Time-reversal acoustics research promises medical breakthroughs

Scientists have not yet found a way to actually make time run backward, but in the cutting-edge world of recent acoustics research, they have shown a way to make sound waves run backward in a kind of ultra-focused reverse echo. By the technology known as time-reversal acoustics, sound waves – in exact reverse order from the original sound – echo directly and very precisely back to their source point.

The technology promises a wide array of applications, including medical applications such as ultra-precise medical imaging, diagnostic techniques using ultrasound, incision-free surgical techniques, and even the potential for a method of recharging the batteries of implanted devices like pacemakers without performing surgery.

Dr. Alexander Sutin, an acoustics expert and senior scientist at Stevens Institute of Technology (Hoboken, NJ), is a co-author of six papers to be presented at the Acoustical Society of America’s 75th Anniversary Meeting in May 2004 (New York City). Four of the papers address time-reversal acoustics systems that have potential breakthrough applications in medicine, nondestructive testing and land mine detection.

“Almost all acousticians think that time-reversal acoustics is the hottest topic in acoustics,” says Dr. Sutin.

One area of his team’s medical research involves a kind of “virtual finger” that could focus on an area inside the body much more precisely than any other known method. One of the challenges of imaging the human body or targeting tumors or gall stones non-surgically is that the body is not homogenous – tissues, fat and bone vary in density – so most ultrasound signals become distorted. The beauty of time-reversal acoustics (TRA), however, is that the technology works even more precisely in an environment that has lots of ultrasound-distorting obstacles. Difficult environments like the human body actually improve the focusing of ultrasounds to a specific location, sharpening the focus and enhancing precision.

One of the other big advantages of TRA in medical applications is that systems work so rapidly, and refocus so quickly, that movement of the body presents almost no problem.

The TRA systems employ two things: sound (or ultrasound), along with a time-reversal “mirror” that uses an array of transducers. The transducers convert sound waves into electrical signals, then a computer reverses their order, and the transducers transfer electrical signals back into sound and aim the reverse sound waves back in the direction from which they came.

One of Dr. Sutin’s papers addresses how TRA may be used for nonlinear imaging. Such imaging would involve several TRA focusing systems. The interaction of the crossing beams allows three-dimensional, nonlinear acoustic images of an object inside a human body or a material structure.

Dr. Sutin, originally from Russia, conducted experiments in Paris with the worldwide leader in the field of time-reversed acoustics, Dr. Mathias Fink, the technology’s pioneer.

The American TRA system was developed and built at Artann Laboratories, headed by Dr. Armen Sarvazyan, a colleague of Dr. Sutin’s. Experiments on medical applications of TRA were conducted by Dr. Sutin at Artann Labs.

Joint work with scientists from Los-Alamos National Laboratory, Purdue University, University of Colorado, and the Russian Institute of Applied Physics will be presented in two papers, reflecting the progress in the development of nonlinear acoustic methods of crack detection.

Dr.Sutin’s research on time-reversed acoustics is supported by the National Institutes of Health, Los Alamos National Laboratory and NASA.

Another application of a TRA/nonlinear technique would be to use ultrasound to measure the blood pressure inside a certain point or chamber within the heart. To make this work, harmless tiny capsules (ultrasound contract agents) would be introduced in the blood stream. They would react differently during the heart’s different pressure conditions and their reactions would be measured by sound waves aimed from different angles and returned to a time-reversal mirror. Variations in harmonics levels resonating from the capsules would be correlated to ambient pressure. The precision of the TRA system would allow highly accurate focusing in one area of the heart, meanwhile the nonlinear acoustic technique would give a diagnostician information about pressure changes as the heart pumps.

A different application would involve the ability to non-surgically check for cracks in older mechanical heart valves, a valuable tool for warding off difficulties in some heart patients. The detection of internal cracks and flaws in materials is a standard use of nonlinear acoustic technology. This application would simply employ it inside the human body with the benefit of TRA focusing.

Dr.Sutin’s blood pressure measurement research involving time-reversed acoustics has been supported by Stevens Institute of Technology as part of its Technogenesis ® initiatives. (Technogenesis is Stevens’ unique environment for education and research, in which students, faculty, and partners in academia, government and industry jointly nurture new technologies and companies from laboratory innovation to marketplace implementation.)

Dr. Sutin’s senior scientist position is within Stevens’ Davidson Laboratory, part of the Charles V. Schaefer School of Engineering with special emphases on ocean engineering and acoustical research.

Established in 1870, Stevens Institute of Technology offers baccalaureate, master’s and doctoral degrees in engineering, science, computer science, management and technology management, as well as a baccalaureate in the humanities and liberal arts and in business and technology. The university, located directly across the Hudson River from New York City, has a total enrollment of about 1,700 undergraduates and 2,600 graduate students. Additional information may be obtained from its web page at www.stevens.edu .