In vivo acceleration of ultrasonic tissue heating by microbubble agent

Ultrasonic power absorbed by a microbubble was calculated through numerically solving Rayleigh-Plesset equation to analyze its nonlinear breathing motion. At an ultrasonic intensity of 0.33 W/cm/sup 2/ and a frequency of 3.2 MHz, a resonant microbubble, approximately 0.75 /spl mu/m in radius, absorbed ultrasonic power of 8.6 /spl mu/W. This calculation predicts that tissue ultrasonic absorption will be increased by more than twice if microbubbles are delivered to the tissue at a concentration in the order of 20 resonant microbubbles/mm/sup 3/. An exteriorized murine kidney was exposed to HIFU at 3.2 MHz in degassed saline and the tissue temperature change was measured. With administration of 0.2 ml/kg Optison, the temperature elevation induced by HIFU exposure was multiplied by two to three times. This effect may have a potential use to enhance the selectivity as well as the throughput of HIFU treatments.