2.4. Extraction of Information from Imaging Modalities

Once the asymmetry metrics are defined, the information used for the calculation of these metrics must be extracted from both imaging modalities. For the OCT, the thickness in each sector is provided by the device, although image processing methods can also be applied; see, e.g., [16,23,24,25].

For the retinographies, although there are many automatic methods in the literature for optic disc and cup segmentation [15,26,27,28], the manual segmentation provided in the PAPILA dataset is used [18]. The CDR, RDR, and thickness measurement have been deployed using the ophthalmologist contours as ground truth. Firstly, these points were fitted with ellipses in order to decrease the variance between the points and the noise resulting from manual marking. In order to establish a similarity with the OCT results, the thickness values have been multiplied by a scaling factor. This results in the mean thickness in the general sector (G) obtained from retinography being as close as possible to that obtained from OCTs. Figure 3 shows a retinal fundus image of the right eye of a healthy patient with all the information extracted from the contours together with the location of the RDR. The image on the right shows in blue the contour of the optic disc, in green the contour of the excavation, and in red the difference between them, in addition to a horizontal line with the average value of each of these curves. The sectors are distributed, as typical OCTs are, with the corresponding labeling. The location of the value with the minimum thickness or RDR is plotted in purple.

(a) Representation on retinal fundus images of the division by sectors and measurement of the thicknesses of the right eye of a healthy patient and (b) Cartesian coordinate representation of the RDR location and the division by sectors. The contour of the optic disc delineated in blue, the contour of the excavation in green, and the difference between the two in red. The horizontal lines show the mean values of each of these curves. The × mark indicates the angle of the RDR.