Advancements in structural and functional imaging using Fourier domain optical coherence tomography
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Optical Coherence Tomography (OCT) is a non-invasive optical imaging modality based on low coherence interferometry that enables high resolution, cross sectional imaging in biological tissues and materials. One major advantage of OCT is that it can be used to obtain both structural and functional information from the same data set. OCT has established itself as a clinically approved imaging technique in applications ranging from excised tissue imaging to high speed ocular imaging and intra coronary imaging. Research is still underway to enable faster volumetric data acquisition, to increase the axial and spatial resolution of imaging and to develop new processing techniques to enhance structural and functional imaging. Conventional structural OCT imaging is based on contrast from spatial variations in tissue scattering that exploit the changes in refractive index within the imaged sample. However, while taking the inverse Fourier transform of the interference signal to reconstruct the OCT structural image, the spatial frequency information which corresponds to small, submicron structure, is lost. This reduces the sensitivity of conventional OCT signal processing to detect submicron changes in the scattering structures. Studying nanoscale structural and dynamic changes in vivo is fundamental to understanding changes occurring at cellular level before the changes manifest at the tissue level. Detecting these submicron structural changes could help scientists and clinicians to diagnose the onset of a disease, its progression and in determining treatment effectiveness of drugs. Along with structural imaging, functional imaging capability has been explored in the recent past especially for high resolution microvascular blood flow imaging. Functional extensions of OCT utilize the endogenous contrast mechanisms within the biological sample or the use of exogenous contrast agents. Functional OCT offers great potential in oncological applications, with emphasis on tumour vasculature, angiogenic processes, photodynamic therapy, targeted drug delivery and monitoring treatment efficacy. The motivation for this thesis is to explore the enhancement of structural and functional imaging capability using OCT based on endogenous and exogenous contrast mechanisms. This thesis guides the reader towards the subject of OCT covering the fundamentals of Fourier domain OCT and the various functional extensions that have been developed over the years. Further, the thesis explores the optimization of correlation mapping OCT angiography using a high speed swept source OCT system for in vivo imaging applications in particular for ocular imaging. Moving on, the thesis describes ocular imaging of a pre-clinical rat burn model using a 200 kHz swept source OCT system. In order to assess the impact of the ocular burn and to monitor the wound healing process in vivo, graph based segmentation technique is developed to extract the central corneal thickness. The estimated central corneal thickness is then used to study the impact of stem cell based treatment for the wound healing process. Further, to map the corneal thickness over a 3D volume, a distance regularized level set based segmentation approach is implemented to generate pachymetry maps and used to assess the impact of corneal injury and the wound healing process. Next, the thesis discusses the application of nano-sensitive OCT to detect the spatial and temporal structural changes within the cornea. The ability of nano-sensitive OCT to map structural changes within the cornea during the injury and subsequent wound healing process using just a single B-frame was confirmed. The study revealed that nano-sensitive OCT was able to detect structural changes with nano scale sensitivity between healthy cornea, injured cornea and also during the reparative phase of the injury at all depths within the cornea with high statistical significance. Further, a novel approach based on time-frequency decomposition of nano-sensitive OCT images is introduced which revealed corneal dynamics which otherwise have not been observed by conventional OCT processing techniques. Analysis of time varying spatial period signals within the cornea revealed five frequency bands corresponding to endothelial (0.-2 – 0.085 Hz), neurogenic (0.085 – 0.25 Hz), myogenic (0.25 -0.5 Hz), respiratory (0.5 – 1.5 Hz) and cardiac band (1.5 -5 Hz). Also, changes in energy content within the endothelial and neurogenic bands were observed between the healthy cornea and injured cornea that demonstrates the potential of the described technique for functional assessment. Analysis of these dynamic changes within the cornea can be used as a tool to study the effects of noxious stimuli on the corneal surface and its effects on the physiological signalling within the organism. Further, the thesis discusses the design and development of a photothermal OCT system to detect the enhanced imaging depth capability of gold nanostars having a plasmon resonance at 1064 nm. Though many studies have reported the use of gold nanorods, nanoshells, nanospheres, nanorose and single walled carbon nanotubes as contrast agents for photothermal OCT imaging, these studies used laser excitation wavelengths of 800 nm or below. Gold nanoparticles that has a localised surface plasmon resonance (LSPR) in the NIR region of the electromagnetic spectrum where both the absorption and scattering would be at their minimum can facilitate deeper imaging and provide better scattering contrast for biological tissue imaging. The characterization and optimization of the developed OCT system for photothermal imaging applications is discussed. Further, the photothermal signal enhancement by silica coating of the gold nanostars is explored. The experimental results demonstrated that silica coated gold nanostars of the same optical density can produce about two-fold higher photothermal signals compared to uncoated nanostars. Also, to investigate the deeper imaging capability of silica coated GNS within a scattering medium, photothermal studies were carried out on capillary phantoms having an inner diameter of 4 mm which demonstrated the enhanced imaging depth beyond 2 mm. In summary, this thesis illustrates the methods in which a high speed optical coherence tomography can be employed as a tool for structural and functional imaging based on endogenous and exogenous contrast mechanisms.
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