Ultrasonic hysteresis in biological media.

Non-linear mechanical response of viscoelastic strain responding media to high amplitude stress-strain is examined from a phenomenological point of view and found to lead to results compatible with empirical observations of high intensity ultrasound irradiation of brain, liver, and eye lens tissues. The proposed hysteresis model provides for most of the observed dependencies such as an intensity dependent absorption coefficient, an absorption coefficient increasing linearly with frequency, and a dispersionless velocity of ultrasound in soft tissues (excluding lung). The non-linear compliance of tissues further predicts production of half-harmonic signals even in the absence of cavitation.