| dc.description.abstract | Corneal diseases such as keratoconus and Fuchs' dystrophy lead to the dysfunction of the cornea, which can result in vision loss. Early-stage detection at the cellular level provides the opportunity for treatment that slows or stops disease progression and potentially for disease cure. Optical coherence tomography (OCT), often described as the optical equivalent of ultrasound imaging, enables high-speed, non-invasive volumetric imaging at a cellular resolution. These advantages of OCT have made it a useful tool in ophthalmology and beyond. High-speed OCT data acquisition is desirable, particularly for volumetric imaging, to reduce involuntary eye and body motion and suppress motion-induced artifacts. Line-scan OCT (LS-OCT) utilizes a 2D lens, such as a cylindrical lens, as the line generator to project a line-shaped detection beam onto the sample instead of the focused pencil beam traditionally used in OCT systems. Combined with high-speed 2D cameras, LS-OCT systems allow for a data acquisition speed that is 1 to 2 orders of magnitude higher than conventional point-scanning OCT systems. The three main goals of this thesis research are: (i) to develop a novel Powell lens-based line-scan OCT system, (ii) to optimize the performance of the Powell lens-based line-scan OCT system for in vivo human studies, and (iii) to develop a line-scan OCT protocol for conducting dynamic OCT (dOCT) studies on various biological tissues.
A Powell lens is used in a line-field spectral domain OCT (PL-LF-SD-OCT) system to generate a line-shaped imaging beam with an almost uniform distribution of optical power along the line direction. This design overcomes the significant sensitivity loss of approximately 10 dB that is observed along the line length direction (B-scan) in LF-OCT systems based on cylindrical lens line generators. The PL-LF-SD-OCT system offers almost isotropic spatial resolution (∆x and ∆y approximately 2 µm, ∆z approximately 1.8 µm) in free space and a sensitivity of approximately 87 dB with only about 1.6 dB loss along the line length for an imaging power of 2.5 mW at an imaging rate of 2,000 frames per second (fps). Images acquired with the PL-LF-SD-OCT system allow for the visualization of cellular and sub-cellular structures of biological tissues.
Following the development of the first PL-LF-OCT system, we present a second-generation system that combines sufficiently high: spatial resolution (2.4 μm × 2.2 μm × 1.7 μm (x × y × z)) to resolve individual cells; sensitivity (approximately 90 dB) to image the semi-transparent human cornea; and image acquisition rate (≥ 2,400 fps) to suppress most involuntary eye motion artifacts. In summary, the second-generation system allows for contactless, in vivo imaging of the cellular structure of the human cornea. Volumetric images acquired in vivo from the corneas of healthy subjects show corneal epithelial, endothelial, and keratocytes cells, as well as sub-basal and stromal corneal nerves. The system's high axial resolution also allows for clear identification and morphometry of the corneal endothelium, Descemet's membrane, and the pre-Descemet’s (Dua) layer.
By characterizing time-dependent signal intensity fluctuations, dOCT enhances contrast in OCT images and indirectly probes cellular metabolic processes. Almost all of the dOCT studies published so far are based on the acquisition of 2D dOCT images (B-scans or C-scans) via point-scanning spectral-domain/swept-source OCT or full-field OCT respectively, due to limitations in the image acquisition rate. Here we introduce a novel high-speed Line-Field dOCT (LF-dOCT) system and image acquisition protocols designed for volumetric dOCT imaging of biological tissues. The imaging probe is based on an exchangeable telecentric lens pair that enables a selection of transverse resolution (1.1 µm to 6.4 µm) and field of view (FOV) (250×250 µm² to 1.4×1.4 mm²) suitable for different biomedical applications. The system offers an axial resolution of 2.6 µm in free space, corresponding to approximately 1.9 µm in biological tissue assuming an average refractive index of 1.38. A maximum sensitivity of 90.5 dB is achieved for 3.5 mW optical power at the tissue surface and camera acquisition rate of 2000 fps. Volumetric dOCT images acquired with the novel LF-dOCT system from plant tissue (English cucumber) and animal tissues (mouse liver and prostate tumor spheroids) allow for volumetric visualization of the tissues’ cellular and sub-cellular structure. | en |
