Temperature rise recorded during lesion formation by high-intensity focused ultrasound

Temperature rise was observed as a function of time in liver and dog prostate tissue ex vivo during heating with high-intensity focused ultrasound. The temperature rise was measured using a needle thermocouple placed at the focus. The temperature vs. time behaviour closely followed the predictions of a model based on bulk and surface heating. When the tissue temperature was raised above 50°C, an increase in heating rate was seen. At higher temperatures, a point was reached at which a marked, irreversible change of tissue properties was observed, consistent with protein denaturation. The change was sometimes accompanied by a sudden further rise in temperature followed by an equally sudden fall. On dissection, regions of tissue damage (lesions) were seen, sometimes containing bubbles consistent with acoustic cavitation or vaporisation.