Characterization of biological media by means of their non-linearity.

This review presents the state-of-the-art in applications of the second order acoustic non-linearity parameter, B/A, for the characterization of biological media. The close relationship between B/A and the molecular structure of certain liquids, as well as the influence of the inhomogeneous nature of biological materials on B/A, are discussed. The finite amplitude and the thermodynamic methods are presented, and their advantages, disadvantages and accuracy of data produced are discussed. A new and promising procedure, real time non-linear parameter tomography, is critically evaluated and its prospective application for in vivo studies is treated in some depth. B/A values covering a broad variety of biological materials are presented and a world-wide collaborative research scheme is suggested for systematic and in-depth studies of non-linearity of biological media and for clinical exploitations of the results of these studies.