Optically transparent gel for experimentally mimicking cavitation enhanced ultrasonic heating of tissue

High intensity focused ultrasound (HIFU) is a noninvasive method for cancer treatment. However, there is a problem of a long treatment time for treating a large volume. It is known that cavitation bubbles, generated by extremely high intensity ultrasound pulses, enhance the heating effect of HIFU. In order to investigate a cavitation-enhanced highly-efficient method of HIFU, an optically transparent gel with both ultrasonic absorption and cavitation threshold similar to biological tissue is being developed. Such a polyacrylamide (PAA) gel was successfully produced by controlling the concentrations of both acrylamide and albumin. The effect of cavitation bubbles enhancing the ultrasonic heating was measured using the gel by exposing it to HIFU. The effect was considered in the bio-heat transfer equation (BHTE) by increasing the ultrasonic absorption coefficient in the region of the cavitation, whose volume was determined by high-speed-camera observation. The absorption coefficient was calculated by fitting between the temperature rise curves at the focal point in the experiment and numerical simulation. The simulation using the obtained absorption coefficients of the gel with and without cavitation showed overall agreement with the experiment using the gel. The developed gel and method will be useful for further development of this HIFU method.