Measurements of bubble-enhanced heating from focused, MHz-frequency ultrasound in a tissue-mimicking material

Time-resolved measurements of the temperature field in an agar-based tissue -mimicking phantom insonated with a large aperture 1-MHz focused acoustic t ransducer are reported. The acoustic pressure amplitude and insonation dura tion were varied. Above a critical threshold acoustic pressure, a large inc rease in the temperature rise during insonation was observed. Evidence for the hypothesis that cavitation bubble activity in the focal zone is the cau se of enhanced heating is presented and discussed. Mechanisms for bubble-as sisted heating are presented and modeled, and quantitative estimates for th e thermal power generated by viscous dissipation and bubble acoustic radiat ion are given. (C) 2001 World Federation for Ultrasound in Medicine & Biolo gy.