Theoretical models of ultrasound absorption in tissues

On the basis of many measurements on attenuation, absorption, and speed of ultrasound in biological materials, one can make the following generalizations: (a) Biological tissues have high attenuation coefficients, about two to three orders greater than their major component, water; (b) the amplitude attenuation coefficient increases with frequency near-linearly in soft tissues and near-quadratically in fluids with frequency in the range 1 to 10 MHz, and possibly even up to 100 MHz; (c) speed dispersion is extremely small (∼0.1 to 1 m/s/ MHz); (d) absorption and attenuation coefficients either increase or decrease with an increase in temperature, depending on the nature of the tissue, and the frequency of irradiatiOn; (e) fluids containing proteins show a sharp increase in excess absorption in acidic and basic pH ranges, and finally; (f) at sufficiently high intensity of irradiation the absorption coefficient increases with intensity. Comprehensive models for the propagation of ultrasonic waves through lossy media should explain these experimental findings. We will review some of the current models and critically examine them in light of the definition of absorption in relation to attenuation and scattering. [Work supported by NIH CA 24085, CA 41324, and NSF 8310626.]