Acoustic diode could improve ultrasound imaging

HONG KONG - An acoustic diode, enabling the one-way transmission of sound waves, could dramatically improve the quality of medical ultrasound imaging and lead to better sound dampening materials. Such a device has now been created by researchers at China’s Nanjing University. The team, led by Prof Jian-chun Cheng, described its work at the Acoustics 2012 meeting in Hong Kong in mid-May*.

Acoustic diodes are analogous to the electric diodes that produce unidirectional flow of current through electronic devices, protecting them from sudden and damaging reversals of flow. Electric diodes, which are akin to the check valves in car engines, work by providing nearly zero resistance to current flow in one direction and very high resistance in another.

However, says associate professor and team member Bin Liang, "there is no analogous method to protect ultrasound sources from the disturbance of backtracking acoustic waves. Indeed, such unidirectional flow is far tougher to achieve with acoustic waves than with electric current because sound waves travel just as easily in both directions along any given path."

The acoustic diode consists of two parts. The first is an ultrasound contrast agent (UCA), made from a suspension of microbubbles. The UCA has a strong acoustic nonlinearity, which means it converts the acoustic energy of an incident wave into a wave with twice as many pulsations per second. Therefore, Professor Bin said, "sound waves enter such a material at a particular frequency and leave with a frequency twice as great." The UCA microbubbles come in a broad range of sizes, so they can produce acoustic nonlinearity over a broad frequency range.

The second part of the acoustic diode is a superlattice consisting of thin alternating sandwich-like layers of water and glass. The superlattice acts like a filter that allows the sound waves with the doubled frequency to pass through the material but not the original sound waves. "Hence, if the sound comes from the side of the nonlinear material, it will hit that material first, creating doubled frequency sound that passes through the filter, while any sound coming from the other side at the original frequency is blocked before it reaches the doubling layer," Professor Bin explained.

In clinical medical imaging using ultrasound, acoustic waves are sent into the body and the reflected waves are received by the scanning instrument and the surrounding sensors to form the ultrasound images of the internal organs. "However, some of the reflecting waves interfere with the ingoing waves, which may lower the brightness and the resolution of the image. Therefore, preventing waves from coming back toward the ultrasound source would help to improve the quality of the ultrasound image," Professor Bin said.

"In general, we hope that the acoustic diode could apply to diverse situations where a special control of acoustic energy flux is required, for example, to improve the quality and effect of medical ultrasound diagnosis and therapy, or the design of unidirectional sound barriers," he added.

* Acoustics 2012, a joint meeting of the Acoustical Society of America (ASA), Acoustical Society of China, Western Pacific Acoustics Conference, and the Hong Kong Institute of Acoustics, was held May 13-18 at the Hong Kong Convention and Exhibition Centre.