SOUND WAVES travel poorly in air. The higher the frequency the worse it is, which is why, when a neighbor plays the stereo, generally more bass than treble is heard at a distance. The ultrasound waves used medically are of such high frequencies that they travel only in liquids and solids.
Imaging researchers have begun to capitalize on that property, using air as an ultrasound contrast agent for echocardiography. The air is encased in tiny bubbles, generally less than 10 μm in diameter, that can be injected into the bloodstream. When ultrasound waves hit the bubbles, they bounce right back, providing a strong signal and a better image.
Stephen Feinstein, MD, an associate professor of medicine at the University of Chicago (Ill), has been developing "microsphere"—some call it "microbubble"—echocardiography since the early 1980s. "There are about 30 of us who have been working on this and pushing hard," Feinstein says.