Phospholipids are a major component of all biological membranes. When placed in water, phospholipids form a bilayer, where the hydrophobic tails line up against each other. This forms a membrane with hydrophilic heads on both sides. This membrane is partially permeable and very flexible.
Phospholipid containing microbubbles are in use as diagnostic ultrasound contrast agents. Phospholipids can be targeted to atheroma and other pathologic components to enhance atherosclerosis imaging. The majority of these echogenic liposomes range in diameter from 0.25 to 5.0 μm.

Retrolenticular afterglow could occur through diffraction and refraction on interfaces. A circular object may act as a lens to the ultrasound beam, showing an artifact region of increased echogenicity.

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.

(UCA / USCA) Ultrasonography is the most commonly performed diagnostic imaging procedure. The introduction of sonographic contrast media into routine practice modifies the use of ultrasound in a variety of clinical applications. USCAs consist of microbubbles filled with air or gases and can be classified according to their pharmacokinetics. Among the blood pool agents, transpulmonary ultrasound contrast agents offer higher diagnostic potential compared to agents that cannot pass the pulmonary capillary bed after a peripheral intravenous injection. In addition to their vascular phase, some USCAs can exhibit a tissue- or organ-specific phase.
The sonogram image quality is improved either by decreasing the reflectivity of the undesired interfaces or by increasing the backscattered echoes from the desired regions.

Different types of ultrasound contrast agents:
Ultrasound contrast agents act as echo-enhancers, because of the high different acoustic impedance at the interface between gas and blood. The enhanced echo intensity is proportional to the change in acoustical impedance as the sound beam crosses from the blood to the gas in the bubbles.

The ideal qualities of an ultrasound contrast agent:
A typical ultrasound contrast agent consists of a thin flexible or rigid shell composed of albumin, lipid, or polymer confining a gas such as nitrogen, or a perfluorocarbon. The choice of the microbubble shell and gas has an important influence on the properties of the agent.
Current generations of microbubbles have a diameter from 1 μm to 5 μm. The success of these agents is mostly dependent on the small size and on the stability of their shell, which allows passage of the microbubbles through the pulmonary circulation. Microbubbles must be made smaller than the diameter of capillaries or they would embolize and be ineffective and perhaps even dangerous.
The reflectivity of these microbubbles is proportional to the fourth power of a particle diameter but also directly proportional to the concentration of the contrast agent particles themselves.
Ultrasound contrast agents produce unique acoustic signatures that allow to separate their signal from tissue echoes and to depict whether they are moving or stationary. This enables the detection of capillary flow and of targeted microbubbles that are retained in tissues such as normal liver.
The new generation of contrast media is characterized by prolonged persistence in the vascular bed which provides consistent enhancement of the arterial Doppler signal. Contrast agents make it also possible to perform dynamic and perfusion studies. Targeted contrast imaging agents are for example taken up by the phagocytic cell systems and thus have liver/spleen specific effects.

See also Ultrasound Contrast Agent Safety, Adverse Reaction, Tissue-Specific Ultrasound Contrast Agent, and Bubble Specific Imaging.