Quantification of Retinal Oxygen Extraction Fraction and Vascular Oxygen Tension Gradients

Energy metabolism in the retinal tissue of the eye is considered to be one of the highest in the body. In order to maintain this highly metabolic demand for visual processing and vision, a constant delivery of oxygen by the retinal circulation is required. Alterations in retinal circulation due to vascular abnormalities are implicated in many retinal diseases and may lead to abnormal oxygenation of the retinal tissue. As a result, parameters that characterize the retinal oxygenation may help advance our understanding in both healthy and diseased retinas. This thesis describes novel methods for determining oxygen extraction parameters based on retinal vascular oxygen tension (PO2) measured by phosphorescence lifetime imaging. First, semi-automated vessel segmentation methods for quantifying the vascular PO2 from two- (2D) and three-dimensional (3D) phosphorescence lifetime images were developed and validated. Inner retinal oxygen extraction fraction (OEF) and vascular PO2 longitudinal gradients (gPO2) of retinal vessels in rats were then derived based on vascular PO2 from 2D and 3D images, respectively. Measurements of gPO2 in retinal vessels were validated by determining its response to hypercapnia. Quantification of inner retinal OEF was established and validated by measuring its alteration due to a severe hypoxic challenge. The effect of light flicker stimulation on OEF was also determined, thus demonstrating a method for assessment for neurovascular coupling in terms of the compensatory capacity of blood flow increase in response to increased oxygen metabolism. Methodologies for measurements of inner retinal OEF and vascular gPO2, and assessment of neurovascular coupling by OEF were demonstrated for the first time. These methods can serve as useful tools for investigating retinal oxygenation in health and disease.