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Miniature endoscope captures 3-D images

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Michael A. Greenwood

An ultraminiature endoscope that offers three-dimensional imaging and enough flexibility to reach particularly delicate regions of the body has been developed by researchers from Harvard Medical School and the Wellman Center for Photomedicine at Massachusetts General Hospital in Boston.

Although the endoscope has been a familiar medical tool for decades, its usefulness is often limited by its size, inflexibility and less-than-perfect picture quality. Miniature endoscopes currently use bundles of optical fibers to transmit a two-dimensional image. However, bundling makes the device larger and the images provided can lack important medical information.

This spectrally encoded miniature endoscope uses micro-optics and a single optical fiber to project colors of light onto portions of the subject.

To overcome these limitations, researchers led by Drs. Guillermo J. Tearney and Dvir Yelin devised a technique called spectrally encoded endoscopy (SEE). Their probe consists of a single optical fiber about 350 μm wide — comparable to a strand of human hair. The fiber is expanded through a 1.8-mm-long silica spacer and focused by a gradient index lens, then diffracted by a transmission grating on the probe’s tip.

Polychromatic light from the endoscope is broken into wavelengths and projected onto various locations on the tissue surface under investigation. The reflected light is then decoded outside the body with a spectrometer. Another device, an interferometer, uses that information to create a three-dimensional figure. The enhanced image not only provides additional diagnostic information, but also helps determine disease states and can serve as a better guide for microsurgical procedures.

The researchers have worked on the technology for almost five years. Recently, they performed an in vivo test on a mouse to demonstrate their model’s potential. Entering through the abdominal cavity, the SEE probe captured detailed images of metastatic ovarian tumor nodules on the peritoneum.

The team’s prototype equipment is custom-built and includes a broad-bandwidth (700 to 900 nm) Ti:sapphire laser, a Michelson interferometer and a high-speed spectrometer; the probe can obtain volumetric images with about 400,000 resolvable points at video rates of 30 fps. With further optimization of the optics, the researchers believe it will be possible to obtain images with 10 times the number of pixels provided by other miniature endoscopes.

Tearney said that his minimally invasive instrument could make it possible for certain operations to move to an outpatient setting, reducing requirements for anesthesia. The probe’s slender design also could minimize tissue damage and allow for safer exploration of hard-to-reach places such as the brain, eye and middle ear. Less-invasive fetal surgery is another potential application.

Although the technique produces images that are inherently monochromatic, Tearney said that color eventually could be introduced by using three separate broad-bandwidth sources, each centered at visible red, green or blue wavelengths. Color is useful because it can provide an additional contrast mechanism for diagnosis.

The researchers also had a particularly difficult time constructing the grating at the tip of the probe, so an outside firm helped engineer the minuscule component.

Tearney said that he hopes the device will be ready for use on human patients within a year. They must further develop the prototype and obtain regulatory approval before that can happen. He also said that he would like to see the miniature endoscope used as a regular part of medical procedures within five years.

The investigators are looking for a commercial partner.

Nature, Oct. 19, 2006, p. 765.

Dec 2006

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