Audible-frequency methods in assessment of soft tissues

One of the reasons why MHz-range ultrasonic compressional waves have become a powerful tool for medical diagnostics is that their wavelength is of the order of a millimeter, adequate for imaging an anatomical structural. Other types of acoustic waves that can be generated in soft tissue, such as shear and surface waves, are two–three orders of magnitude slower and, respectively, have a millimeter range wavelength in the audible-frequency range. These waves can be effectively used for imaging and soft tissue characterization in terms of the shear elasticity modulus. This paper is a summary of the studies on applications of the surface and shear waves for diagnostic assessment of soft tissue elasticity and dynamic viscosity performed by the author and his coworkers over three decades. A number of techniques for measuring shear elastic moduli of soft tissue using both wave propagation and the acoustic impedance methods are described. Audible-frequency methods and devices for assessment of skin in dermatological applications, for the detection of breast cancer, for imaging of brain and muscle tissues are analyzed. Various embodiments of the Shear Wave Imaging modality are considered. The reasons for a limited success of the audible-frequency methods despite their great potential in ultrasonic medical imaging and diagnostics are discussed.