| Pathologic changes on the surface of human organs can be detected by use of an endoscope. However, undersurface of organs cannot be observed by one traditional endoscope; no realizing early diagnosis of pathologic changes. The ultrasonic imaging can be used for getting the undersurface pathologic information because of great penetrating power. Resolutions in the ultrasonic imaging are very low. Optical coherence tomography (OCT) is an imaging technique with the rapid development in recent ten years. OCT is combined with the electronic endoscope, which makes it possible to observe the pathologic changes under the internal organs.In this paper, the principle of spectral domain optical coherence tomography (SDOCT) is studied and endoscopic system with rotated scanning is based in order to get the surface and depth information. We propose possible solutions to minimize the structure of rotating scanning which uses min-ultrasonic-motor to drive reflecting prism according to alimentary tract endoscopic feature. And interferometric Synthetic Aperture Microscopy, which can increase transverse resolution with fixed depth of field in the SDOCT, is analyzed.The main researches of this paper are shown as the following:1 Spectral domain OCTThe principle and system of time domain and frequency domain OCT is introduced. According to the rapid imaging of SDOCT, the cause of interference and the way of reconstruction are studied. The equations of transverse resolution and detection depth are deduced. The relationship between signal noise ratio and split ratio is computed and simulated.2 Interferometric Synthetic Aperture MicroscopyInterferometric Synthetic Aperture Microscopy (ISAM), which can increase transverse resolution with fixed depth of field, is used in the spectral domain OCT. The approximate wavenumber domain algorithm is applied, which can save much rebuilding time without the Stolt interpolation. A frequency shift can replace the Stolt interpolation and remove the remnant phase error. The approximate algorithm just reduces the reconstruction time from 10 min to 50 s to finish one 320 pixel×265 pixel image and greatly reduces computational complexity to enhance the real-time.3 Deign of rotated scanning probeA rotated scanning probe, which can be placed into the hose of endoscope, is proposed. The light from the fiber end-face in the reference arm of the interferometer is collimated by an angle-polished gradient index (GRIN) lens and then guided to a high-speed depth-rotating scan unit containing a prism with medium film plated and mini-motor. In order to implement radial scanning a mini-motor driving prism to rotate is fixed in front-end of the transparent housing. 4 Design of SDOCT endoscopic imaging systemThe SDOCT system including interferometer, probe and spectrum sampling is introduced, which is mainly composed of the following parts:1) A free simple Michelson's interferometer is set up for principle experiment and an all-fiber interferometer, which can increase system integrated level as a result of combination with an endoscope, is set up against free system's limitation.2) According to the paraxial meridian line equation, GRIN lens is designed for OCT probe. By use of modulate transfer function, the effect of GRIN lens on imaging quality is analysed to improve imaging. A prism with medium film plated for high index of reflection is designed for increasing utilization of light energy. Multi-transversal scanning ways are analysed and tested; and then rotated scanning is selected for SDOCT endoscope.3) A spectrum sampling system, based on embedded micro-processor S3C2443 and widely-used real-time multi-task operating system WinCE5.0, is designed. The split-optics table SGM200-M and linear array charge-coupled device (CCD) G9214-512S is chosen.4) By use of above endoscope imaging system, images of mirror, multi-layer cover-glasses, and onion's primary and secondary cuticula are gained. Compared with free SDOCT system, all-fiber imaging system based on embedded sampling makes it integrated and flexible.
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