Diabetic Retinopathy occurs in patients who suffer from diabetes for many years and as a result of which the vision gets affected. The affect can be low to severe depending on the extent to which the disease has occurred. There are two stages of the disease. The early stage is Non proliferative diabetic retinopathy (NPDR) and later is Proliferative diabetic retinopathy (PDR). In NPDR, various problems may occur such as macular edema which is swelling in the central retina and retinal ischemia which occurs due to poor blood flow. In PDR, the advanced stage of NPDR, new blood vessels starts growing in the retina known as neovascularization.
The extraction of retinal blood vessels if done at early stages can be very helpful in diagnosing the severity of the disease and accordingly the treatment can be followed. In later stages, treatment is not very effective. In this paper various blood vessel segmentation techniques are discussed. Besides segmentation techniques, retinal image enhancement techniques are also discussed. The evaluation of techniques is done on publically available databases DRIVE and STARE. These databases contain retinal images along with the ground truth images which are accurately marked by the experts for the purpose of evaluation of the techniques. The paper also discusses various metrics which are frequently used for the evaluation of image segmentation techniques.