Analysis of images of the human retina

Abstract

The microvasculature is implicated in the development of cardiovascular disease, and the eye offers a unique window through which the retinal microvasculature can be imaged and evaluated. Retinal fundus photography allows computer aided quantitative measurements characterising vascular geometry that are believed to relate to vascular disorder and cardiovascular risk. Measurement of vessel diameters from non-fluorescein images is challenging for numerous reasons. A novel measurement technique, the Sliding Linear Regression Filter (SLRF) is proposed to overcome deficiencies in earlier approaches, and confidence gained in its performance by comparison with manual measurements by a skilled clinician. Non-dimensional parameters are preferred to characterise the vascular geometry to avoid dependence on refraction of the eye. Departure of diameter relationships from optimal conditions at arterial bifurcations is hypothesised to indicate endothelial dysfunction and to predict risk of atherogenesis and cardiovascular risk. Other parameters of interest include vascular length/diameter ratios and tortuosity. A computer based semi-automatic grading tool using the SLRF method has been developed and applied to images from the Beaver Dam Eye Study in a prospective case-control study to explore associations between retinal vascular geometry and incident death from ischemic heart disease (IHD) and stroke over a 10 year period. Results were obtained from 126 IHD and 28 stroke cases, together with 528 age and gender matched controls. Disordered arterial bifurcation optimality was associated with death from IHD, consistent with the underlying hypothesis. Furthermore, this association appears independent of other recognised risk factors, suggesting that it adds additional prognostic value. Elevated conventional arterial L/D ratio was associated with death from stroke, but this association was not maintained after adjustment for systolic blood pressure. An interesting but unexpected association between reduced simple tortuosity and death from IHD was found, although this should be treated with some caution, due to eccentricities in behaviour of this parameter.