Research On Imaging And Extraction Of Optical Parameters For Turbid Tissues With Optical Coherence Tomography

The optical properties of biomedical tissues clearly relate with ingredients, structure and status of human health. Optical Coherence Tomography (OCT) is an imaging technologies with noninvasive property, high resolution and high dynamic range. This technology is of great value for measuring optical parameters of turbid materials, such as tissues. In this thesis, the basic theories of OCT have been researched. The signal of OCT system is simulated by Monte Carlo numerical algorithm. The paper also discusses the possibility of reconstruction of optical parameter of turbid samples by OCT system. Verification based on experiment has been presented. This thesis makes research on the following aspects.First and foremost, the principle and signal characteristics were explained and discussed. Derivative process for the dispersion properties of rapid scanning optical delay line (RSOD) was given out in detail. The extended Huygens-Fresnel principle is quoted for analyzing signal to noise ratio and maximum probing depth in turbid samples. Theoretical conclusions have been verified by experiments in phantom liquid sample.A time-domain OCT system has been build up with RSOD for tissue measurement with 1550 nm optical source. The system could execute depth scanning at speed of 100 times per second with resolution of 14μm and maximum probing depth of 2 mm. We performed numerical dispersion compensation for water and RSOD up to the second order. The refractive index of transparent and turbid liquid was measured by OCT accurately.We analyzed transformation of photon in a systematic way with consideration of optical scheme. The geometry model of focused optical beam was established by describing with myriads of hyperboloids of one sheet. The initial status of photons can be determined in accordance to Gaussian distribution the way a focused beam performs. OCT signals were simulated by means of Monte Carlo and the propagating trajectory, weight and position on correlate plane of each photon will be recorded. With assistant of Monte Carlo simulation, the relationship of OCT signal and optical parameters was revealed by adding up effects of a large number of photons. Numerical results have presented relation of signal intensity and depth of target layer with different scattering coefficient, absorption coefficient and anisotropy parameter. A condensed method based on scaling path and weight of photons was introduced to accelerating simulation procedure, which makes the fast inverse Monte Carlo reality.A new algorithm of measuring optical parameters of turbid material was put forward by OCT system and depth dependent signals. The method is a inverse process of Monte Carlo and reconstruction of parameters by optimization methods or artificial neural networks. Liquid mixture of IntralipidTM and India ink was measured and analyzed by inverse Monte Carlo. IntralipidTM with glucose and milk were also used as samples. The influence of NaCl, KCl and glucose on refractive index of liquid is measured by OCT system. Results of experiments show that the degrading speed of OCT signal is sensitive with concentration of fat, color agent and glucose. These ingredients change scattering or absorption properties of turbid phantoms. Some suggestions about theory and experiment were put forward at the end of this thesis.