[1] Contrast agents absorb or alter external electromagnetism or ultrasound, which is different from radiopharmaceuticals, which emit radiation themselves.
Microbubbles are used as contrast agents for sonographic examination, specifically echocardiograms, for the detection of a cardiac shunt.
These microbubbles are composed of agitated saline solution, most of which are too large to pass through the capillaries (blood vessels) of the lungs.
In addition, pharmaceutically prepared microbubbles are composed of tiny amounts of nitrogen or perfluorocarbons strengthened and supported by a protein, lipid, or polymer shell.
The drop in density on the interface between the gas in the bubble and the surrounding liquid strongly scatters and reflects the ultrasound back to the probe.