Sharp Views from Dissonant Ultrasound

Scientists have invented a new type of ultrasound probe that jostles tissue or other material then listens for sounds generated by the movement. The technique, described in today's Science, can reveal subtle tissue abnormalities or flaws within materials.

Traditional ultrasound images, such as those of fetuses, are made by sending out high-frequency pings and detecting the echo. But now bioengineers Mostafa Fatemi and James Greenleaf of the Mayo Clinic in Rochester, Minnesota, have developed a new way to take ultrasound-generated pictures by focusing two ultrasonic beams of slightly varying frequencies on an object. The beams' interference vibrates the target, which as a result emits low-frequency sound.

By incrementally varying the frequencies of the ultrasonic waves, the duo can pinpoint the frequency at which a target resonates--like the amplification of a particular hum in a shower stall. Because this frequency depends in part on an object's mass and stiffness, the researchers can use this approach to measure these properties. For example, calcified portions of a human arterial sample looked markedly brighter than adjacent healthy areas. The technique is sensitive enough to detect a glass bead less than half a millimeter wide on a latex sheet.

"It's a good idea, with some promising applications," says Scott Morris, an engineer working at the University of Illinois at Urbana-Champaign's Bioacoustics Research Laboratory. He says it could lead to ultrasound scans that reveal much more subtle variations in tissue or other materials than can currently available scans. Greenleaf and Fatemi hope to launch clinical trials in several years; in the meantime, they plan to image damaged arteries of pigs.