Effect of image quality, color, and format on the measurement of retinal vascular fractal dimension.
OBJECTIVE
Fractal dimension of retinal vasculature is a global summary measure of retinal vascular network pattern and geometry. This study was conducted to examine the effect of variations in image color, brightness, focus, contrast, and format on the measurement of retinal vascular fractal dimension.


METHODS
A set of 30 retinal images from the Blue Mountains Eye Study was used for a series of experiments by varying brightness, focus (blur), contrast, and color (color versus monochrome). The original and the modified images were graded for fractal dimension (D(f)) using dedicated retinal imaging software (IRIS-Fractal). A further set of 20 grayscale images was used to compare image format (.jpg versus .tif) with regard to the resultant D(f) and processing time.


RESULTS
The mean D(f) of original images in this sample was 1.454. Compared with the original set of images, variations in brightness, focus, contrast, and color affected the measurements to a small to moderate degree (Pearson correlation coefficient, r, ranged from 0.47 to 0.97). Very dark or blurry images resulted in a substantially lower estimate of D(f). Monochrome images were also consistently associated with lower D(f) compared with that obtained from color images. Using .jpg or .tif image formats did not affect the measurement or the time needed to process and measure D(f).


CONCLUSIONS
Variations in image brightness, focus, and contrast can significantly affect the measurement of retinal vascular fractals. Standardization of image parameters and consistent use of either monochrome or color images would reduce measurement noise and enhance the comparability of the results.
Wainwright
Alan
A
Department of Ophthalmology, Centre for Vision Research, Westmead Millennium Institute, University of Sydney, Sydney, Australia;
Liew
Gerald
G
Burlutsky
George
G
Rochtchina
Elena
E
Zhang
Yong Ping
YP
Hsu
Wynne
W
Lee
Janice MongLi
JM
Wong
Tien Yin
TY
Mitchell
Paul
P
Wang
Jie Jin
JJ
D003116:Color; D017709:Fractals; D006801:Humans; D007089:Image Enhancement; D007091:Image Processing, Computer-Assisted / Q000592:standards*; D010781:Photography; D015203:Reproducibility of Results; D012171:Retinal Vessels / Q000033:anatomy & histology*