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/cm² and a frequency of 3.2 MHz, a resonant microbubble, approximately 0.75 μm in radius, absorbed ultrasonic power of 8.6 μW. 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/mm³. 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.