Automatic optic disk boundary extraction by the use of curvelet transform and deformable variational level set model
Highlights
► Optic disk (OD) segmentation is an important task in automated retinal screening. ► Curvelet transform is employed to get probable location of OD. ► Based on the size of yellowish objects two curvelet-based methods are proposed. ► We use some morphological operations to remove undesired pixels in edge map. ► The boundary of the OD is extracted by using level set deformable model.
Introduction
Optic disk (OD) is characterized as bright yellowish disk, from which, blood vessels and optic nerves emerge. It is a brighter region than the rest of the ocular funds and its shape is approximately round. The OD location can be used as a reference to identify the fovea in retinal images, for blood vessel tracking and other tasks. Many schemes have been proposed to detect OD. Tamura et al. [1] and Liu et al. [2] find the largest clusters of pixels with the highest intensities, while meets their difficulty when large hard exudates coexist in retinal image. Sinthanayothin et al. [3] identify the area with the highest intensity variation of adjacent pixels as OD. Li and Chutatape [4] use principal component analysis (PCA) to locate OD even for retinal images having bright lesions. Barrett et al. [5] propose applying Hough transform in order to locate OD. This method finds the circular shape with fixed radius in a thresholded edge image of the fundus and estimates the OD-contour by employing the Hough transform on detected edges. Niemeijer et al. [6] use a model of the geometrical directional pattern of the retinal vascular system, and implicitly embed the information on the OD position as the point of convergence of all vessels. Lalonde et al. [7] propose an OD detection method based on pyramidal decomposition and Hausdorff-based template matching. They estimate the OD-contour using Hausdorff-based matching between detected edges and a circular template. In general, the individual attributes are not sufficient to decide final OD location. For instance, in uneven illumination conditions or in the presence of bright lesions brightness, it is necessary to combine all attributes to handle all possible variants of a retinal image.
In this paper, we present a new curvelet-based method for OD detection which is a hybrid approach that characterizes OD using intensity and vessel structures. The pipeline of the presented method is illustrated in Fig. 1 (the left column shows the proposed algorithm for extraction of blood vessels, and the right column shows the pipeline of proposed algorithm for determination of OD candidate locations). Candidate regions for OD location are determined by modification of curvelet coefficients of enhanced image, and performing Canny edge detector and some morphological operations on reconstructed image with this modified coefficients. We choose the candidate region that has maximum summation of pixels in strongest edge map, as final location of OD. After detection of OD location, we use variational level set deformable model and apply the initial level set contour around the detected OD center and extract the whole OD boundary.
The paper is organized as follows. After a short review on digital curvelet transform (DCUT) via wrapping, the proposed method in this paper is introduced in Section 2. Our curvelet-based technique includes 4 steps including: (1) contrast improvement and non-uniform illumination correction, (2) curvelet-based determination of candidate regions, (3) OD detection, and (4) OD boundary extraction. In Section 3, the results and discussion are presented, and finally this paper is concluded in Section 4.
Section snippets
Proposed method
DCUT was introduced to represent edges and other singularities along curves much more efficiently than other traditional transforms such as Fourier transform, ridgelet transform and wavelet transform [8], [9]. Ridgelet analysis may be constructed as wavelet analysis in the Radon domain. The Radon transform translates singularities along straight lines into point singularities. Then, the wavelet transform can be used to effectively handle these point singularities. In images, edges are typically
Results
The proposed method for detection of OD location has been evaluated on three publicly available datasets, the STARE [13], DRIVE [24], and DIARETDB1 [25]. We considered the OD detected center within 7 pixels (with average OD within 40×40 pixels) of its actual position as successful detection.
The quantitative results obtained from the proposed method are shown in Table 1.
The experimental result shows that the proposed algorithm is robust to variable appearance of retinal images and can give the
Conclusion
Because the brightness, contrast and color of OD vary a lot among different patients, some previous methods for OD detection would not work well in all the images used in clinical environment. In this paper, we introduced a new method for OD detection based on the DCUT. In the curvelet domain, some features like exudates and OD in retinal images are correspond to large coefficients in curvelet domain. So, we perform Canny edge detector to extracted image from strengthen curvelet coefficients
Acknowledgement
The authors would like to thank Dr Alireza Dehghani from Ophthalmology Department, Isfahan University of Medical Sciences for his contribution in data collection and evaluation of results.
Hossein Rabbani was born in Iran in 1978. He received the B.Sc. degree in Electrical Engineering (Communications) from Isfahan University of Technology, Isfahan, Iran, in 2000 with the highest honors, and the M.Sc. and Ph.D. degrees in Bioelectrical Engineering in 2002 and 2008, respectively, from Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran. From January 2007 to July 2007, he was with the Department of Electrical and Computer Engineering, Queen's University, Kingston,
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Cited by (0)
Hossein Rabbani was born in Iran in 1978. He received the B.Sc. degree in Electrical Engineering (Communications) from Isfahan University of Technology, Isfahan, Iran, in 2000 with the highest honors, and the M.Sc. and Ph.D. degrees in Bioelectrical Engineering in 2002 and 2008, respectively, from Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran. From January 2007 to July 2007, he was with the Department of Electrical and Computer Engineering, Queen's University, Kingston, ON, Canada, as a Visiting Researcher. Then he joined Department of Biomedical Engineering and Medical Image & Signal Processing Research Center, Isfahan University of Medical Sciences as an Assistant Professor. He is also currently a Postdoctoral Research Scholar at University of Iowa.
His main research interests are medical image analysis, multidimensional signal processing, multi-resolution transforms, probability models of sparse domain's coefficients and image restoration.