Transient cavitation in tissues during ultrasonically induced hyperthermia.

An isospherical array of ultrasonic transducers designed for hyperthermia treatment of malignant tumors in humans was employed in studies of transient cavitation in tissues during ultrasonic heating. Both in vitro and in vivo studies revealed the presence of transient cavitation within tissues during ultrasonic heating, as indicated by the detection of the first half-order subharmonic of insonifying ultrasound with an interrogating transducer. The applied power to the focal region of the array required to exceed cavitation thresholds in tissue was 75 +/- 8 W/cm2, a level considerably higher than that required to heat tissues to the temperature range used for cancer treatment (43-45 degrees C).