Contrast agents are not used for most diagnostic ultrasound examinations. Because ultrasound relies on the detection of sound waves reflected from tissue interfaces, gases can create prominent acoustic interfaces near body tissues and can be adapted for use as contrast agents. Microbubbles containing air or other gases that are introduced into the bloodstream, act by increasing the ultrasound signal reflected from the vessel contents or from the structures fed by the vessels. Ultrasound contrasts agents relying on microbub-bles may be described as (a) encapsulated microbubbles, (b) gaseous liposomes, or (c) colloidal suspensions and emulsions. These agents allow visualization of small vessels, identification of tumor vascularity, improved contrast visualization of solid organs, and improved visualization of heart chambers and vascular grafts. Ultrasound contrast agents are generally very safe. They are designed so that no large bubbles form in the body (95% or more are <10 ^m), and they have relatively short life spans before breaking down and dissipating. Microbubble decomposition occurs during passage through the heart and lungs and limits the time of beneficial imaging enhancement. Recent warnings regarding severe cardiopulmonary reactions have led to a change in the label for ultrasound microspheres.51
Perflutren (Protein-Type A Microspheres—Optison or Lipid Microspheres—Definity). Perflutren (also known as octafluoropropane) is a volatile gas that efficiently reflects sound waves. This gas is encapsulated as a protein or lipid microsphere that is generated when a solution of protein or lipids is agitated with the gas immediately prior to use. The milky white suspension of microspheres must be injected shortly after activation for maximal contrast enhancement. Protein shells consist primarily of human albumin associated with small amounts of N-acetyltryptophan and caprylic acid. Lipid shells consist of a phospholipid mixture, one of which is a polyethylene glycol phospho-lipid. Perflutren microshperes are indicated in suboptimal echocardiograms to improve the delineation of left ventricular borders.
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