Development Of Swept Source Optical Coherence Tomography

Optical, coherence tomography (OCT) is a non-destructive, non-invasive, high-resolution biomedical imaging technique, which can be used to obtain the structure and functional images of the biological internal tissues. Swept source OCT is a subtype of Fourier domain OCT, and compared with Time domain OCT it has the advantages of higher imaging rate and sensitivity. Swept source OCT has become a hot topic of the biomedical imaging research, and play an important role in the field of biological tissue imaging and endoscopic imaging. This dissertation focuses on the imaging method and system technology of SS-OCT, and the main works and innovations are listed as below:1. The SS-OCT system working at 1300nm is developed with imaging software based on VC++. Based on a high speed scanning laser source, the constructed SS-OCT system realizes high speed A-Scan rate of 20 KHz compared to 500Hz A-Scan rate of Time-Domain OCT. The SS-OCT system is mainly comprised of a swept laser source, a primary interferometer and a calibration interferometer. The SSOCT system realizes optical coherence tomographic imaging at axial scanning speed of 20 KHz with axial resolution of 7.2μm and depth range of 8 mm in air. We get some high speed optical coherence tomography images of finger-pad organism, togue tissue, fish eye and mineral stone using the SS-OCT system successfully.2. A sinusoidal waveform driven resonant fiber cantilever based one dimensional scanning probe is proposed and developed based on the constructed SS-OCT system. The probe exploits resonance of a fiber cantilever which is excited by one piece of piezoelectric bimorph through a sinusoidal driving signal whose frequency is near the resonant frequency of the fiber cantilever. The system axial resolution achieves 8.3μm. Based on the fiber cantilever probe, at 20 KHz A-Scan rate, the SSOCT system can get OCT images of sample at a frame rate of 20fps, the lateral range of lmm, and the lateral resolution of 10μm. Some preliminary images of mutilayered sample and biological sample are obtained and presented demonstrating the feasibility of the scheme and settle a steady frame for endoscopic research.3. A fiber based probe that is capable of two-dimensional scanning applicable in optical coherence tomography is proposed and developed. Based on the resonance of the fiber cantilever with asymmetry structure which has two distinguished resonant frequencies in orthogonal directions, Lissajous pattern is produced suitable for two-dimensional scanning upon a sample. Orthogonal resonances of the fiber cantilever are simultaneously excited by single piezo bender actuator with one driving signal consisting of two components corresponding to above-mentioned two resonant frequencies. By integrating a backward-placed two-dimensional position sensitive detector (PSD) into the probe, real-time lateral position of the scanning pattern is registered simultaneously for image reconstruction. Dynamical characteristics of the fiber cantilever are experimentally studied with special consideration on factors determining the resolution of the scanning pattern, including frequency and amplitude ratios between two components of the driving signal and fetching duration used for an cn face image. With the developed probe implemented in the established OCT system, en face OCT images of typical samples are obtained with satisfying resolution and contrast, demonstrating the feasibility of such fiber cantilever with asymmetry structure for realizing two dimensional scanning by single actuator, potentially applicable to endoscopic OCT imaging.4. To ensure the axial resolution approaching the practical value, the image reconstruction method needs the spectral interference signal is evenly distributed in the wave-number (k) space. Based on the constructed SS-OCT system, a calibration method in advace based on the primary interfereometer is proposed and experimentally tested. Non-uniform Discrete Fourier Transform (NDFT) method is introduced in the SSOCT system for data processing. Spectral calibration method based on Mach-Zender Interferometer (MZI) and conventional data interpolation method is also adopted in the system for comparison. OCT images from SSOCT based on the NDFT method, the MZI method and the interpolation method are illustrated. A direct k-space interpolation based on the spectral phase of MZI calibration signal is proposed and developed. The image quality by different calibration methods is compared and ensured the advantage of the proposed calibration method.5. A high speed complex conjugate mirror image elimination method based on the high speed swept source optical coherence tomography system is proposed and developed. The system introduces group delay modulation into the spectral interference signal by a transmissive grating based optical delay for separating the complex conjugate artifact. The autocorrelation function corresponding to each wave number of the source remains a higher level by tilting a small angle of the mirror in transmissive grating based optical delay compared with simply shifting reference mirror. The method is validated experimentally for in vivo imaging.