Make medical imaging effervesce....

Patients with heart problems may one day get an injection of tiny bubbles as part of ultrasound technology for imaging blood flow. No ordinary bubbles, these microspheres exist as protein envelopes that encase the air bubbles as they bounce through the heart's turbulent channels, says Kenneth S. Suslick, a chemist at the University of Illinois at Urbana-Champaign.

Different materials reflect sound differently, but the similar acoustic properties of blood and muscle make them difficult to distinguish. "It's just not that easy to see anything," says Suslick.

Years ago, however, other researchers demonstrated that adding bubbles to blood makes its flow stand out because air reflects ultrasound so strongly. They also discovered that albumin-coated bubbles -- made by zapping dissolved albumin with long pulses of low-frequency, high-intensity sound -- worked well for this purpose.

Now, Suslick and Mark W. Grinstaff have learned how these sound blasts make albumin, a common body protein, encircle air to form tough microbubbles. They first found that the process requires oxygen and involves chemical modification of the albumin. The blast of sound draws air into the albumin solution and disperses it, like a milkshake, Suslick explains. At the same time, this energy produces a reactive, negatively charged oxygen molecule called a superoxide. The superoxide attacks te albumin that has wrapped around the air bubbles.

The Albumin contains lots of the amino acid cysteine. The superoxide breaks the chemical bonds between the cysteine's sulfur atoms. Those bonds then reform, this time between more distant sulfur atoms, thereby crosslinking the albumin and locking it into position around the air, suslick and Grinstaff conclude in the Sept. 1 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES.

"[This research] tells you what kinds of proteins you can use and how to make the microspheres more efficiently," says Suslick.

Last year, Molecular Biosystems Inc. in San Diego applied for FDA approval to market the bubbles as a contrast agent for echocardiography, which uses short, high-frequency sound pulses for imaging. Suslick suggests microbubbles may also prove useful as drug delivery systems or as oxygen carriers in artificial blood. "They are very biologically compatible, so that's one of the advantages of the protein bubbles," he adds.