Micro/nanoparticles in ultrasound imaging and therapy

This chapter focuses on microbubbles, gas-filled bubbles with diameters <10 μm that are administered intravenously. Unlike radiographic or MRI contrast agents, microbubbles remain in the vasculature, and thus are good blood flow tracers. They are also unique in that the applied ultrasound pulses can 'activate' microbubbles by causing them to oscillate and even rupture. Their oscillation can be highly non-linear, generating microbubbles-specific signals that have facilitated the development of various contrastenhanced ultrasound (CEUS) imaging techniques. This chapter reviews some of the physical characteristics and biological effects that are associated with microbubbles. In particular, we discuss how a microbubble responds to an acoustic pulse by examining bubble resonances and bubble dynamics. We also examine the microbubble shell, an important characteristic of microbubbles that keeps them stable, but also affects their response. A major part of this chapter discusses how a microbubble responds inside a real blood vessel, and how its response affects the surrounding elastic vessel. There is a coupled interaction in which a microbubble pushes and pulls on the vessel, and in turn, gets pushed and pulled by the vessel. Finally, we briefly look at the safety of microbubbles in relation to other common cardiovascular tests.