Comparison of procedures for determination of acoustic nonlinearity of some inhomogeneous materials

The widespread use of ultrasonic energy in medical diagnosis and therapy and the recent research showing the existence of nonlinear acoustic effects in inhomogeneous materials, like biological media, have led to a strongly increased interest in investigations aiming at providing enough information for an evaluation of the second order nonlinearity ratio B/A as a tissue characterizing parameter on a par with attenuation, impedance, etc. The several techniques used for the determination of B/A roughly fall into two main groups, finite-amplitude methods and thermo-dynamic-acoustic methods. The B/A values determined using various techniques of the two main groups are strongly scattered, showing a dependence on the procedure used, which makes it difficult to draw any valid conclusions. The paper discusses advantages and disadvantages of the techniques used hitherto, aiming at an explanation of the reasons for the deviations between the present B/A results for biological media. The necessary accuracy in the determination and the reliability in the exploitation of B/A as a tissue characterizing parameter for clinical use are emphasized.