Study On Some Key Technologies Of High-speed Imaging Complex Spectral Domain Optical Coherence Tomography

Optical coherence tomography (OCT) is a novel optical tomography technique which has many advantages, such as non-contact and non-invasion detection, high resolution, high sensitivity for the imaging of living biological tissue. It has been possessed of great development potential application in clinical medicine. In order to make OCT become a kind of high-speed, practical medical diagnosis system, the study on some key techniques of high-speed imaging of OCT system is carried out on both theoretical and practical sides.On theoretical side, an OCT Monte Carlo simulation model based on confocal mode is put forward. In this new OCT model, the Gaussian beam with practical size is applied. Since OCT System in general use confocal scanning mode, incident beam is always the focused cone-shaped beam. However, in the conventional OCT Monte Carlo simulation model, the incident beam is an ideal beam with infinity narrow size in stead of focused cone-shaped beam. Consequently, the simulation result is incapable of present the special characteristic of optical system in detail. The OCT Monte Carlo simulation model based on confocal mode is designed to make up the deficiency of conventional model. The confocal microscope structure and focused Gaussian beam is introduced and the strategy on classifying back scattering photons is improved.On practical side, experiment based on complex spectral Fourier-domain OCT is carried out. Conventional time-domain OCT with single-point detection technique acquires the interference signal point and point. It is a time-consuming process to create a cross-sectional image, so this kind of scanning mode may be one of the main constraint factors for high-speed imaging. When Fourier-domain technique is performed in OCT system, depth-scan information is provided by an inversion Fourier transformation of the spectrum of the back scattering light. The time-consuming mechanical OCT depth-scan can be put aside and the video rate imaging could be possible. In order to realize high-speed imaging of OCT for complex biological tissue, eliminate the alias phenomena in the image and rebuild the true tomographic structure of specimen, the phase shifting interference technology is introduced and a full range complex spectral OCT experimental system is created. A special coating beam splitterand a high precision phase shifter are used in this system. The condition for high speed imaging is provided by these aborative designs.The main innovative point can be summarized as follows:1. An OCT Monte Carlo model based on confocal mode is put forward for the first time. The effect of numerical aperture of confocal system for the image depth of OCT is studied with new model. The diffuse reflection of back scattering light satisfied with OCT coherence condition is also estimated with this model. Simulation software based on this model is developed and can be used to modeling many optical systems with the confocal scanning structure.2. A novel interferometer with special coating beam splitter is put forward for the first time. The beam splitter is coating with high reflective metal membrane except a small circle with 1mm diameter in the center, so the most back scattering light is reflected from the total reflection region of beam splitter. By applying such beam splitter, on the one hand the intensity of incident light to tissues can be increased and the back scattering light dissipation can be decreased to the last degree. On the other hand the difference between the intensity of reference light and that of signal light will be reduced. As a result, the image contrast is enhanced and the video-rate imaging becomes possible.3. The phase shifter driven by voice coil is firstly applied to extract the phase of spectral interferogram. High precision calibration proved that phase shifter can achieve nanometer linear displacement. The error forπ2 phase step is below 2%, which meets the requirement of phase shifting interferometry. And what is More, it is possessed of other advantages, such as low cost, easy control and quick response. Combining the Fourier-domain OCT with phase shifting interferometry, the full-range measurement can be achieved, so imaging space and the bandwidth of detection system can be extended and put to good use.4. Multithreading synchronous execution and cyclic utilization buffer mode are applied firstly in the image reconstruction of Fourier-domain OCT. In the procedure of image reconstruction, two threads of hardware acquisition and data processing are proceeding synchronously and two buffers with the same size are assigned in computer memory. When the hardware acquisition thread store the image data to one of buffer, in the mean time the data processing thread handle the image data on the other buffer. This procedure is performed alternately until the end of image reconstruction. With this advanced method, the utilization efficiency of memory isincreased and the speed data processing is accelerated. As a result, the imaging speed will be increased.