A microbubble shell, designed to reduce diffusion into the blood, can be stiff (e.g., denatured albumin) or more flexible (phospholipid), varying in thickness from 10-200 nm.
The shell stabilizes against dissolution and coalescence with additional materials at the gas-liquid interface. This material can be an elastic solid shell that enhances stability by supporting a strain to counter the effect of surface tension. Also a surfactant, or a combination of two or more, improves the stability by a high reduction of the surface tension at the interface. Current ultrasound contrast agents are micron-sized bubbles with a stabilizing shell.

From Bracco Diagnostics, Inc.
SonoVue® was first launched in October 2001 and is now available in all European countries.
SonoVue is a second generation USCA, designed and optimized with regard to the resistance to pressure. SonoVue is an example of an important family of microbubbles whose membrane consists of phospholipids. SonoVue microbubbles are filled with sulfur hexafluoride (SF6), a gas which has a low solubility and diffuses slowly in blood for the gaseous phase of the microbubbles.
In particular, the SonoVue microbubbles, thanks to the high flexibility of their shell, are strongly echogenic in a wide range of frequencies and acoustic pressure and therefore can be used with both destructive and conservative contrast bubble specific imaging methods.

See also Coherent Contrast Imaging.

(UCA / USCA) Ultrasonic contrast agents, also called ultrasound contrast agents, are encapsulated bubbles on the order of 1-10 μm in diameter. These gas bubbles are injected into the blood stream in order to increase blood/ tissue contrast during an ultrasonogram. These microbubbles are filled with air or a gas with a lower solubility in blood than air, such as perfluorochemicals. The microbubble shell consists of albumin, phospholipid, or other material and encapsulates the gas core. Due to this construction, ultrasonic contrast agents are highly compressible, and have a high echogenicity.

See also Ultrasound Contrast Agent Safety.