High-resolution spectral domain optical coherence tomography system and structural characterization of ex vivo biological tissues using nanosensitive OCT

Dey, Rajib
Optical Coherence Tomography (OCT) is a rapidly growing imaging modality which is based on the principle of low coherence interferometry. It can perform high-resolution, cross-sectional imaging of the microstructure of biological tissues by detecting the coherent spectrum from the backscattered light. One significant advantage of OCT is that it can extract structural and functional imaging information in real time leading to a wide range of clinical and industrial imaging applications. Integration of OCT technology for imaging with high axial and spatial resolution to enhance the structural visibility into clinical practice is ongoing. During the FDOCT image construction, after inverse Fourier transform, high spatial frequencies, corresponding to small structural information is removed from the interference signal. All the high spatial frequency information exists in the OCT signal but does not exist in the constructed OCT image and it reduces the sensitivity to detect small structural changes. To detect this high spatial frequency information in each voxel of the three-dimensional OCT image, nanosensitive OCT (nsOCT) method has been invented. The nsOCT method provides the small size structural information with nanoscale sensitivity from all depth profiles of one single B-frame. The motivation of the thesis is to develop a high-resolution SDOCT system and enhance the structural imaging capability of ex vivo biological tissues using the developed nanosensitive OCT. The first part of this thesis summarizes the fundamentals of OCT with a focus on the principles and key performance metrics. The various stages of development of OCT imaging since its invention is also summarized. Further, the thesis explores a development of the simple characterization scheme using fibre-based near-isometric resolution OCT system at a centre wavelength of 1300 nm with 400 nm bandwidth. Despite the challenges in transporting a broadband spectrum using fibre-optics, the system investigation was motivated by the ever-increasing demand for commercialization of high-resolution OCT systems and simplification of construction. Furthermore, we evaluated and demonstrated a direct measurement method for axial resolution using an air wedge. Imaging of biomedical and other samples is demonstrated using a high numerical aperture objective lens and compared with images from a commercial OCT system. Further, the effect of the improved structural visibility by achieving image voxels closer to an isometric shape with a high NA sample lens is presented using our bespoke high-resolution OCT system. The second part of this thesis discussed the application of nanosensitive OCT to improve the structural visibility with nanoscale sensitivity and broader dynamic range of detected spatial frequencies. The thesis demonstrates numerical and experimental detection of a few nanometres structural difference using the nsOCT method from single B-scan images of phantoms with sub-micron periodic structures acting like Bragg gratings. After that a single en face image is used to confirm the ability of nanosensitive OCT to map structural changes within the skin tissue with an intervening margin area at clinically relevant depths. In addition, the thesis compared the nsOCT en face image with a high-resolution confocal microscopy image from the same tissue. Different bandwidths of structural sizes confirm the structural differences between the healthy and lesional/cancerous region which further allow detection of the skin cancer margin. A corresponding study with other bandwidths of the lower spatial frequencies was done using histological images. As an another application of nsOCT, further the thesis demonstrated the structural changes with nanoscale sensitivity inside ex vivo bovine cornea associated with CXL treatment and clearly detected by the proposed over-sampling nsOCT method. This study shows that the spatial periods inside corneal stroma increased slightly after 30 minutes riboflavin instillation but decreased significantly after 30 min UVA irradiation. En face nsOCT images at different corneal depths have also confirmed the consistent consequences, demonstrating that the nanoscale structural size decreases after the CXL treatment. In summary, this thesis demonstrates that the broad bandwidth high-resolution spectral domain OCT can be used as a tool for structural imaging of biomaterial in conjunction with the nsOCT approach.
NUI Galway
Publisher DOI
Attribution-NonCommercial-NoDerivs 3.0 Ireland