- Grating Shrinks Endoscope
Brent D. Johnson
Endoscopes provide high-resolution color images in real time for a range of medical applications, but these instruments are too large for procedures such as fetoscopy and pediatric surgery. A possible solution is to employ a single optical fiber rather than the standard fiber bundle, but this requires a high-speed scanning element that again makes the size of the probe an issue.
Researchers have developed a miniature endoscope that can image previously inaccessible parts of the body. Here, a finger is viewed through a 1-mm probe: A) anterior side with the nail, B) posterior side.
Nicusor Iftimia and Milen Shishkov, working under the supervision of Brett E. Bouma and Gary J. Tearney at the Wellman Laboratories of Photomedicine in Boston, are using a technique called spectrally encoded endoscopy that eliminates the need for high-speed scanning and thereby reduces the diameter of the probe.
Recent advances in hybrid imaging techniques and in microendoscopes have witnessed a dramatic improvement in the imaging of structural and functional aspects of pathological processes. But hybrid techniques, still in their infancy, are expensive and could take years to satisfy the promise of a truly compre- hensive, noninvasive diagnostic tool. Today's endoscopes, in contrast, offer the required images at the point of interest.
A typical probe consists of bundles of optical fibers, each bundle conveying one pixel of an image. However, the practical dimensions of the probe, which reduces the field of view and the resolution of the image, limit the number of fibers. The smallest, state-of-the-art endoscopes have diameters of approximately 1.0 mm.
ftimia explained how images that require fast acquisition, similar to confocal devices, must be scanned rapidly in one direction to acquire an image. This can be accomplished with a galvanometer or with a device based on microelectromechanical systems technology.
ftimia, however, has selected a volume phase diffraction grating to spread light across the sample. The grating illuminates the sample with an array of focused spots, where each position is encoded by a different wavelength. The reflected light is then transmitted back through the fiber and is externally decoded. A line of the image may be acquired by measuring the reflected spectrum using a 15-kHz spectrometer. To obtain a two-dimensional image, transverse mechanical scanning of the optical fiber and distal optics in the conjugate direction must occur.
Shishkov selected a grating from Wasatch Photonics Inc. of Walnut Creek, Calif., that was designed for telecom applications because it had a diffraction efficiency of 95 percent. When coupled to a Ti:sapphire laser with a bandwidth of 225 nm centered at 830 nm, the probe can achieve a much wider field of view with greater resolution at 150,000 resolvable points, compared with 30,000 with a fiber bundle probe.
The result is an ultraminiature, single-fiber endoscope with a diameter of 250 µm and a probe diameter of 500 µm that can image parts of the body that were previously inaccessible. Iftimia believes that the device will minimize the tissue damage and bleeding sometimes associated with endoscopes and that it will subsequently reduce the risk of these procedures by decreasing the need for anesthesia.
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