University of California, Berkeley, scientists have found a way to overcome one of the main limitations of ultrasound imaging the poor resolution of the picture.
Everyone who has had an ultrasound, including most pregnant women, is familiar with the impressionistic nature of the images. One of the limits to the detail obtainable with sonography is the frequency of the sound: The basic laws of physics dictate that the smallest objects you 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 1-5 megahertz far higher than what humans can hear which imposes a resolution limit of about a millimeter.
In a paper appearing online this week in the journal Nature Physics, physicists at UC Berkeley and Universidad Autonoma de Madrid in 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 damp 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 Jie Zhu, a post-doctoral fellow in the Center for Scalable and Integrated NanoManufacturing (SINAM), a National Science Foundation-funded Nano-scale Science and Engineering Center at UC Berkeley.
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 centimeter (6 inch) bar with a square cross-section of 6.3 cm (2.5 inches). Placed close to an object, the structure captures the evanescent waves and pipes the
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University of California - Berkeley