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This paper presents a modified Canny edge detector to detect retinal blood vessels especially small vessels. The detector is designed as a local dynamic hysteresis thresholding value generator based on Canny edge detector. It adapts knowledge of the location of major vessels to define a small neighborhood and generate the local hysteresis threshold values to detect meaningful edges especially the edges of small blood vessels that may be missing using Canny edge detector alone. The effectiveness of the modified Canny edge detector is demonstrated by the preliminary experimental results obtained with the proposed method. A comparative test is also presented to highlight the performance differences between the modified Canny edge detector with origin Canny edge detector.