Development And Application Of Doppler Optical Coherence Tomography

Optical Coherence Tomography (OCT) is a potential optical imaging technology, which uses coherence gating for high-resolution tomographic imaging of in vivo tissue structure, and have been extended for functional imaging. Based on system supplementation and algorithm processing, OCT can provide physiological information of tissue in addition to morphological structure, such as blood flow, birefringence and absorption spectra. In this dissertation, the work is focused on Doppler OCT imaging, including Doppler algorithm study, vector velocity measurement and animal application. The main contents are as follows:1. The established spectral domain OCT system has been functional extended for Doppler imaging. Different Doppler OCT algorithms for time domain and spectral domain OCT have been studied and compared theoretically and experimentally.2. A method to measure the absolute velocity is developed in the spectral-domain Doppler OCT system, where Doppler angle is determined by combined information on the measured Doppler shift and Doppler bandwidth, and then the flow velocity is determined from Doppler angle and Doppler shift. Theoretical analysis relevant to Doppler measurement and absolute velocity determination is given. Measurement on flow velocity of aqueous solution of polystyrene beads in a capillary tube is done, in good agreement with the expected values. Finally, the developed method is applied to the in vivo measurement of rat's cerebral arteries.3. A transit-time based method to ascertain the azimuth angle of a velocity vector by spectral-domain Doppler OCT is proposed, so that three-dimensional velocity vector can be quantified. A custom-designed slit plate with predetermined slit orientation is placed into the sample beam to create three delay-encoded sub-beams of different beam shape for sample probing. Based on the transit-time analysis for Doppler bandwidth, the azimuth angle within 90°range is evaluated by exploitation of the complex signals corresponding to three path length delays.3-D velocity vector is quantified through further estimating of Doppler angle and flow velocity by combined Doppler shift and Doppler bandwidth measurements. The feasibility of the method is demonstrated by good agreement between the determined azimuth angles and the preset ones, and further confirmed by velocity vector measurement of flowing solution inside a capillary tube.4. Application in cerebral microcirculation has been studied. Rats with cranial window are used as a model, and changes in blood flow velocity of cerebral arterioles in sensory cortex are measured in real time with the established Doppler OCT system, under light stimulation, electrical stimulation and drug administration. The results show significant differences in blood flow velocity between experimental groups and control groups, demonstrating the feasibility of Doppler OCT technique in cerebral microcirculation study.5. Optical superresolution is adopted for axial resolution enhancement. Three-zone phase and amplitude pupil filters are designed and implemented in the sample arm of the established time domain OCT system. Axial response curves measured under different conditions are in good agreement with the theoretical analyses. Comparison results shows that with the implemented pupil filter, the axial resolution is improved by more than 10%, demonstrating the feasibility of axial superresolution in OCT by use of appropriate pupil filters.