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Tomography is a term used to refer to imaging techniques that produce a 3-dimensional map of a structure. Typically, it refers to medical imaging techniques resulting in images of organs and tissue structures within the human body, often with the aim of locating tumours or other malformations of the tissue. Tomographic techniques that produce maps of deeply buried structures include magnetic resonance imaging, positron emission tomography, ultrasound and X-ray, but none of these can resolve detail with dimensions less than about 100 micrometres (roughly the same diameter as a human hair).Optical coherence tomography OCT is a technique that allows sub-surface structure to be imaged with much higher resolution than any of the other available methods. In the highest resolution systems, individual cells, a few micrometres in diameter, can be seen. OCT uses visible or infra-red light, which is strongly scattered by tissue, and does not penetrate far into the body. Information can only be obtained from a surface layer about 2 mm deep. Although this appears initially to be a disadvantage, the information obtained using OCT is extremely useful. A majority of cancers begin in the surface, or epithelial, tissues of the body. These include the skin and internal surfaces of the gastro-intestinal tract. By using flexible optical fibre endoscopes internal surfaces, as well as the skin, are accessible to OCT probes, which can detect minor tissue changes indicative of pre-cancerous conditions.Most OCT probes currently being developed carry light to the measurement region using a single optical fibre. A lens achieves focusing to a diameter of a few micrometres in the tissue. At boundaries between different tissue types, light is reflected back towards the probe, and re-coupled into the optical fibre, from where it mixes on a detector with a reference beam. If the paths travelled by the two beams match, and the reflection from tissue is strong, a large signal is seen by the detector. Thus by scanning the reference beam path length, boundaries within the tissue are detected as signal bursts corresponding to various depths.If 3D information is required, measurements must be made at many positions across the sample surface. This is often achieved by scanning the beam rapidly with a small moving mirror. However, it is difficult to construct scanning systems for use inside the body, as these must be miniaturised to fit inside an endoscope, and electrical power must be delivered to the endoscope tip to drive the scanner. We propose a new technique that uses a component called a coherent fibre bundle. This can be several metres long, comprising many thousand fibres arranged in a regular array a few millimetres in diameter. A pattern of light focused onto one end is transmitted unchanged to the other end. This allows the use of each individual fibre as a separate OCT system at an array of positions across the sample surface, with a digital camera used to view the image. Because the fibre bundle is extremely flexible, it can be used endoscopically. Mechanical scanning is now required only in the part of the system external to the body, reducing the complexity of the probe.We intend to investigate techniques for constructing OCT systems based on optical fibre bundles. Light from the source will be used to illuminate either a row of fibres, or each fibre in rapid succession, and the performance achieved using the two techniques will be compared. Processing techniques exist to extract simultaneous information from the full sample depth. A broad spectrum source is used, and a diffraction grating separates the interference fringe frequencies produced by many narrow, discrete wavelength bands within this broad spectrum. Alternatively, the information from each wavelength can be separated in time, by rapidly sweeping narrowband light from a laser across a broad spectral range. Mathematical processing extracts the depth-dependent structural data.
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