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Medical imaging technique

[[File:Contrast enhanced ultrasonography of benign lesion. jpg|thumb|Unspecific cortical lesion on CT is confirmed cystic and benign with contrast enhanced renal ultrasonography using image fusion.

An echocardiogram is a study of the heart using ultrasound. A bubble echocardiogram is an extension of this that uses simple air bubbles as a contrast medium during this study and often has to be requested specifically. Although colour Doppler can be used to detect abnormal flows between the chambers of the heart (e. g., persistent (patent) foramen ovale), it has a limited sensitivity. When specifically looking for a defect such as this, small air bubbles can be used as a contrast medium and injected intravenously, where they travel to the right side of the heart. The test would be positive for an abnormal communication if the bubbles are seen passing into the left side of the heart. (Normally, they would exit the heart through the pulmonary artery and be stopped by the lungs.) This form of bubble contrast medium is generated on an ad hoc basis by the testing clinician by agitating normal saline (e. g., by rapidly and repeatedly transferring the saline between two connected syringes) immediately prior to injection.

There are a variety of microbubble contrast agents. Microbubbles differ in their shell makeup, gas core makeup, and whether or not they are targeted. Microbubble shell: selection of shell material determines how easily the microbubble is taken up by the immune system. A more hydrophilic material tends to be taken up more easily, which reduces the microbubble residence time in the circulation. This reduces the time available for contrast imaging. The shell material also affects microbubble mechanical elasticity. The more elastic the material, the more acoustic energy it can withstand before bursting. Microbubble gas core: The gas core is primary part of the ultrasound contrast microbubble that determines its echogenicity. Gas bubbles that are subjected to ultrasound pulsate and scatter a characteristic signal. This signal manifests itself as a high amplitude entity in a contrast enhanced sonogram. Gas cores can be composed of air, or heavy gases like perfluorocarbon, or nitrogen. Regardless of the shell or gas core composition, microbubble size is fairly uniform. They lie within a range of 1–4 micrometres in diameter. That makes them smaller than red blood cells, which allows them to flow easily through the circulation as well as the microcirculation.

Perflutren lipid microspheres (brand names Definity, Luminity) are perfluorocarbon emulsions composed of octafluoropropane encapsulated in an outer lipid shell. Octafluoropropane gas core with an albumin shell (Optison) is another perfluorocarbon emulsion that is an Food and Drug Administration (FDA) approved microbubble and made by GE Healthcare). Sulphur hexafluoride microbubbles (SonoVue Bracco (company)). It is mainly used to characterize liver lesions that cannot be properly identified using conventional (b mode) ultrasound. It remains visible in the blood for 3 to 8 minutes, and is expired by the lungs. Air within a lipid/galactose shell (formerly Levovist, an FDA approved microbubble that was made by Schering).

Targeted microbubbles are under preclinical development. They retain the same general features as untargeted microbubbles, but they are outfitted with ligands that bind specific receptors expressed by cell types of interest, such as inflamed cells or cancer cells. Current microbubbles in development are composed of a lipid monolayer shell with a perfluorocarbon gas core. The lipid shell is also covered with a polyethylene glycol (PEG) layer. PEG prevents microbubble aggregation and makes the microbubble more non reactive. It temporarily "hides" the microbubble from the immune system uptake, increasing the amount of circulation time, and hence, imaging time.