. This wave, named head wave, runs at the tendon-skin interface, axially along the tendon; thus, this propagation mode is defined axial propagation. The head wave leaks energy through the interface back into the skin generating the so-called lateral wave. The latter can be detected with a mechano-electric transducer at the skin surface.
The interest in the lateral wave lies in the fact that it propagates through the tendon, at the tendon bulk wave velocity, so it reflects the tendon mechanical properties although it is generated and detected noninvasively at the skin surface. The lateral wave model has been studied and demonstrated for skin-bone complexes, and has been applied to bone characterization (bone mass density, elastic modulus, ultimate strenght, etc.). However, there are many similarities between long bones and tendons, although they represent two different propagating mediums, so the same formalism developed for bone, at first approximation, can be applied to tendons.
See scheme below for a representation of the wave propagation involved, and see the original paper for more information (Pourcelot et al., 2005).