Page 94 - Computational Retinal Image Analysis

86 CHAPTER 5 Automatic landmark detection in fundus photography

The best results for this method occur when the fuzzy model is calculated on
the illumination corrected image using a multiscale approach to vessel segmenta-
tion (vessel segmentations that target different widths of vessels in the image). The
method is run on six different scales of vessel segmentations. If there is no consensus
on the vessel voting, the largest bright region from a predetermined number of pixels
in the illumination corrected image is used as the found OD. The method was tested
on the STARE database (81 images) and achieved 89% accuracy (100% accuracy on
the 31 healthy retinas).

5.3 Detection of optic disc in retinal images by means of a
geometrical model of vessel structure (Foracchia et al., 2004 [21])
This method is noted both for being state-of-the-art at its time of publication and
beyond, but also for its ability to detect the OD even when it was not present in the
image. This method is similar to the method developed by Hoover and Goldbaum
(2003) by which they both are looking for the convergence of vessels at the OD. This
method, however, creates a geometric model that is fit to the directional information
of all the blood vessels in the image. This model, which is essentially two parabolic
arches, finds the OD center as the convergence of these two parabolas, symbolizing
the convergence of the retinal vasculature.
To develop the model, an arbitrary vessel tracking algorithm can be used to pro-
vide vessel centerline position and direction. The two parabolas, which come to-
gether at the OD and represent the main vessel paths across the retina can be describe
by the following locus of points:
Γ = ( ) ay 2 = } (3)
{ xy
,
x
:
Where a controls the aperture of the parabola. This only describes the course of
the main vessels in the image. To capture the fact that the vessels branch and bifur-
cate, with vessels inside the parabola bending toward the center of the image and
vessels outside the parabola bending toward the periphery. To account for this, the
complete model for vessel direction in the image is:

  (4)

θ mod ( xy,,p) = arctan sgn x () sgn y () 1 + ( 

dx y,,p)
  2 ax 

Where
y − sgn y () x
(
dx y,,p) = a (5)
c 1 + c 2
1 + e − x 1 + e x
p contains the model parameters a and the optic disc center points. In a cartesian
coordinate system, the x and y values in these formulas should be shifted by values of