Microbubbles as ultrasound contrast agents and in targeted drug delivery

The development of microbubbles has had considerable impact on the field of diagnostic ultrasound. These minute, gas- or lipid-filled spheres have enabled imaging in the kidney, liver, heart, and myocardium with resolutions that were previously unachievable. Insonation of these microagents generates high-energy cavitational oscillations, which, in addition to providing contrast, can increase local drug diffusivity through microstreaming or bubble collapse. The ability of focused insonation to induce such collapse on ligated microbubbles offers opportunities to deliver targeted therapies in novel ways. Microbubbles have had considerable impact on cancer research in terms of both imagining tumors and through the delivery of therapeutic agents and on the delivery of substances across biological obstructions such as the blood-brain barrier. This review offers a discussion of current approaches used in microbubble construction, of the underlying physics involved in their creation and destruction, and of current applications for these particles, the latter demonstrating their importance for high-contrast medical ultrasound. Challenges to overcome and future areas of microbubble application are also discussed.