ABSTRACT
It is well known that ultrasound may influence biologic tissues through thermal and mechanical effects, and it should be noted that teratogenicity has been reported as a consequence of the exposure of animal embryos and fetuses to high temperatures. 1 No hazardous effect is expected when the temperature rise is less than 1.5°C or the thermal index (TI) † is less than 1 for a long exposure, while 5 minutes’ exposure of young tissue to 41°C can be hazardous. 2 , 3 The hazard threshold for pulsed ultrasound output power was found to be spatial peak temporal average (SPTA) 240 mW/cm 2 for cultured cells, with no suppressive effect being found below this power level in our previous studies. 4 The non-thermal effects of ultrasound include inertial cavitation and other mechanical effects. Although cavitation in liquids can lead to free-radical formation, that does not occur in the cytoplasm because of its high viscosity, and free radicals formed in the extracellular liquid may not reach cells because of their short lifetime. However, intense mechanical effects may produce hemorrhage in the neonatal animal lung.
