Abstract:
Significant progress has been made in recent years in the development of effective ultrasonic contrast agents for inducing better image contrast of certain anatomic structures in the body for diagnostic purposes. An ideal ultrasonic contrast agent should have the following characteristics: (1) be nontoxic, (2) be capable of producing sufficient contrast via intravenous injection, and (3) have a long persistence time. Gaseous contrast agents such as Albunex[sup (registered)] (Molecular Biosystems), Echovist[sup (registered)] and Levovist[sup (registered)] (Schering), and EchoGen[sup (registered)] (Sonus) have been shown to satisfy a majority of these requirements. Albunex[sup (registered)] was approved in 1994 by the FDA for clinical use. The most intriguing application of these contrast agents is in harmonic pulse-echo and Doppler imaging since nonlinear resonance of microairbubbles would yield echoes at the harmonics of the incident frequency. If detection is made at one of these harmonics, ideally only the signals produced by the bubbles will be seen, allowing a means to discriminate against echoes produced by surrounding tissue structures. In this paper the physical and ultrasonic properties of these agents will be reviewed along with principles of harmonic imaging. Clinical applications of these agents including harmonic imaging and Doppler flow measurements will be discussed. [Work supported by NIH Grant No. HL28452.]