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Hair-thin endoscope delivers 3D images

23 Oct 2006

Optical endoscopy may soon be possible in delicate and difficult-to-access areas, thanks to a new single fiber technique.

Optical endoscopy provides a minimally invasive way of viewing organs and structures within the body. This enables real-time diagnosis and "keyhole" interventions that are generally safer, cheaper, and involve shorter patient recovery times than open surgery. Such procedures may soon be possible in delicate and difficult-to-access areas, thanks to a new endoscopy technique developed at the Wellman Center for Photomedicine at Massachusetts General Hospital, Boston, MA.

The smallest endoscopes currently on the market use a bundle of optical fibers to supply light to, and transmit images from, internal areas of interest. These multifiber endoscopes are reasonably rigid, making it difficult to navigate tortuous ducts and vessels. They can also suffer from suboptimal image quality caused by honeycomb-type artefacts from cladding on each individual fiber, and by the limited number of fibers squeezed into the submillimetre diameter endoscope. Image quality can be improved by switching to solid-state, charge-coupled-device cameras, but endoscopes using this technology can be 10 mm or more in diameter.

The Mass General team has side-stepped these difficulties by developing an operable miniature endoscope from a solitary optical fiber (Nature 443 765). The spectrally encoded endoscopy (SEE) device takes polychromatic light from the single fiber and projects each colour onto a different part of the tissue. Light reflected back from the tissue's surface is decoded by a spectrometer outside the body to form one line of the endoscopic image. Moving the fiber, using an external motor or galvanometer, provides data for an interferometer to calculate structural information.

Researchers demonstrated the technology's validity by imaging metastatic ovarian tumour nodules on the abdominal wall of a live mouse. A 350 µm diameter probe was inserted into the animal's abdominal cavity through a modified 23-gauge needle. The resulting black-and-white images showed a number of raised tumour nodules. The imaging findings were later verified by histological analysis of the tissue.

Attaining 3D images with such a small endoscope is unprecedented, corresponding author Guillermo Tearney told medicalphysicsweb. "There may be some commercially available larger endoscopes that do 3D imaging, but certainly nothing in the miniature-endoscope domain."

The researchers believe that, in principle, they could shrink their probes to the size of an 80-250 µm diameter optical fiber. Such devices could be used in applications requiring navigation through very small, delicate passages, such as the salivary, mammary or pancreatic ducts, or the fallopian tubes. They may also be used to reduce risks associated with foetal endoscopy procedures.

"Safety is a main concern," said Tearney. "The smaller the size of the device, the safer it is. We feel we could put our endoscope in places like the brain or the inner ear, and that it would be very safe and very capable."

Tests of the prototype endoscope on human subjects are now planned. The team needs to identify which applications to trial the first SEE probe on, and secure the necessary approvals, Tearney says. Meanwhile, work will continue to produce higher-resolution images from still-smaller probes.

The researchers will also be trying to develop a miniature probe that can image in colour, just as larger endoscopes can. The move from black-and-white to colour is complicated by the fact that SEE already uses wavelength information to encode position. One option to gain colour images is to use three separate broad-bandwidth sources, each centred at visible red, green or blue wavelengths.

Meanwhile, commercial partners are being sought to take the technology forward, though Tearney acknowledges that there is still a long way to go before commercial products emerge: "Clinicians are used to seeing colour images, and to some extent their whole knowledge is based on colour. When you ask them whether they could do the same job with a monochrome image, many times they could. But for this device to be widely accepted, certainly for some applications there should be colour information."

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