Microbubbles in therapeutic ultrasound

Abstract

Microbubbles have been developed as ultrasound contrast agents as an emulsion of stabilized gas bubbles that can be injected systemically and circulate throughout the body. Contrast between vascular structures and other tissues is due to the high echogenicity of the bubbles and the significantly different nonlinear response of these bubbles compared to nonlinear signals due to finite amplitude distortion of the ultrasound field. Microbubbles can also be introduced as superheated perfluorocarbon droplets that when activated by ultrasound, vaporize to form gas bubbles in a process termed acoustic droplet vaporization (ADV). The droplets have a range of sizes depending on their purpose in diagnostic and therapeutic application. For therapy, such droplets can be used as vascular agents, can be extravascular such as for delivery in tumor tissue or even formulated into implantable constructs used in tissue engineering and used for spatial/temporal control of factors affecting incorporation of the construct. In each case, formulation can include encapsulate drugs for local release. When bubble are present in an ultrasound field, microbubbles can also affect tissue heating and can undergo stable and/or inertial cavitation resulting in physical effects to tissue ranging from petechial hemorrhage sites to complete cellular disruption. The latter is termed Histotripsy and has developed into an alternative treatment strategy to ultrasound therapies based on the heating of tissue. This presentation will discuss how microbubbles are being used in medical ultrasound and the advances being made in both diagnosis and therapy.