6.4. DISCUSSION 69

Figure 6.7: (a) Test image 06 from the DRIVE dataset. Each number marked is the rank order of the corresponding peak in the node map. Only the peaks on or near the ONH and macula are shown. (b) Training image 28. (c) Training image 38. (d) Training image 40.

6.4.1COMPARATIVE ANALYSIS

The two methods described in the present book for the detection of the ONH have their own advantages and disadvantages. From Table 6.3, we see that the method based on phase portraits has success rates of 100% and 69.1% for the DRIVE and the STARE datasets, respectively, which are higher than those provided by the method based on the Hough transform.

By comparing Table 6.1 and Table 5.1, we see that the mean distance related to the results of the method based on phase portraits is larger than that for the method based on the Hough transform (for the DRIVE dataset). The ophthalmologist marked the center of the ONH and not the focal point of convergence of the central retinal vein and artery, which often appear to be different in

70 6. DETECTION OF THE ONH USING USING PHASE PORTRAITS

Figure 6.8: (a) Image im0004 from the STARE dataset. The black square is the first peak detected in the node map that also met the condition based on 50% of the reference intensity. The black triangle is the center of the ONH marked by the ophthalmologist. Distance = 375 pixels. (b) Image im0027. Distance = 368 pixels. (c) Image im0139. Distance = 312 pixels. (d) Image im0239. Distance = 84 pixels. Reproduced with permission from R.M. Rangayyan, X. Zhu, F.J. Ayres, and A.L. Ells, “Detection of the optic nerve head in fundus images of the retina with Gabor filters and phase portrait analysis”, Journal of Digital Imaging, 23(4): 438-453, August 2010. © Springer.

fundus images of the retina, especially in the case of glaucoma. With erosion of the optic cup due to glaucoma, the central retinal artery and vein are displaced to a side of the ONH. If the focal point of convergence of the central retinal vein and artery is considered as the reference, the mean distance of the results of detection provided by the method using phase portraits could be lower.

Figure 6.9 shows the results of detection of the ONH with four examples from the STARE dataset. The detected circle in part (a) of Figure 6.9 is considered to be a successful detection with

6.4. DISCUSSION 71

Figure 6.9: (a) The result of detection of the ONH for the image im0009 from the STARE dataset using the Hough transform. The black dashed circle corresponds to the highest local maximum in the Hough space that also meets the condition based on 90% of the reference intensity. The cyan contour in solid line is the contour of the ONH marked by the ophthalmologist. Distance = 3 pixels, overlap = 0.87 with the intensity condition. (b) The result of detection for the image im0009 using phase portraits. The square represents the first peak detected in the node map that also meets the condition based on 50% of the reference intensity. The triangle indicates the center of the ONH marked by the ophthalmologist. Distance = 47 pixels. (c) Results for the STARE image im0010 using the Hough transform. Distance = 51 pixels, overlap = 0.22 with the intensity condition. (d) Results for the image im0010 using phase portraits; distance = 2.2 pixels.