MEDICAL ultrasound equipment tends to be bulky. But tiny devices micromachined from silicon may one day slim the technology down so much that crucial parts could be placed inside the body. Then doctors might be able to gather images of artery-harming plaque, for instance, from inside the arteries themselves.
A Stanford University scientist, Butrus T. Khuri-Yakub, has developed a prototype of a highly miniaturized ultrasound device. Dr. Khuri-Yakub, a professor of electrical engineering, said that arrays of the devices, a type of transducer fabricated using standard semiconductor technology, might one day appear in many medical applications, from portable prenatal screeners to hand-held scanners used on battlefields to check the injured for internal bleeding.
Other researchers, too, are working on developing technology for these tiny silicon transducers, known generally as capacitive micromachined ultrasonic transducers.
Dr. Khuri-Yakub has obtained research financing from the National Institutes of Health to use the transducers to go inside arteries to search for plaque. ”We are building a three-dimensional medical imaging system that will fit on a catheter we are hoping to fit inside arteries,” he said.
General Electric was a sponsor of Dr. Khuri-Yakub’s transducer technology as he developed it over the last decade and demonstrated its advantages over the piezoelectric materials presently used widely for ultrasound transducers. G.E. is now beginning to integrate the new silicon transducers with its own scanning technology, primarily in medical applications.
Dr. Khuri-Yakub’s transducers lend themselves well to applications like a portable hand-held ultrasound scanner, said Kai E. Thomenius, chief technologist for ultrasound and biomedical imaging at G.E. Global Research in Niskayuna, N.Y. ”I’d like to see the scanners used in triage,” he said. ”People could place them on the abdomen of a wounded soldier and look for sites where there might be bleeding from veins and arteries.”
The scanners could be wireless, Dr. Thomenius said, so that radiologists at a distant location could help workers on the battlefield.
The new transducers take advantage of silicon manufacturing techniques and can be built into arrays of many shapes and integrated smoothly with electronics. ”We plan to replace the piezoceramic materials with silicon devices,” he said.
Dr. Khuri-Yakub said the ultrasound transducer technology was based on a simple act: wiggling. ”Put your hand in the water and wiggle it, and you’ll send a wave,” he said. ”That’s the business of ultrasound, to wiggle something that will then entrain movement in a surrounding medium.”
To produce a powerful wiggle, the research group fabricated arrays of the tiny drumlike transducers from bulk silicon, etching wells in the material and putting membranes or drumheads of silicon nitride on top. The drumheads are small, about 10 microns to 50 microns in diameter, with a gap between the drum membrane and the silicon substrate of only a few hundred nanometers.
When an electric signal is applied between the metalized membrane and the silicon substrate, an electric field is generated that causes the drumheads to vibrate, sending out ultrasonic waves.
Because the operating frequencies of ultrasound scanners are typically in the range of one million to 15 million cycles per second, the researchers apply electrical signals in that frequency range and the array converts the signal to the acoustic equivalent, said Dr. Thomenius. ”In this way,” he said, ”thin acoustic beams are created that can then hit structures like tumors and artery walls.”
When the acoustic energy is reflected back from the structures, the waves must be detected and converted to electricity. Again the wiggling drumheads are the tiny transducers. Because there are negative and positive charges on the drumheads, as they wiggle they generate the electrical signals that are amplified and used to construct images.
Dr. Khuri-Yakub’s research on the transducers has been sponsored throughout the last decade by several organizations, including the Office of Naval Research. ”As we developed the technology slowly through the 90’s,” he said, ”the medical ultrasound community became aware of its potential,” and financing for the research widened.
He currently has research money from the National Institutes of Health to use the silicon micromachined ultrasound transducers to search for plaque in arteries.
General Electric’s research arm has received grants, including one from the Army, to pursue applications of the technology in ultrasound imagers. ”We are interested in ultrasound arrays that can be used for monitoring of patient conditions,” Dr. Thomesius said, but also for other kinds of monitoring, including nondestructive evaluation of materials, pipelines, and nuclear power plants.
The broad frequency range of operation is one of the transducers’ advantages over piezoelectric materials. ”With these devices, we can go more easily to higher frequencies,” Dr. Khuri-Yakub said. ”As you increase the frequency, you see smaller things, because the wavelengths are shorter.”
F. Levent Degertekin, an assistant professor of mechanical engineering at Georgia Institute of Technology, is a former student of Dr. Khuri-Yakub who is now pursuing research on micromachined ultrasonic transducers. The whole field is so new, he said, that it is only now, as the technology starts to mature, that engineers are responding, adapting micromachined transducer arrays for different needs.
”For example,” Dr. Degertekin said, ”doctors are telling us what capabilities they would like to have in medical imaging, and we are responding.”
He and his group are using a manufacturing process that they hope will make the transducers more compact. They would also like to put pressure sensors on arrays so that blood pressure can be measured exactly where imaging of blood vessels occurs.
The real innovation in Dr. Khuri-Yakub’s work, one scientist said, is that his devices store a lot of energy. ”He can generate very high electric fields without breakdowns,” said Calvin F. Quate, a professor at Stanford and one of the developers of the acoustic microscope. This is possible because of the tiny gap between electrodes and silicon substrate in the drumhead devices, he said.
Dr. Quate suggests that Dr. Khuri-Yakub’s work will have far broader ramifications than generating ultrasound. ”You can make a very small electric motor and other transducers,” he said. ”He has changed the field of transducers.”