Scientists have found a way to overcome one of the major limitations of ultrasound imaging--the poor resolution of the image.

One of the limits of ultrasound imaging is that the detail obtainable is the frequency of the sound: The fundamental laws of physics dictate that the smallest objects one can "see” are about the size of the wavelength of the sound waves. For ultrasound of deep tissues in the body, for example, the sound waves are typically 1 MHz - 5 MHz--much higher than what humans can hear--which imposes a resolution limit of approximately 1 mm.

In an article appearing online in November 2010 in the journal Nature Physics, physicists at the University of California, Berkeley (UCB; USA) and the Universidad Autonoma de Madrid (Spain) demonstrate how to capture the evanescent waves bouncing off an object to reconstruct detail as small as one-fiftieth of the wavelength of the sound waves. Evanescent sound waves are vibrations near the object that dim out within a very short distance, as opposed to propagating waves, which can travel over a long distance.

"With our device, we can pick up and transmit the evanescent waves, which contain a substantial fraction of the ultra-subwavelength information from the object, so that we can realize super-resolution acoustic imaging,” said first author Dr. Jie Zhu, a postdoctoral fellow in the Center for Scalable and Integrated NanoManufacturing (SINAM), a National Science Foundation-funded Nanoscale Science and Engineering Center at UCB.

The researchers refer to their device for capturing evanescent waves as a three-dimensional, holey-structured metamaterial. It consists of 1,600 hollow copper tubes bundled into a 16-cm bar with a square cross-section of 6.3 cm. Placed close to an object, the structure captures the evanescent waves and pipes them through to the opposite end.

In a practical device, according to Dr. Zhu, the metamaterial could be mounted on the end of an ultrasound probe to improve the image resolution. The device would also improve underwater sonography, or sonar, as well as nondestructive assessment in industry applications.

"For ultrasound detection, the image resolution is generally in the millimeter range,” said coauthor Dr. Xiaobo Yin. "With this device, resolution is only limited by the size of the holes.”

In the researchers' study, the holes in the copper tubes were approximately 1 mm in diameter. Using acoustic waves of about 2 kHz, the resolution of an image would normally be limited to the wavelength, or 200 mm. With their holey-structured metamaterial, they can resolve the feature size as small as 4 mm, or one-fiftieth of a wavelength. "Without the metamaterial, it would be impossible to detect such a deep subwavelength object at all,” Dr. Yin stated.

 

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