Nonlinearities in the Bioeffects of Ultrasound

Biological effects of ultrasound involve nonlinear phenomena in qualitatively different ways. (1) The propagation of sound itself is nonlinear. Most nonthermal bioeffects of interest to users of diagnostic ultrasound require acoustic pressures great enough that the wave becomes distorted by nonlinear propagation. Under 1imi ting conditions this process can increase the absorption parameter of weakly absorbing medta by orders of magnitude. (2) Acoustic cavitation is highly nonlinear. At low amplitudes the response of a bubble is dominated by the acoustic pressure. At a critical acoustic pressure, however, the inertia of the surrounding medium becomes controlling. At this threshold, a 10 or 20% increase in acoustic pressure Ieads to increases in the collapse pressures in the bubble by orders of magnitude. (3) Very little of the biochemistry, physiology and pathology of the biological system is linear. me rates of biochemical processes, including thermal denaturation of biological macromolecules, are exponential functions of the temperature. Wether the physical process of heating by ultrasound is linear or nonlinear, this leads to a very strong non] inear dependence of thermal tissue damage and teratologica[ effects upon the leveIs of ultrasound,