Acoustic Hyperlens Could Sharpen Ultrasound Imaging

26 October 2009—In the past few years, researchers have created artificial materials known as metamaterials, which bend and focus light in unnatural ways. While a microscope's glass lens can detect only objects larger than half a wavelength of light, metamaterials could enable ultrahigh-resolution imaging of much tinier features.

A new device made by researchers at the University of California, Berkeley, does a similar trick with sound waves. Just as with its optical counterpart, imaging with sound is limited by the wave's length, a phenomenon called the diffraction limit. But the acoustic metamaterial that mechanical engineering professor Xiang Zhang and his colleagues presented yesterday on the online version of the journal Nature Materials magnifies and detects features that are just one-seventh of the sound signal's wavelength.

When light (or sound) waves fall on an object, features smaller than the wavelength scatter the light as evanescent waves, which quickly fade as they travel away from the object. Conventional lenses cannot amplify evanescent waves, so the object's fine details aren't seen. But the precisely calculated shapes of metamaterials allow them be formed into so-called hyperlenses, which capture and preserve those waves so that they can be amplified. "In a way, it's like a very special kind of magnifying glass that can transmit very small subwavelength features and make them appear bigger," says Steven Cummer, an electrical and computer engineering professor at Duke University, in Durham, N.C., who has done theoretical work on acoustic hyperlenses.

Acoustic hyperlenses could allow higher-resolution medical ultrasound imaging. They could also help bridge and building inspectors detect tiny cracks or defects deep inside a structure.

Nicholas Fang, a mechanical science and engineering professor at the University of Illinois at Urbana-Champaign, says that acoustic hyperlenses could make it possible to see objects just a few micrometers in size. Currently, ultrasound scans have a resolution of a few millimeters, and even those are achieved by using short-wavelength, or high-energy, signals. "For better resolution, you dump more energy into the tissue or structure under inspection," Fang says. "[But] with more ultrasound energy, you're at risk of harming or destroying healthy tissue or healthy structures."

A hyperlens that magnifies tiny features can be used in reverse to focus sound, says Guy Bartal, a researcher in Zhang's laboratory who was involved in the new work. That means you could focus longer-wavelength (lower-energy) signals down to a fraction of their wavelengths and get high-resolution ultrasound scans.

The new device is made of 36 brass fins, each 3 millimeters tall, radiating outward from the center of a semicircle. The researchers put two sound sources transmitting signals with frequencies between 4.2 and 7 kilohertz at the center of the semicircle. The 1.2-centimeter distance between the sources corresponds to one-seventh of the 4.2-kHz signal's wavelength. That's too small a separation for an acoustic detector to normally be able to tell the sources apart. But the evanescent waves are captured and amplified as they travel along the air channels between the brass fins so that the two sources are clearly distinguishable in two dimensions at the outer edge of the lens.

Fang says that this is an elegant demonstration, showing that acoustic metamaterials could be used to focus sound beyond the diffraction limit and proving that the devices could work for high-resolution imaging. Earlier this year, he made an acoustic hyperlens that focuses ultrasound waves but not beyond the diffraction limit. "It's the very first proof of concept…but the result is solid and very promising," he says.

Practical use will take time. Bartal says the researchers are trying to make the device compatible with ultrasound frequencies greater than 20 kHz. They are also working on a device for creating three-dimensional ultrasound images. Then they need to adapt their device—make it smaller, for instance, according to Cummer—for use in a practical ultrasound imaging system.

Given that acoustic metamaterials have been around only for the past two years, Cummer believes progress has been quick. "High-quality demonstrations like this suggest to me that we could see applications within a handful of years," he says.