Optic Disk Detection

M. Foracchia, E. Grisan, A. Ruggeri. Detection of optic disc in retinal images by means of a geometrical model of vessel structure. IEEE Trans Med Imag 23:1189-1195, 2004.

We present here a new method to identify the position of the optic disc in retinal fundus images. The method is based on the preliminary detection of the main retinal vessels by means of a vessel tracking procedure. All retinal vessels originate from the optic disc and then follow a parabolic course towards retinal edges. Thus, a geometrical parametric model was proposed to describe the direction of these vessels and two of the model parameters are just the coordinates of the optic disc center. Using samples of vessels directions (extracted from fundus images by the tracking procedure) as experimental data, model parameters were identified by means of a simulated annealing optimization technique. These estimated values provide the coordinates of the center of optic disc. A Matlab prototype implementing this method was applied to a set of 40 images of both normal and pathological subjects. In all these images, the optic disc position was correctly identified, even in rather difficult pathological situations. An extensive validation on a set of 81 images (STARE project data set) is currently in progress to assess the robustness of the proposed technique.

Fig. 1. Retinal fundus images with vessels and optic disc (yellowish circle on the right-hand side).
Fig. 2. The course of the main vessels originating from the optic disc can be modeled as two parabolas, having a common origin inside the optic disc. This can be described by the geometrical locus Γ.
Γ only describes the general course of main vessels. Moving away from the optic disc, these vessels bifurcate, and branch vessels tend to diverge from main vessel direction: vessels inside the parabolas bend towards the center of the image, while those outside bend towards the external edges of the image
Fig. 3. Complete model of vessels direction.
Fig. 4. Plot of RSS values as a function of model parameters (xOD, yOD).
Complete model for vessel direction at any point (x,y) of the image (Fig. 3) where d is a term modelling the increasingly diverging direction when moving away from the parabolas; (a,b) are model parameters, and vector b includes the optic disc center coordinates (xOD,yOD).
Minimization of the residuals sum of squares RSS is performed with respect to model parameters (a,β) (minus sign indicates a direction difference)

Fig. 4. Some pathological images with estimated parabolas (blue line) and optic disc position (white cross).

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