Physical principles of microbubbles for ultrasound imaging and therapy.

Microbubble ultrasound contrast agents have been in clinical use for more than two decades, during which time their range of applications has increased to encompass echocardiography, Doppler enhancement, perfusion studies and molecular imaging, as well as a number of therapeutic applications including drug delivery, gene therapy, high-intensity focused ultrasound treatments and sonothrombolysis. The aim of this article is to review the different types of microbubble agent, their physical behaviour and the mechanisms underlying their effectiveness in imaging and therapeutic applications.