Vessel Enhancement

In order to calculate the vessel density and the Hausdorff fractal dimension, the image must first be filtered in order to achieve vessel enhancement. To do so, the Frangi vesselness filter13 was applied on the input image. This filter performs a type of contrast enhancement, which emphasizes structures (such as blood vessels) that can be regarded as tubular.

It should be mentioned that the filter introduced by Frangi fits a case in which the tubular objects are darker than their surroundings. In OCT angiograms the blood vessels appear as bright tubular objects, and therefore the filter was applied on the negative of the images.