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  • Material converts ultrasound waves into optical signals

Jun 2013
LONDON – Ultrasound images have never looked sharper, thanks to a new metamaterial that converts ultrasound waves into optical signals, providing high-resolution images for biomedical applications.

Conventional ultrasound technology relies on generating images by converting ultrasound waves into electrical signals. Although the technology has advanced throughout the years, it is still largely constrained by bandwidth and sensitivity – obstacles to producing diagnostic-quality images.

A new material could lead to considerable improvements in ultrasound technology, enabling the production of high-quality, high-resolution images in biomedical applications. Courtesy of Ryan McCarron, King’s College London.

Now, scientists at King’s College London, in collaboration with colleagues at Texas A&M, Queen’s University Belfast and University of Massachusetts Lowell, have engineered a material that is not subject to those limitations, primarily because it converts ultrasound waves into optical signals rather than electrical ones. The optical processing of the signal does not limit the bandwidth or sensitivity of the transducer.

“The high bandwidth allows you to sample the change of distance of the acoustic waves with high precision,” said Dr. Wayne Dickson of the department of physics, King’s College London. “Greater sensitivity enables you to see deeper in tissue, producing visuals in much greater detail than is currently possible.”

The metamaterial consists of gold nanorods embedded in a polymer known as polypyrrole. An optical signal is sent into this material, where it interacts with, and is altered by, incoming ultrasound waves before passing through the material. A detection device then reads the altered optical signal, analyzing the changes in its properties to process a higher-resolution image.

“We developed a material that would enable optical signal processing of ultrasound,” said Vladislav Yakovlev, a Texas A&M professor. “Nothing like this material exists in nature, so we engineered a material that would provide the properties we needed. It has greater sensitivity and broader bandwidth.”

“The greater sensitivity and broader bandwidth [mean] we can go from 0 to 150 MHz without sacrificing sensitivity,” Dickson said. “Current technology typically experiences a substantial decline in sensitivity around 50 MHz. This means the metamaterial can efficiently convert an acoustic wave into an optical signal without limiting the bandwidth of the transducer, offering exciting potential in biomedical applications.”

While these advances are not yet ready for integration into ultrasound technology, Dickson and his team have successfully demonstrated how conventional technology can be improved by using the newly engineered material.

“The potential our findings offer is tremendously exciting, as up until now, the most sensitive ultrasound detector, despite being based on conventional optical materials, has both been bandwidth-limited and difficult to engineer into a real device due to the stringent requirements on the optical alignment,” Dickson said. “Conversely, our material operates in a configuration that should prove relatively straightforward to integrate into a working device, heralding the next generation in ultrasound sensors for this extremely important technique in medical diagnostics and therapeutics.”

The research appeared in Advanced Materials (doi: 10.1002/adma.201300314).

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