Optometry and Vision Sciences - Theses
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ItemMapping the Parafoveal Capillary Network and Its Flow Characteristics in Healthy Eyes( 2022)Capillary blood flow plays an essential role in the nourishment and maintenance of healthy neural tissue while, in disease, altered capillary flow patterns form the earliest signs of diabetic vasculopathy and are implicated in other major conditions including stroke and dementia. Despite its obvious importance, the difficult-to-meet demands of high spatial and temporal imaging resolution have hitherto limited detailed characterisation of how blood flows through normal capillary networks to maintain healthy retinal structure and function. In this thesis, capillary flow characteristics were studied in the central retina of three healthy young individuals using an adaptive optics ophthalmoscope to provide the required cellular-level spatial resolution, combined with fast frame rates (200-300 frames/second) adequate to capture the single-file flow of red blood cells in capillaries over the course of about 3 seconds. In the first part of the thesis, an automated kymograph method was validated against manual tracking of single red blood cells over successive movie frames for the measurement of instantaneous flow velocities. The automated method proved much faster, and arguments are presented to demonstrate its superior accuracy and robustness, particularly for vessels in which manual tracking is challenging due to high flow speeds. In the second part of the thesis, using the validated kymography approach, velocities were estimated contemporaneously from many neighbouring vessels of the parafoveal capillary network for each subject. Our findings show that capillaries universally exhibit a pulsatile flow pattern with alternating peaks and troughs in velocity with every heartbeat. A high degree of inter-vessel variability over a range of flow parameters (such as the peak, trough velocities, pulsatility, abruptness and peaktime), within a single subject and even within each retinal field, was `noted. This variability could not be explained by “local” vessel factors such as the vessel diameter, tortuosity, vessel length, linear cell density and hematocrit of the vessel. However, within a vessel, a moderate relation between velocities and hematocrit was noted, suggesting a redistribution of plasma between cells with changes in flow. Given the failure of local vessel factors to explain flow variability, the final part of the thesis explored associations between flow and capillary network variables including vessel depth, branch order, and distance from the feeding arteriole of a network. A detailed network analysis to establish the vessel connections and classifications are also presented. Most of the vessels studied were of terminal capillary type with collecting and supplying junctions on either side. Nearly 47 % of the upstream and downstream vessel junctions were amenable to fitting with a model of relative branch diameters based on Murray’s Law, with only a few adhering to modelled expectations. However, a key parameter of the model (the junction exponent) was found to be inversely related to the average velocity and trough velocity in downstream vessels. Cellular flow velocities were also moderately correlated with the length of vessel segments, and with distance to the upstream “feeding” arteriole. In summary, this thesis presents a validated method for studying retinal capillary flow characteristics in normal subjects and provides insights on flow variability within individual vascular networks.