High-resolution spectral domain optical coherence tomography system and structural characterization of ex vivo biological tissues using nanosensitive OCT
Date
2023-09-12Author
Dey, Rajib
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Abstract
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.