It was recently reported that not only the cavitational bioeffect but also the thermal bioeffect of ultrasound can be enhanced with microbubble agents. If the ultrasonic absorption of tissues can be significantly increased in a well-controlled manner with administration of such a microbubble agent, it will be especially useful for the ultrasonic power high enough for the treatment is difficult to deliver. The possibility of this approach was checked in a preliminary in vitro ultrasonic exposure experiment of Levovist suspension. Ultrasonic power absorbed and scattered by a microbubble was calculated through numerically solving Rayleigh-Plesset equation to analyze its nonlinear breathing motion. It is estimated that approximately 3 microbubbles in the resonant size per cubic millimeter of tissue are needed to double the tissue absorption due to ultrasonic exposure at 2 MHz. This microbubble concentration is achievable in a human body. It is also predicted that the microbubble size may need to be controlled small enough to maintain the ultrasonic penetration for efficient ultrasonic heating of tissues.
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