Research On Spectral Domain Optical Coherence Tomography And Tissue Scattering Theory

Optical Coherence Tomography (OCT) was first developed in 1990s', which is becoming an emerging, noninvasive, and non-destructive imaging technique. The technique is based on Michelson interference theory. When the focused beam propagating through a biological tissue, the time delay of the backscattering light reflected by different layers can be retrieved by the low coherence interferometer. Temporal domain OCT can obtain the imaging information by point-to-point movement of reference arm because the interference is only observed when the optical pathlengths between the reference and the sample arms are matched to within the coherence length of the light source used. However, the requirement of mechanical scanning in the reference arm makes the imaging speed too slow for most of in vivo imaging applications. Recently, researchers are increasingly focusing on using spectral domain OCT because the mechanical scanning in the reference arm to obtain OCT images is no longer necessary, leading to much improved scanning speed. However, it is difficult for spectral domain OCT to obtain the anisotropic information of the biological tissue. In this regards, polarization sensitive OCT (PSOCT) has been developed so that the optical birefringent properties of tissues can be imaged, in addition to the structural information of the tissue. This dissertation focuses on the development of a PSOCT system for applications of imaging birefringent properties of biological tissue.The researcher will first introduce briefly about some basic theories on the interaction between light and tissue, along with some relevant and important optical parameters. Then, the theory of OCT and the characteristics of its signal will be then discussed. Based on these, a fiber-based spectral domain OCT system is introduced and analyzed. In this system, a spectrometer is used to obtain directly the spectral interference signal, while the structure information is obtained by Fourier transformation. To eliminate the noise, a phase shifting method is introduced, the development of which is based on averaging technique. The method is compared with different phase shifting techniques. It is found that there is different sensitivity to PZT shifting error for phase shifting methods that use more than three-phase steps. It is concluded that five phase shifting method is most immune to the noise. Secondly, Mie scattering theory is studied and analyzed, and also is calculated with special parameter assumption, and estimated the polarization patterns of light in homogeneous medium with Monte Carlo Simulation. In Mie scattering theory, the scattering event is regarded as the result of interaction between photons and scatterers in medium. Energy of photon is attenuated after each event, and the direction of scattering is determined by scattering angle and azimuth angle. The whole scattering process can be described by single scattering matrix. After calculation, it is found the scattering process is mainly forescattering and natural incident light will be change into partially polarized light. Then, based on Mie scattering analysis, Monte Carlo simulation is used to analyze the absorption and scattering properties of different media, and simulated Mueller matrix and degree of polarization of homogenous medium.In the study, a spectral-domain polarization-sensitive OCT system is also constructed, and is used to perform experiments. This system can scan in high speed, belonging to spectral domain OCT. Based on polarization theory and birefringent property, the interference signal is split into horizontal and vertical components by a polarization beam splitter. Phase delay and fast axis angle of tissues can be obtained by these two components, furthermore, Stokes vectors and the degree of polarization can also be obtained. In the end, some in vivo tissue experiments were imaged by using this system to demonstrate the feasibility of the developed PSOCT system.