Photonics and ultrasound engineering researchers from Duke University and The George Washington University have designed an optical scanner that is small enough to be inserted into the body. Light beams from the device could potentially detect abnormalities hidden in the walls of the colon, bladder or oesophagus.
Once approved for use in hospitals and clinics, the experimental device, called an ‘electrostatic micromachine scanning mirror for optical coherence tomography,’ would provide physicians with a new diagnostic tool for endoscopy procedures.
Using tiny electrically activated artificial muscle fibres to vibrate a gold-covered mirror only about 2 millimetres 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.
Researchers Jason Zara and Stephen 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 metre wide. The mirror quivers in response to the action of more than one-and-half million microscopic energy-storing capacitors arranged in parallel strips of polyimide, a flexible plastic.
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 fibre optic cable is reflected onto a tissue surface in a scanning pattern. This repeat scanning produces optical images of the tissues’ outer layers.
The idea of using light as a deeper probe, called Optical Coherence Tomography (OCT), was pioneered at the Massachusetts Institute of Technology, where Joseph Izatt, an associate professor of biomedical engineering at Duke’s Pratt School of Engineering, was a postdoctoral scientist.
‘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 millimetre 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. However, wavelengths of light are much shorter than those of sound. Consequently, 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 colour frequency like complete lasers can.
The special combination of features permits OCT investigators to use it in interferometry, 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.
OCT currently has US 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.