Detecting optic disc on asians by multiscale gaussian filtering

Abstract

The optic disc (OD) is an important anatomical feature in retinal images, and its detection is vital for developing automated screening programs. Currently, there is no algorithm designed to automatically detect the OD in fundus images captured from Asians which are larger and have thicker vessels compared to Caucasians. In this paper, we propose such a method to complement current algorithms using two steps: OD vessel candidate detection and OD vessel candidate matching. The first step is achieved with multiscale Gaussian filtering, scale production, and double thresholding to initially extract the vessels' directional map of various thicknesses. The map is then thinned before another threshold is applied to remove pixels with low intensities. This result forms the OD vessel candidates. In the second step, a Vessels' Directional Matched Filter (VDMF) of various dimensions is applied to the candidates to be matched, and the pixel with the smallest difference designated the OD center. We tested the proposed method on a new database consisting of 402 images from a diabetic retinopathy (DR) screening programme consisting of Asians. The OD center was successfully detected with an accuracy of 99.25% (399/402).

Figure 1

Comparison of Caucasian retina (a) to Asian (b).

Figure 2

Thick (a) and (b); thin (c) and (d) vessel cross-sections. The solid line is Asian while the dotted comes from Caucasian.

Figure 3

(a), (c), and (e) Normal fundus image from HIT. (b), (d), and (f) Fundus diagnosed with moderate NPDR from HIT.

Figure 4

Steps of the proposed method shown in rectangles and its control flow given by the arrow.

Figure 5

Steps of the proposed method applied to a fundus image (a). (b) is the scale production of (a). The result of double thresholding on (b) is (c). The thinned vessels of (c) is (d). (e) is the OD vessel candidates after removing pixels with low intensities. The detected OD center is illustrated in (f) by a cross.

Figure 6

A vessels' directional matched filter designed for HIT.

Figure 7

Results of the proposed method where a white cross represents the detected OD center.

Figure 8

A small vessel map is covered with the 61 × 21 template (a). Medium-to-large maps are covered using the 121 × 41 template (b).

Figure 9

The result of average distance compared with various scale factors applied to the templates. From right to left, the first point is +1.5, next is the current template size, and the left point is −1.5.

Figure 10

Visual comparison of the OD detection results (proposed method with others).