Targeted ultrasound contrast agents: diagnostic and therapeutic potential

Future clinical applications of contrast-enhanced ultrasound will likely expand beyond the assessment of microvascular perfusion. One promising direction is the development of the field of "molecular imaging" with microbubbles or other ultrasound contrast agents. This application of contrast-enhanced ultrasound requires the development of site-targeted contrast agents that are retained within regions of a specific disease process, thereby allowing phenotypic characterization of tissue. Since microbubbles are pure intravascular tracers, the disease processes to be assessed must be characterized by unique antigens or "fingerprints" that are expressed within the vascular compartment. Accordingly, the pathologic states that have been or will be imaged include inflammation, atherosclerosis, neoplasms, thrombus formation, and angiogenesis, all of which are initiated and supported by molecular events within the vascular space. This review describes: 1) different strategies that have been employed to direct microbubbles to regions of disease, 2) the unique challenges for imaging targeted ultrasound contrast agents, and 3) some of the early experience imaging molecular events in animal models of disease.