Detection and Characterisation of the Optic Disk in Glaucoma and Diabetic Retinopathy

The optic nerve head is that region of the retina where nerve fibres and blood vessels pass through the sclera. It is sensitive to changes associated with glaucoma that can occur without other symptoms. Diabetes can also be the cause of changes in the appearance of the retina. The changes from the normal appearance can be diagnostic of the diseases, tracking of the changes in sequential images can be used to assess treatment or the progress of the illness. We are concerned with developing automated techniques of generating quantitative descriptions of the retina that might be used in diagnosis and treatment. Normal and abnormal images were collected from a range of sources, to simulate the mass screening process. Although full colour images were collected, only the green channel was processed as it had the greatest contrast. Blood vessels were removed using morphological techniques. The boundary of the optic disk was estimated using a simple edge detector and the circular Hough transform. Heuristic methods were defined to identify true and false positives. Although we do not expect the optic disk to be circular, this technique identifies it with sufficient accuracy to identify its approximate centre, which is used as the origin of 24 uniformly distributed radial vectors. The images were resampled along these vectors. The resulting image was filtered using the Lee filter and thresholded. In one or other of these images, normal subjects had a region of maximum grey value that extended the full width of the image (at the centre of the optic disk) but did not extend the full height. Using a chi-squared test, the separation of normal and abnormal images using this test was found to be highly significant (p < 0.05, n = 60).