4.4. ANNOTATION OF IMAGES OF THE RETINA 39
4.4ANNOTATION OF IMAGES OF THE RETINA
In the present work, the performance of each described method is evaluated by comparing the detected center and boundary of the ONH with the same as marked independently by an ophthalmologist and retina specialist (A.L.E.). The center and the contour of the ONH were drawn on each image, by magnifying the original image by 300% using the software ImageJ [69]. When drawing the contour of the ONH, attention was paid so as to avoid the rim of the sclera (scleral cresent or peripapillary atrophy) and the rim of the optic cup, which in some images may be difficult to differentiate from the ONH. When labeling the center of the ONH, care was taken not to mark the center of the optic cup or the focal point of convergence of the central retinal vein and artery .
Figure 4.2 shows the difference between the optic cup and the ONH.
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Figure 4.2: (a) Image 04 of the DRIVE dataset. (b) The circle in solid line approximates the boundary of the ONH and the dashed ellipse approximates the boundary of the optic cup. (c) Contour of the ONH marked by an ophthalmologist (A.L.E.).
Several images in the STARE dataset do not contain the full region of the ONH. In such cases, the contour of the ONH was drawn only for the portion lying within the effective region of the image. The mask obtained in the preprocessing step (see Section 3.1) was used to get the part of the contour on the edge of the effective region. Figure 4.3 shows an example with incomplete ONH. In one of the STARE images, the center of the ONH is located outside the FOV. In this case, the center was marked at the estimated location outside the effective region.
4.5EVALUATION OF THE RESULTS OF DETECTION
4.5.1MEASURES OF DISTANCE AND OVERLAP
The results of detection of the ONH consist of the center point and the radius of a circle (in the case of the method based on the Hough transform). In order to evaluate the accuracy of the results, for each image, the Euclidean distance between the detected center and the corresponding center marked by the ophthalmologist was computed in pixels and converted to mm. In addition, the overlap
40 4. DATASETS AND EXPERIMENTAL SETUP
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Figure 4.3: (a) Image im0003 of the STARE dataset. (b) The manually marked center and contour of the ONH. The mask obtained in the preprocessing step (see Section 3.1) was used to obtain the part of the contour on the edge of the effective region.
between the circular approximation and the contour of the ONH drawn by the ophthalmologist was computed as
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(4.2) |
overlap |
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∩ B |
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where A is the region marked by the ophthalmologist and B is the region detected by the described method; see Figure 4.4. The value of overlap is limited to the range [0, 1].
For the images in the STARE dataset which do not contain the complete ONH, the regions used to compute the overlap were limited to their parts lying within the corresponding effective regions of the fundus images, determined by applying the mask generated for each image as described in Section 3.1.
4.5.2FREE-RESPONSE RECEIVER OPERATING CHARACTERISTICS
Free-response receiver operating characteristics (FROC) provide details of the variation of the sensitivity of detection with respect to the mean number of false-positive responses per image [70,71]. FROC analysis is applicable when there is no specific number of true negatives, which is the case in the present study.
For the images in the DRIVE dataset, a result in the present work is considered to be successful if the detected ONH center is positioned within the average radius of 0.8 mm (40 pixels for the DRIVE images) of the manually identified center; otherwise, it is labeled as a false positive. Using the scale factor provided by ter Haar [27] (see Section 4.3), for the images in the STARE dataset, the corresponding distance between the detected ONH and the manually identified center for successful