Acoustic radiation force in vivo: A mechanism to assist targeting of microbubbles

(T)he goal of targeted imaging is to produce an enhanced view of physiologi cal processes or pathological tissue components. Contrast agents may improv e the specificity of imaging modalities through selective targeting, and th is may be particularly significant when using ultrasound (US) to image infl ammatory processes or thrombi, One means of selective targeting involves th e attachment of contrast agents to the desired site with the use of a speci fic binding mechanism. Because molecular binding mechanisms are effective o ver distances on the order of nanometers, targeting effectiveness would be greatly increased if the agent is initially concentrated in a particular re gion, and if the velocity of the agent is decreased as it passes the potent ial binding site, Ultrasonic transmission produces a primary radiation forc e that can manipulate microbubbles with each acoustic pulse. Observations d emonstrate that primary radiation force can displace US contrast agents fro m the center of the streamline to the wall of a 200-mm cellulose vessel in vitro. Here, the effects of radiation force on contrast agents in vivo are presented for the first time, Experimental results demonstrate that radiati on force can displace a contrast agent to the wall of a 50-mm blood vessel in the mouse cremaster muscle, can significantly reduce the velocity of flo wing contrast agents, and can produce a reversible aggregation. Acoustic ra diation force presents a means to localize and concentrate contrast agents near a vessel wall, which may assist the delivery of targeted agents. (C) 1 999 World Federation for Ultrasound in Medicine & Biology.