Quantification of retinal microvasculature

Optical coherence tomography angiography (OCTA) is a relatively novel three-dimensional imaging modality, that provides images of the blood perfusion in the retina. This thesis focuses on image analysis methods for quantifying micro vascular structures in the retina based on OCTA images. By segmenting the microvasculature in to capillaries, vessels, foveal avascular zone,and background, we can accurately quantify structural features in the retina. The proposed methods demonstrate the potential of using OCTA to assess the state of patients with eye conditions. The thesis is composed of an introductory part and four research contributions including a technical report, two conference papers (Paper I and II), and a journal paper (Paper III). Our first contribution is a quantification algorithm for computing vascularmetrics from OCTAdata using a dictionary-based segmentation model (Paper Iand the technical report). We validated the method on healthy subjects and cataract patients and showed that it is possible to distinguish subjects based on the extracted features. Next, we investigated how our quantification can be used for detecting vascular changes in OCTAdata. Patients undergoing cataract surgery might develop a postoperative complication called macular edema, which is a build-up of fluid in the center region of the retina. These patients might have an abnormal microvasculature preoperatively or might develop vascular changes within a few weeks after the operation. Therefore,we proposed another approach for investigating vascular changes in patients with hematological disorders (PaperII). As opposed to the cataract patients, for whom we are unsure which vascular changes to expect, it has previously been shown that patients with abnormal blood compositions might cause abnormal bloodflow. Hence, we have used OCT Aimages of these patients to investigate the properties of our method. Before reasons for edema development after cataract surgery can be investigated,the effects of uncomplicated cataract surgery on the vasculature must be analyzed, which we have done in Paper III. Based on the findings in Paper I and II, we have developed a quantification method to observe potential vascular changes in a larger subset of cataract patients (Paper III). The importance of good quality data was seen in Paper I, and hence a grading protocol was developed in Paper III to aid the statistical analyses. In conclusion, this thesis contributes with a method that allows for precisely quantifying the retinal microvasculature. Furthermore, it provides studies of patients with hematological conditions and a longitudinal study of a large group of cataractoperated patients. These studies showed that vascular changes can be detected in hematological patients undergoing treatment, while only small microvascular changes were detected in cataract operated patients, although a significant growth of the macular thickness was observed.