Improving optical coherence tomography theories and techniques for advanced performance and reduced cost

• Main Author: Wang, Nanshuo
• Other Authors: Liu Linbo
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: Engineering::Electrical and electronic engineering; Engineering::Bioengineering
• Online Access: https://hdl.handle.net/10356/87187; http://hdl.handle.net/10220/49872
• Institution: Nanyang Technological University

Optical coherence tomography (OCT) has been developed as a high-resolution three-dimensional imaging technique for clinical diagnosis. This thesis proposes several theoretical discoveries and technical improvements to the current OCT systems aiming to drastically reduce system costs while achieving the state-of-the-art performance. Polarization-sensitive OCT (PS-OCT) is a functional extension of conventional OCT which offers the depth-resolved birefringence imaging capability. To resolve local retardation in tissue samples, the current PS-OCT system employs two multiplexed polarization states to illuminate the sample under investigation and detects the backscattered light with two detection channels. We observed that the polarization state of light after round-trip propagation through a birefringent medium frequently aligns with the employed input polarization state but "mirrored" by the horizontal plane of the Poincaré sphere. We explore the predisposition for this mirror state and demonstrate how it constrains the evolution of polarization states as a function of the round-trip depth into weakly scattering birefringent samples, as measured with PS-OCT. The constraint enables measurements of depth-resolved sample birefringence with PS-OCT using only one input polarization state, which offers a critical simplification and cost reduction compared to the use of multiple input states. We demonstrated the capability of polarization mirror state in local birefringence restoration with birefringent phantom imaging and swine retina imaging ex vivo. If we take transpose for both sides of the Jones transmission equation of a PS-OCT system with two input states and two detection channels, the mirror state constraint still holds while the input and output states exhibit interchangeability, which implies that depth-resolved birefringence imaging is possible with only one detection channel, if the sample is illuminated with two orthogonal polarization states. Considering the high cost of spectrometers and the difficulty of pixel alignment between two detection channels, the transposed mirror state constraint helps to cut the cost and complexity of spectral domain PS-OCT system. Inspired by the mirror state constraint, we realized that the polarization states of detected sample light, after propagating through a stochastically moving reciprocal optical path, would not distribute evenly on the Poincaré sphere, but tend to aggregate around the mirror state. This can be used to solve a long-standing problem that the detected interferograms of fiber-based OCT suffer from the fringe attenuation effect due to the polarization state mismatch between the light from the two interference arms which results in sensitivity deterioration and intensity fluctuation. This problem was conventionally solved by expensive polarization diverse detection. The polarization mirror states implies the existence of an optimal polarization state for the reference light. If the reference light is aligned with the polarization mirror state of the input light, a 3.5dB signal-to-noise ratio improvement is demonstrated without additional hardware costs. The axial resolution, being a critical parameter to the resolving power of OCT system, is determined by the light spectral shape and bandwidth. The pursuit of higher axial resolution leads to exponentially increasing system cost, and sometimes is even impossible because of the unavoidable gaps in the wavelength range of illumination, transmission, and detection. We demonstrate that the axial resolution deteriorated by gaps in OCT spectra can be restored by adopting the gapped amplitude and phase estimation (GAPES) method. The algorithm estimates the missing parts between separated spectral bands and obtains a tissue axial profile with reduced sidelobe artifacts and significantly improved axial resolution over the individual bands. This technique may make it possible to combine spectrally separated sources and detectors to improve axial resolution in OCT images, hence greatly reduce the cost of ultra-high-resolution OCT system. In conclusion, this thesis proposed methods to reduce the cost of current OCT system with advanced performances in terms of state-of-the-art contrast, sensitivity and resolution, which has been demonstrated with phantom imaging and biological tissue imaging.