Abstract
BACKGROUND: Static vessel analysis is a method to determine the diameter of
retinal vessels in images of the ocular fundus. The suitability of non-mydriatic
and mydriatic images for that method and the influence of mydriasis on the
results were examined. MATERIALS AND METHODS: In the prospective study, 30 eyes
of 15 patients (10 women, mean age 51.6 +/- 13.2 years) were examined. At first,
3 images were taken of each eye with the retinal camera Topcon NW 200
(magnification 1). After pupil dilation with tropicamid eye drops, 3 more images
were taken using the Topcon and 3 others using the system Visualis (IMEDOS,
Jena/Germany, FF450plus, 535-561 nm, 30 degrees image, 1840 x 1360 pixel). The
vessel diameters were measured with the software Vesselmap2 (IMEDOS). The
investigator assigned vessels to arteries or veins and their diameters were
calculated automatically by the software. There is the possibility to define the
vessel edge manually in cases of a poor image quality. The calculation of the
central retinal arterial and venous equivalent (CRAE, CRVE) as well as the
arterio-venous ratio (AVR) were made according to the formula of Parr-Hubbard.
Furthermore, the nasal retinal vessels > 60 microm were examined to estimate the
influence of tropicamid on the vessel diameter. RESULTS: Because of insufficient
illumination and poor contrast in 21 \% of the non-mydriatic images not all
vessels could be detected automatically. Additionally, fewer vessels could be
detected in 7 \% of the non-mydriatic images compared to the mydriatic images. The
average coefficient of variation of CRAE and AVR of each triplet of images was
higher in non-mydriatic images (2.6 \% and 3.2 \%, respectively) than in mydriatic
images of the Topcon (1.8 \%; 2.3 \%) and the FF 450 (1.7 \%; 1.8 \% ANOVA p < 0.05).
No significant differences were found between the various examination methods for
both the coefficient of variation of CRVE (1.9 \%; 1.8 \%; 1.7 \%) and the average
values of CRAE, CRVE and AVR. With regard to their diameters, the nasal retinal
arteries and veins > 60 microm, were depicted sufficiently in all images, and
only differed insignificantly between the three methods. CONCLUSION: The quality
of non-mydriatic images is often lower than that of mydriatic images. This fact
can account for the high variance of measured parameters in the non-mydriatic
images. The depiction of all relevant vessel segments is a precondition for the
image-based analysis. An influence of the mydriasis caused by tropicamid on the
retinal vessel diameters > 60 microm was not found.
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