Identifying the neuroretinal rim boundary using dynamic contours

The neuroretinal rim forms the outer boundary of the optic nerve head: that region of the retina where blood vessels and nerve fibres pass out of the eye. It is normally a circular structure, but is known to change shape due to nerve damage in glaucoma. Its shape can therefore be used in the diagnosis and assessment of the treatment of this disease. Automatically finding the boundary would be useful as it would allow reliable quantitative shape measurements to be made. However, it is a difficult problem as the boundary is ill defined and partially obscured by blood vessels. In this paper we present an algorithm that successfully identifies the boundary using dynamic contours (snakes). The success of the algorithm is very dependent on preprocessing the image to enhance the contrast between the retina and the optic nerve head. We therefore describe the preprocessing in some detail. The algorithm has been tested on numerous images and found to be successful, as judged by an optometrist, in every case.

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