Although ultrasound imaging is ubiquitous in the medical field, it has been limited by an inability to obtain high-resolution, detailed images. By using a 3-D metamaterial to achieve deep-subwavelength imaging, however, scientists at the University of California, Berkeley (UC Berkeley), and the Universidad Autonoma de Madrid (Spain) believe that they can enhance ultrasound resolution by a factor of 50. If realized, the metamaterial could be incorporated into current ultrasound probes to capture high-resolution medical images, thereby improving patient care.

In acoustic imaging, the size of the smallest feature is diffraction limited. This essentially restricts resolution of the object to the scale of the wavelength of sound waves. But the diffraction limit is valid only in the far-field. Thus, higher-resolution imaging can be achieved by capturing the information contained within the evanescent waves of the near field, according to Jie Zhu, a postdoctoral fellow in the Center for Scalable and Integrated Nanomanufacturing at UC Berkeley involved in the research.

"The evanescent wave in the near-field carries a lot of information about the deep-subwavelength features of this imaging object," Zhu says. "What we want to do is transfer this evanescent wave to a remote location so we can achieve deep-subwavelength imaging there, which means achieving better resolution with lower-frequency ultrasonic waves." Using a lower frequency allows for better penetration and a stronger signal, he adds. These benefits, in turn, allow for better performance.

To effectively capture and transmit the information on an evanescent wave, the scientists created what they describe as a '3-D holey-structured metamaterial.' The metal metamaterial is composed of 1600 hollow copper tubes measuring roughly 1 mm in diameter; the tubes are bundled into a 6-in. bar featuring a 2.5-in. square cross section.

When placed close to the imaging object, this metamaterial allows for the evanescent wave emitting from the object to penetrate inside its structure. Coupling strongly with Fabry-Pérot resonances inside the holey plate, the deep-subwavelength information from the object is efficiently transmitted through the structure and collected on the other side. As a result, feature sizes measuring 1/50th of a wavelength can be recovered in images, Zhu comments. "The principle of deep-subwavelength imaging is simple, but what it can achieve is amazing."

Zhu collaborated with principal investigators Xiang Zhang, a professor of mechanical engineering at UC Berkeley, and F.J. Garcia-Vidal of the Universidad Autonoma de Madrid on the project, which was funded by the U.S. Office of Naval Research and the Spanish Ministry of Science.