Engineers design new optical microprobe to detect subsurface organ abnormalities

Photonics and ultrasound engineering researchers from Duke University and The George Washington University have collaborated to design an optical scanner miniaturized enough to be inserted into the body, where its light beams could someday detect abnormalities hidden in the walls of the colon, bladder or esophagus.

The experimental device, called an “electrostatic micromachine scanning mirror for optical coherence tomography,” is described in an article published in the April 15, 2003, issue of the research journal Optics Letters. Once approved for use in hospitals and clinics, it would provide a new capability for endoscopy procedures.

Using tiny electrically activated artificial muscle fibers to vibrate a gold–covered mirror only about 2 millimeters wide, the prototype device broadcasts a special kind of quasi-laser light that can not only scan internal organ surfaces but also penetrate just beneath the surface.

Key researchers in the miniaturization effort are Jason Zara, an assistant professor of engineering and applied science at George Washington, and Stephen Smith, a professor of biomedical engineering at Duke’s Pratt School of Engineering in Durham, N.C.

“This new device has shown great promise for new diagnostic applications,” said Zara, Smith’s former graduate student at Duke who is lead author of the Optics Letter report. Co-authors include Smith; Joseph Izatt, an associate professor of biomedical engineering at Duke’s Pratt School of Engineering; and Izatt’s former graduate student Siavash Yazdanfar and former postdoctoral research associate K. Divakar Rao.

Izatt, who leads biophotonics research activities at the Pratt School’s Fitzpatrick Center for Photonics and Communications Systems, is a leader in the budding optical scanning technology that Zara and Smith have scaled down to fit into catheters.

Zara and Smith designed and fabricated a system that includes a tiny mirror that vibrates up to 2,000 times a second on hinges just 3 millionths of a meter wide. The mirror quivers in response to the action of more than one-half million microscopic energy-storing capacitors arranged in parallel strips of the flexible plastic polyimide.

This arrangement acts like artificial muscle, Smith said. “When a voltage is applied to each of these capacitors, they contract. That pulls the mirror to the right. When the voltage is turned off, the mirror then swings back to the left.” As the voltage rapidly switches on and off and the mirror vibrates, a beam of light from a fiberoptic cable is reflected onto a tissue surface in a scanning pattern. This repeat scanning produces optical images of the tissues’ outer layers.

The artificial muscle was made at MCNC, a Research Triangle Park microelectronics and computer research institution founded by the state of North Carolina. Zara and Smith have also founded a startup company, Memscept, Inc., to market the research.

The idea of using light as a deeper probe, called Optical Coherence Tomography (OCT), was pioneered at MIT, where Izatt was a postdoctoral scientist. He continued developing the concept while on the faculty of Case Western Reserve University before coming to Duke.

“The standard endoscope gives a physician an internal view of hollow organ surfaces with white light,” Izatt said. “What OCT does is look below those surfaces.

“It can look up to about a millimeter and a half deep into the walls of organs,” he added. “That’s sufficient to detect cancers such as carcinomas which grow near tissue surfaces, while they are still small enough to be completely removed. A physician’s normal view of the surface would not see a cancer there, but we can see it with OCT because we are looking underneath.”

Izatt acknowledged that light waves cannot penetrate near as far into the skin as ultrasound, a competing technology that uses sound waves to image internal structures. On the other hand, wavelengths of light are much shorter than those of sound. As a result, “OCT’s resolution is much greater,” Izatt said.

Rather than using the white light of normal endoscopy, this version of OCT harnesses infrared light from a laser-diode that has had one key laser feature disabled. “Strictly speaking, it is not a laser, but it’s close to being a laser,” Izatt said.

While this modified “superluminescent diode” has laser-like “spatial coherence,” meaning that its beam remains more focused than normal light, it does not emit light of a single color frequency like complete lasers can.

The special combination of features permits OCT investigators to use it in interferometry. Interferometry is a technique to create visual images by rapidly scanning surfaces with light of various wavelengths while interpreting the return reflections from various depths.

Using a superluminescent diode with interferometry is the “cheapest” form of OCT, Izatt said. And the similarities between this light scanning method and ultrasound delivery systems spurred a natural collaboration, added Smith, who is part of Duke’s ultrasound research program.

The Optics Letters article also included results of several micromachine OCT scans of biological tissue. One examined the lining of an excised pig colon. A second scan probed the cornea and iris of an excised pig’s eye. A third imaged the underside of a human fingertip.

OCT currently has U.S. Food and Drug Administration clinical approval only for scanning the eye’s retina, where the procedure is widely used, Izatt said. It is also being evaluated for various possible imaging uses in the gastrointestinal tract, the lungs, the bladder, the cervix and in coronary arteries, he added.