Optic Disc Segmentation by Weighting the Vessels Density within the Strongest Candidates

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1 Optic Disc Segmentation by Weighting the Vessels Density within the Strongest Candidates Ali Mohamed Nabil Allam Information Systems Department Arab Academy for Science & Technology (AASTMT) Cairo, Egypt Atef Zaki Ghalwash Computer Science Department Helwan University Cairo, Egypt Aliaa Abdel-Halim Youssif Computer Science Department Helwan University Cairo, Egypt Abstract Optic disc segmentation is a key element in automatic screening systems, which facilitates the detection of lesions that affect the interior surface of the eye (i.e. fundus). Therefore, this paper aims to provide a fully automated technique for detecting and segmenting the optic disc. First, the fundus image is preprocessed in order to estimate the approximate location of the optic disc, excluding the positions that doubtfully contain the optic disc. Consequently, the top candidates for the optic disc are detected and then ranked based on their strengths. Afterwards, the vessels density within each candidate is calculated and then weighted according to the candidate s strength, in which the one having the highest score is chosen to be the segmented optic disc. The performance of the proposed segmentation algorithm is evaluated over nine heterogeneous datasets of fundus images, achieving a sensitivity of 94.72%. Keywords candidates weighting, circularity strength, fundus image, optic disc segmentation, vessels density. I. INTRODUCTION The eye fundus is the only organ of the central nervous system of the human body that can be imaged directly since it can be seen through the pupil [1]. Accordingly, experts have been applying digital image processing techniques to fundus images with the aim of identifying, locating, and analyzing the fundus landmarks such as the optic disc, macula, and blood vessels. This computer-aided image analysis, in turn, facilitates the detection of retinal lesions and abnormalities that significantly affect the general appearance and semblance of these landmarks, such as glaucoma, diabetic retinopathy, and macular edema. Fig. 1 shows these landmarks of an eye fundus: Retina The optic disc is the blind spot of the eye where the optic nerve enters the retina, coming from the brain. The optic disc is found towards the right or left side of a fundus image. It is round or vertically oval, measuring about one-sixth to one-tenth the width of the fundus, and typically appears as a bright yellowish or white area. Also, the optic disc appears as the convergent area of the vascular tree of blood vessels [1], [2]. Detecting the optic disc and analyzing its spatial structure serves as an indicator of various pathologies such as diabetic retinopathy and glaucoma. As because of confusing between exudates and the optic disc (Fig. 2-b), diabetic retinopathy can be diagnosed by isolating and omitting the optic disc which in turn improves the identification of exudates, one of the main retinal abnormalities occurring in diabetic retinopathy [2], [3]. Fig. 2. Normal retina Also, the cup-to-disc ratio (CDR) is an important structural indicator for assessing the presence of glaucoma. The pathology of glaucoma is characterized by the cupping of the optic disc which becomes ischemic due to the imbalance between the intraocular pressure and the perfusion pressure in the vessels of the retina [1], as shown in Fig. 3. Diabetic retina (with exudates) Macula Optic Disc Arteries Veins Fig. 1. Eye fundus image Fig. 3. Normal optic disc Glaucomatous optic disc 1 P a g e

2 The remainder of this research paper is organized as follows. Section II reviews the previous work pertaining to optic disc segmentation. Section III describes our proposed system for segmenting the optic disc. Finally, the paper presents the results of the proposed technique, followed by a concluding section. II. RELATED WORK This section reviews the literature of optic disc segmentation, in particular, since the optic disc is considered a key landmark in fundus images that shows changes related to diseases including glaucoma and diabetic retinopathies, as mentioned in section I. Moreover, the optic disc also serves as a landmark in order to locate other fundus landmarks such as the macula and blood vessels [2]. Allam et al. [4] presented a survey of optic disc segmentation algorithms through which the main methods reviewed in the literature were categorized as follows: A. Property-based methods This category of methods is based directly on the location, size, color or shape of the optic disc. In the early work of the optic disc segmentation, Goldbaum et al. [5] utilized most of these properties in order to jointly locate the optic disc. They combined three properties of the optic disc: the convergence of blood vessels at the optic disc, the brightness of the optic disc, and entrance of large vessels above and below the optic disc. Walter and Klein [6] approximated the centroid of the optic disc as the center of the largest and brightest connected object in the fundus. Their method achieved a success rate of 58% on the dataset. Zhu et al. [7], [1] used the Circular Hough Transform to detect the circles in which the best-fitting circle for the optic disc was chosen by using a method of intensity-based selection. They achieved a successful detection rate of 90% and 44.4% on the DRIVE and datasets, respectively. Lu [8] captured both the circular shape of the optic disc as well as the image variation across the optic disc boundary, simultaneously. They determined the pixels with the maximum variation along multiple evenly-oriented radial line segments of specific length, which were then exploited to locate both the center and the boundary of the optic disc. Experimental results showed that the boundary of the optic disc was accurately segmented in 97.5%, 99.75% and 98.76% of the ARIA, MESSIDOR, and datasets, respectively. Yu and Yu [9] localized the optic disc through a method based on extracting the brightest pixels, iteratively, in order to overcome the presence of any other bright artifacts such as large exudates. Both, the thresholds for the area and the circular degree were employed in order to select the optic disc region among other candidates. The proposed method localized the optic disc in 95% of a subset of the images. B. Convergence of blood vessels An alternative approach to detect the optic disc is to exploit the vascular tree of the retina taking advantage of the spatial relationship between the optic disc and blood vessels, since the optic disc is considered the convergence point of the few major blood vessels that split into many thinner vessels throughout the retina [2]. Hoover and Goldbaum [10] developed a voting-type algorithm in order to detect the convergence point of blood vessels which was considered as the center of the optic disc in a fundus image. This technique successfully detected 89% of the images in the dataset. Also, ter Haar [11] applied the Hough Transform to the pixels on or close to the binary image of the retinal vasculature achieving a success rate of 96.3% on a local dataset and 71.6% on the dataset. Fleming et al. [12] detected the approximate region of the optic disc using an elliptical shape of the major blood vessels. The approximate location of the optic disc was then refined via the Circular Hough Transform achieving a success rate of 98.4% of the 1056 fundus images. In the work of Rangayyan et al. [13], [1], the blood vessels were first detected using Gabor filters, and then phase portrait modeling was applied to detect the convergence points of the vessels, in which the best-fitting circle for the optic disc was chosen by using an intensity-based condition. This approach achieved success rates of 100% and 69.1% for the DRIVE and datasets, respectively. C. Model-based methods (template-matching) This type of methods is based on comparing a template image (model) with a set of candidates in order to determine the best-matching candidate. Li and Chutatape [14] created an optic disc model (disc-space) by applying Principal Component Analysis (PCA), in which the candidate regions having the highest 1% gray-level were selected and matched to the discspace, where the optic disc was successfully detected in 99% of the images. Also, Foracchia et al. [15] proposed a geometrical model to describe the general direction of retinal vessels at any given position using the coordinates of the optic disc center which were two model parameters (i.e. the convergence point for the blood vessels). The position of the optic disc was correctly located in 97.53% of the images. Closely related, Youssif et al. [2] proposed another model-based approach by matching the expected directional pattern of the retinal blood vessels found in the vicinity of the optic disc. They obtained a vessels direction map of the retinal vessels that were segmented using 2D Gaussian matched filter, achieving a successful detection rate of 100% on the DRIVE dataset and 98.77% on the dataset. Aquino et al. [16] used morphological and edge detection techniques followed by the Circular Hough Transform to obtain an approximate circular optic disc boundary. Their methodology required an initial pixel located within the optic disc, and for this purpose, a location procedure based on a voting-type algorithm was utilized. The algorithms were evaluated over the MESSIDOR dataset, achieving a success rate of 86%. Zhang & Zhao [17] identified a number of candidate vertical windows for the optic disc based on three characteristics of blood vessels at the vicinity of the optic disc: vessels high density, compactness of vertical vascular segments, and the vessels uniform distribution. Consequently, the y-coordinate of the optic disc was identified according to the vessels direction via parabola curve fitting using the General Hough Transform. The optic disc was correctly detected in all images of each of DIARETDB0, DIARETDB1, DRIVE and datasets, except only one image in. III. PROPOSED SYSTEM The block diagram shown in Fig. 4 illustrates our proposed architecture of optic disc segmentation, showing the data that is employed, the steps that are followed and the procedures that are applied in manipulating the eye fundus images. Each of the four block components of this architecture is discussed in detail at the subsequent sections (A, B, C, and D): 2 P a g e

3 Mask Generation Blood Vessels Extraction Vertical Splitting Contrast Enhancement A. Input Dataset Fundus Images Ground Truth D. Segmentation Evaluation Calculate Sensitivity Segmented Optic Disc B. Fundus Image Preprocessing C. Optic Disc Segmentation Detect OD Candidates based on Brightness / Size / Roundness Calculate Vessels Density within each Candidate Weight Vessels Density based on Candidates Strengths Select the Candidate having the Highest Score Value SAI Computing Conference 2016 B. Fundus Image Preprocessing Unfortunately, the raw digital fundus images are not always ready for immediate segmentation. Therefore, an image should be first preprocessed in order to make it more suitable for further segmentation. The following subsections present the main preprocessing steps utilized in the proposed segmentation approach. 1) Mask Generation Masks are usually created and then used in order to exclude the dark background of the image from further calculations and processing. In other words, only the region belonging to the semi-oval fundus image is included for processing. The mask image is a binary image in which the background pixels are assigned the value of zero, whereas the pixels within the region of interest are assigned the value of one [11]. Our proposed segmentation method exploits the masking technique presented by ter Haar [11]. The mask is created by thresholding the red channel of the fundus image using a threshold value determined empirically (t=35). Afterwards, the thresholded image is morphologically processed through opening, closing and erosion operators using a 3 3 square structuring element, as shown in Fig. 5. Fig. 4. Block Diagram of Proposed System Architecture A. Input Dataset: fundus images and ground truth The fundus images are the raw material to be preprocessed, segmented and evaluated. Fundus images are usually accompanied with a ground truth used for comparing the results achieved by some algorithm against the true results provided by medical experts (i.e. ophthalmologists). Table I shows a chronological list of the publicly available datasets, which are all used for testing and assessing our proposed segmentation method. Dataset Name (2000) [18] DRIVE (2004) [19] MESSIDOR (2004) [20] ONHSD (2004) [21] ARIA (2006) [22], [23] DIARETDB0 & DIARETDB1 (2008) [24] DRIONS-DB (2008) [25] TABLE I. EYE FUNDUS DATASETS No. of Images FOV Images Size (in pixels) Images Format Ground Truth PPM OD & BV TIFF BV Set1: Set2: Set3: TIFF Retinopathy and Macular Edema BMP OD TIFF PNG OD, BV and Fovea MA, HE and Soft & Hard Exudates JPG OD HRF (2009) [26] JPG BV FOV = field of view, OD = optic disc, BV = blood vessels, MA = microaneurysms, HE = hemorrhages (c) Fig. 5. Mask Generation A color image generated mask (c) excluded background 2) Blood Vessels Extraction The vasculature is roughly extracted via the Canny edge detector using the intensity image specified by the mask, as shown in Fig. 6. This step of extracting the vascular tree will be furtherly benefited twice. First, it will be utilized to split the fundus into two vertical halves indicating which half contains the optic disc (part B.3). The second benefit is that these detected retinal vessels will be utilized to calculate the density of vessels within each of the segmented optic disc candidates (part C.2). Fig. 6. Vessels Extraction Reference image Extracted blood vessels 3 P a g e

4 3) Vertical Splitting As mentioned previously in section I, the optic disc is usually located towards the left-hand side or the right-hand side of the fundus, and from which the major blood vessels originate. Thus, the optic disc is normally located at the half containing more blood vessels than the other (as noticed in Fig. 1 and Fig. 6). Thereby, the fundus image is vertically split into two halves (i.e. left and right), and the half with much blood vessels is selected for further image processing whereas the other one having less vessels is excluded (Fig. 7). This splitting approach kills two birds with one stone; first, it greatly reduces all further processing-time to almost 50%. Secondly, it reduces the number of false candidates of the optic disc which may occur in the excluded half. These false candidates may result from artifacts that are similar to the optic disc such as exudates, or may also sometimes occur due to non-uniform illumination (i.e. vignetting effect). 4) Contrast Enhancement In the direction of segmenting the optic disc, the contrast of fundus image is first enhanced in order to improve the segmentation process. At the beginning, the color fundus image is converted to a grayscale intensity image by eliminating the hue and saturation information while retaining the luminance. Consequently, the intensity pixels in this grayscale image are mapped to new values such that 1% of the data is saturated at low and high intensities of the grayscale image. This in turn increases the contrast of the output image, as shown in Fig. 8. Fig. 7. Vertical Splitting Left and right extracted vessels Selected half (c) Fig. 8. Contrast Enhancement Original image Intensity image of selected half (c) Adjusted contrast of intensity image C. Optic Disc Segmentation This component is the backbone of the system architecture. First, the top three candidates for the optic disc are detected and then ranked based on their circularity strengths. Consequently, the vessels density within each candidate is calculated and then weighted according to the candidate s strength. At the end, the candidate having the highest score is selected as the segmented optic disc. 1) Detecting Optic Disc Candidates Identifying the optic disc candidates is the most import step that greatly affects the accuracy of the whole segmentation process. The objective of this process is to detect the top three candidates of the optic disc based on its main spatial properties: its brightness, size, roundness, and before all, its approximate location within the fundus (i.e. left half or right half). Since the approximate location of the optic disc had been already determined previously, therefore this step is only concerned with the other three properties: - Brightness: the contrasted intensity image is converted to a binary image by thresholding it at t=255, in order to detect only the bright (white) artifacts in the fundus image. - Size: as mentioned before, the optic disc constitutes onetenth to one-sixth of the width of the whole fundus image. Accordingly, the algorithm estimates the radius range of the optic disc relative to the width of the fundus image, rounded to the greater integer. For instance, in the DRIVE dataset whose fundus widths are 584 pixels, the radii of the optic discs are estimated within a range of (30 to 50) pixels. The radius range (min-to-max) is estimated as follows: min = ceil ( width 100 ) 10 2 max = ceil ( width ) Roundness: because the optic disc is normally round or vertically oval in shape, the algorithm first applies some morphological opening operators in order to structurally enhance the circular objects in the thresholded image. Consequently, the Circular Hough Transform is utilized in order to detect and localize all the circular shapes specified by the estimated radii. Based on all these previous spatial properties, the strongest three circular shapes are chosen as the top candidates of the optic disc, and these candidates are then ranked according to their circularity strengths. 2) Calculating Vessels Density It was mentioned before that the major blood vessels radiate from the center of the optic disc. Therefore, the vessels density within each candidate is calculated in order to determine how much blood vessels each candidate contains. For each of the three optic disc candidates, the corresponding vessels density is calculated as follows: Density = Size (BV) Size (OD) (1) (2) (3) 4 P a g e

5 where, Size (BV) is the number of pixels of the blood vessels inside the detected candidate, and Size (OD) is the number of pixels within the candidate optic disc (i.e. the area of the oval). 3) Weighting Vessels Density Having the top three ranked candidates of the optic disc, as well as the vessels density within each candidate, the vessels density is weighted accordingly. First, each candidate is assigned a weight based on its circularity strength. These weights are determined empirically as w 1 = 1.2, w 2 = 1.1, w 3 = 1.0, for the strongest, moderate and weakest candidates, respectively. Consequently, the calculated vessels densities are weighted according to the candidates strengths, as follows: Score 1 = w 1 Density 1 (4) Score 2 = w 2 Density 2 (5) Score 3 = w 3 Density 3 (6) 4) Selecting the Segmented Optic Disc The final step in the segmentation procedure is to choose one of the candidates to be the optic disc. Each of the figures from Fig. 9 to Fig. 14, show various examples of selecting the optic disc candidate within healthy and pathological images. (Note: these figures are best viewed within the paper when zoomed to 200% or more). The top three candidates of the optic disc in a fundus image are: the red circle (strongest candidate), the green circle (moderate candidate), and the blue circle (weakest candidate). Fig. 9 and Fig. 10 show examples of how the strongest candidate is selected (red circle). In Fig. 9, for instance, the red candidate was promoted and selected as the optic disc due to weighting, although the blue circle has denser blood vessels. Also, Fig. 10 shows a good example of how vertical splitting succeeded in excluding the right half which contained a false candidate (i.e. large exudate) that looked like the optic disc. Fig. 10. Example of selecting the strongest candidate in a pathological image (: im0008.ppm ) Fig. 11 and Fig. 12 show how the moderate candidate is selected (green circle). These examples emphasize the importance of the vessels density together with the candidates weights, in order to accurately localize the optic disc in the fundus image. For instance, although the moderate candidate in Fig. 11 was not the one with the densest vessels nor the highest weight among other candidates, yet this green candidate was weighted in such a way that it scored a value surpassing those scored by both, the strongest and the weakest candidates. Fig. 9. Example of selecting the strongest candidate in a healthy image (DRIVE: 32_training.tif ) Fig. 11. Example of selecting the moderate candidate in a healthy image (DRIVE: 22_training.tif ) 5 P a g e

6 Fig. 12. Example of selecting the moderate candidate in a pathological image (: im0011.ppm ) In the examples shown in Fig. 13 and Fig. 14, the blue circle was selected although it is the weakest candidate. This is because neither of the two other stronger candidates were weighted highly enough to achieve a score greater than that achieved by the weak (blue) candidate. Fig. 14. Example of selecting the weakest candidate in a pathological image (: im0043.ppm ) D. Segmentation Evaluation In order to evaluate the proposed segmentation algorithm, our produced segmentations are compared against the provided ground truths. The evaluation of the algorithm is measured via sensitivity which is the true positive rate of detecting the optic disc. Fig. 15, Fig. 16, and Fig. 17 show samples from different datasets of fundus images and their corresponding ground truths of the optic disc. It is worth to mention that the ground truth is not always represented in the same manner within all datasets; even some datasets, such as HRF, represent this ground truth textually, rather than showing it graphically. Fig. 15. Example of a fundus image and its corresponding ground truth in dataset Fig. 13. Example of selecting the weakest candidate in a healthy image (DRIVE: 03_test.tif ) Fig. 16. Example of the fundus image and its corresponding ground truth in ARIA dataset 6 P a g e

7 Fig. 17. Example of the fundus image and its corresponding ground truth in DRIONS-DB dataset IV. EXPERIMENTAL RESULTS The proposed algorithm was implemented using MATLAB R2013a under a platform of Windows 10 Pro with a CPU i5-3230m of 2.60 GHz. The system was tested over nine miscellaneous datasets having extremely heterogeneous properties (e.g. image spatial size, image quality, FOV, format, etc.). The purpose of using heterogeneous datasets was to prove the effectiveness of the segmentation algorithm, over any fundus image whatsoever. Table II distinguishes the results achieved by our proposed approach over the nine different datasets, mainly in terms of the sensitivity and the processing time. TABLE II. RESULTS OF PROPOSED METHOD No. of No. of Correct Processing Time Avg. Processing No. of Pixels No. Dataset Name Sensitivity Images Responses (in seconds) Time per Image per Image 1 ARIA ,368 2 DIARETDB ,728,000 3 DIARETDB ,728,000 4 DRIONS-DB ,000 5 DRIVE ,960 6 HRF ,185,344 7a MESSIDOR (set1) ,382,400 7b MESSIDOR (set2) ,333,120 7c MESSIDOR (set3) ,538,944 8 ONHSD , ,500 ALL DATASETS Table III shows a comparison of our proposed method against the different optic disc detection methods reviewed in section II, from the perspective of the detection approach, the employed datasets, and the sensitivity of detecting the optic disc. TABLE III. RESULTS OF OPTIC DISC DETECTION METHODS No. Detection Approach Dataset(s) Sensitivity 1 Goldbaum et al., (1996) [5]: Vessels convergence, optic disc brightness, and entrance of large vessels None - 2 Walter & Klein, (2001) [6]: Largest brightest connected object Local dataset (30 images) Hoover & Goldbaum, (2003) [10]: Fuzzy convergence of vessels Foracchia et al., (2004) [15]: Geometrical model of vessel structure Li & Chutatape, (2004) [14]: Template matching via PCA Local dataset (89 images) ter Haar, (2005) [11]: Hough Transform applied to the pixels on or close Local dataset (191 images) to the retinal vasculature Fleming et al., (2007) [12]: Elliptical form of blood vessels Local dataset (1056 images) DRIVE 8 Youssif et al., (2008) [2]: Vessels direction matched filter Aquino et al., (2010) [16]: Morphological operators, edge detection and circular Hough transform MESSIDOR Additional results for the dataset were obtained from the comparative study done by ter Haar [11]. 7 P a g e

8 No. Detection Approach Dataset(s) Sensitivity 10 Rangayyan et al., (2010) [13]: Vessels convergence by Gabor filters and DRIVE phase portrait modelling Zhu et al., (2010) [7]: Circle detection via Hough transform DRIVE ARIA subset (120 images) Lu, (2011) [8]: Circular transformation and image variation along MESSIDOR multiple radial line segments Yu & Yu, (2014) [9]: Iterative brightest pixels extraction subset (40 images) DIARETDB0 14 Zhang & Zhao, (2014) [17]: Vessels distribution and directional characteristics DIARETDB1 DRIVE Proposed Method, (2015): Weighting vessels density within the strongest optic disc candidates ARIA DIARETDB0 DIARETDB1 DRIONS-DB DRIVE HRF MESSIDOR (3 sets) ONHSD V. CONCLUSION The segmentation methods that are solely based on the spatial properties of the optic disc (e.g. its shape, color, size, etc.) achieve good and quick results in normal fundus images that contain no abnormalities. But, such approaches usually fail to detect the optic disc in pathological images where abnormalities, such as large exudates, are confused with the optic disc due to their similar appearance. Therefore, alternative approaches based on vesselsconvergence or template matching proved to achieve better sensitivity rates, since the number of false responses are greatly reduced in the presence of other similar abnormal artifacts. However, such approaches obviously take more processing time and normally require pre-customized templates or imagedependent parameters. Thereby, our segmentation approach proposed in this paper benefited the advantages of these different alternatives, in a simple and straightforward way. It relied on the spatial properties of the optic disc, and at the same time, it reduced the number of false candidates in order improve the sensitivity rates within pathological images, and above all, in a fully-automated manner. The proposed algorithm was tested and evaluated over nine public datasets containing a total of 1933 images. The segmentation algorithm proved its effectiveness by segmenting the optic disc correctly in 1831 images achieving a total sensitivity of 94.72% which was comparable to the results achieved by the other approaches. But more importantly, the implementation of the segmentation algorithm was fully automated regardless of the extreme heterogeneity of the tested datasets (e.g. spatial size, FOV, image quality, etc.); as no image-dependent parameters are adjusted nor predefined templates are used for the sake of customizing the proposed algorithm over certain datasets. VI. REFERENCES [1] X. Zhu, R. M. Rangayyan, and A. L. Ells, "Digital Image Processing for Ophthalmology: Detection of the Optic Nerve Head," in Synthesis Lectures Biomed. Eng.: Morgan & Claypool Publishers, [2] A. A. A. Youssif, A. Z. Ghalwash, and A. A. S. A. Ghoneim, "Optic Disc Detection From Normalized Digital Fundus Images by Means of a Vessels' Direction Matched Filter," IEEE Trans. Med. Imag., vol. 27, no. 1, pp , [3] G. Schaefer and A. Clos, "Image Analysis for Exudate Detection in Retinal Images," in Biocomputation and Biomedical Informatics: Case Studies and Applications.: Medical Information Science Reference, 2010, pp [4] A. M. N. Allam, A. A. Youssif, and A. Z. Ghalwash, "Automatic Segmentation of Optic Disc in Eye Fundus Images: A Survey," Electron. Lett. Comput. Vision Image Anal., vol. 14, no. 1, pp. 1-20, [5] M. Goldbaum et al., "Automated diagnosis and image understanding with object extraction, object classification, and inferencing in retinal images," in Proc. IEEE Int. Congr. Image Process., 1996, pp [6] T. Walter and J. Klein, "Segmentation of Color Fundus Images of the Human Retina: Detection of the Optic Disc and the Vascular Tree Using Morphological Techniques," in Proc. 2nd Int. Symp. Medical Data Analysis (ISMDA '01), 2001, pp [7] X. Zhu, R. M. Rangayyan, and A. L. Ells, "Detection of the Optic Nerve Head in Fundus Images of the Retina Using the Hough Transform for Circles," J. Digital Imaging, vol. 23, no. 3, pp , June P a g e

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