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On the Use of the Second-Order Acoustic Nonlinearity Parameter B/A for Ultrasonic Tissue Characterization
Book chapter

On the Use of the Second-Order Acoustic Nonlinearity Parameter B/A for Ultrasonic Tissue Characterization

Leif Bjørnø and Peter A. Lewin
Ultrasonic Tissue Characterization
1982
DOI: https://doi.org/10.1007/978-94-009-7666-5_8

Abstract

Biological Medium Harmonic Amplitude Monochromatic Wave Source Distance Ultrasonic Wave
The nonlinear character of nature is for instance reflected in the fact that the fundamental mechanisms of sound propagation in liquid media are nonlinear (1). Nevertheless, a linearization of the governing equations has in a number of cases led to mathematically less complicated expressions with solutions showing surprisingly good agreement with experimental results. However, experimental evidence is now accumulating indicating that a linear treatment is not always sufficient for an exhaustive description of for instance ultrasonic wave propagation in biological media (2,3). In spite of the fact that earlier works showed no intensity dependence for ultrasonic attenuation in tissue, recent and more careful studies have proven the existence of intensity dependent attenuation as well as the formation of higher harmonics to finite-amplitude monochromatic waves in biological liquids and tissues (4,5, 6,7). The ultrasonic wave distortion course leading to the formation of the higher harmonics is due to two main sources, the material nonlinearity of the medium being expressed by the nonlinear character of its equation of state and the convection nonlinearity expressed by the fact that the local particle velocity is a function of the local pressure amplitude in the wave. Since the high frequency components of an ultrasonic wave are absorbed more readily than are the lower frequency components, the effective absorption of the distorted wave is greater than the absorption of a monochromatic wave of the fundamental frequency.

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