Optical Coherence Tomography For Retinal Imaging

Optical coherence tomography (OCT) based on low coherence interferometer and confocal scanning microscopy combining with heterodyne detection for high signal-to-noise ratio, provides non-invasive cross-sectional image with micrometer resolution and millimeter imaging depth in real time. What's more, depth and transverse resolutions are decoupled. OCT has been the research focus of biomedical imaging since it was first introduced.OCT has made its most significant clinical contribution in the field of ophthalmology, which provides images of retinal structure that cannot be obtained by any other noninvasive diagnostic technique. Many groups are doing research for high performance economical OCT retinal imaging system. In order to overcome some existing challenges of OCT retinal imaging system and meet the requirements of native development of OCT, the theory, design, alignment, performance optimization and functional detection about oxygen saturation of OCT retinal imaging system are studied to finally improve the resolution and signal-to-noise-ratio (SNR) of OCT and generate our own intellectual properties.The main work and innovations in the dissertation are summarized as following:1. To meet the demands of heterodyne detection and depth scanning in OCT, investigation on parameter optimization of the rapid scanning optical delay line (RSOD) as phase modulator and depth scanner is conducted. The optimized design of RSOD ensures the high carrier frequency of heterodyne detection required for accurate envelope demodulation, and meets the demands of range and rate of depth scanning. The dispersion of the system can also be matched simultaneously by RSOD for high depth resolution.2. A novel design of transmissive RSOD is proposed for balanced detection. In our design, a right angle prism is used for double-pass reflecting and exit displacement shifting of the beam. The design of transmissive RSOD is characterized as high beam coupling efficiency, convenience for fixing and adjusting, and cost economy. The transmissive RSOD possesses all the merits of double-pass reflective RSOD, and can be suit for time-domain OCT and Fourier-domain OCT for balanced detection to achieve high SNR and dynamic range.3. To resolve the conflict between the transverse resolution and depth imaging range, an approach to realize dynamic focusing in OCT using liquid crystal Fresnel zone lens is proposed . Fresnel zone lens suitable for dynamic focusing in OCT is designed based on optical characteristics of twisted nematic liquid crystal spatial light modulator (TN-LCSLM). Experiments on dynamic focusing control of the designed Fresnel zone lens are conducted. Measured focal lengths are in good agreement with the designed ones.4. The single-mode fiber-designed OCT retinal imaging system based on the optimized RSOD as phase modulator is developed. The experiment results indicate an axial resolution of 6.7μm, approaching the ideal resolution. The depth imaging range is about 2.5mm in the sample. Although less than 500μW optical power is incident on the sample, the system SNR is above 88dB. In addition, the animal retinal imaging experiments are conducted both ex vivo and in vivo, and the specimens of human retinal are also imaged by our system. Dual-wavelength optical coherent interferometry (OCI) is also proposed to measure the oxygen saturation in human retinal blood vessel and proved in theory.5. A scheme for swept-source optical coherence tomography (SS-OCT) based on wavelength-encoded endoscopic imaging is presented. The working principle is analysed in detail, and the feasibility of endoscopic imaing is proved in theory. The mathematical analysis of the interference signal pattern in terms of 'temporal impulse response' and 'spectral response' is conducted. The simulated interferograms of simple samples and the decode results by Morlet wavelet are presented. The system key parameters such as transverse imaging resolvable points, imaging range, resolutions are detailed discussed. It is found that the transverse resolution and depth resolution are affected by each other, and an effective method to improve the system resolutions is to enlarge the swept range of the swept source.