Digital image analysis in retinopathy of prematurity: a comparison of vessel selection methods.

PURPOSE: To evaluate vessel selection methods to distinguish between eyes with and without retinopathy of prematurity (ROP) and between different stages of ROP when quantifying the associated vessel changes in width and tortuosity semiautomatically from digital retinal images. METHODS: Color digital images from 75 infants screened for ROP were cropped to a standardized diameter of 240 pixels and evaluated by semiautomated vessel analysis software, Computer-Aided Image Analysis of the Retina (CAIAR), to measure retinal vessel width and tortuosity. Two methods of vessel selection were used: (1) clinical observer selecting the most prominent arteriole or venule in each retinal quadrant (4-vessel analysis) and then separately the 4 most prominent arterioles and venules from each quadrant (8-vessel analysis); (2) CAIAR selecting, regardless of retinal quadrant, the 4 widest or most tortuous arterioles or venules. Selected vessels were measured by CAIAR for tortuosity and width. RESULTS: When comparing ROP stages, whether observer or CAIAR selected and whether 4 or 8 vessels were analyzed, we found that arteriolar tortuosity was significantly greater with advancing ROP stage for stage 0 versus stage 2; stage 0 or 1 versus stage 3; stages 1+2 combined versus stage 3; and stage 0 versus 1+2+3 combined (P < 0.01). Venular tortuosity was significantly greater with advancing ROP stage for stage 0 versus stage 3 and stage 0 versus stages 1 and 2+3 combined (P < 0.01). Width parameters did not help us to distinguish between stages. CONCLUSIONS: Distinguishing between arterioles and venules is not necessary to differentiate stage 0 ROP from stage 2 or 3 ROP when one is using CAIAR. Tortuosity shows more promise than width at providing a reliable vessel parameter for distinguishing between eyes without and with ROP.

Comparison of expert graders to computer-assisted image analysis of the retina in retinopathy of prematurity.

AIMS: To determine correlation of width and tortuosity between expert graders and computer-assisted image analysis of the retina in narrow-field images of eyes with retinopathy of prematurity. METHODS: 11 digital images were selected based on severity of retinopathy of prematurity (ROP). Narrow field images were analysed for width and tortuosity of vessels using computer-aided image analysis of the retina (CAIAR), an image analysis software, and by four ROP experts. Spearman correlation coefficients (ρ) assessed the correlation of CAIAR grading with expert grading. Intra-class correlations assessed agreement among graders. Width and tortuosity were compared among severity of ROP and treatment status using analysis of variance and generalised estimating equations. RESULTS: Expert measurements correlated well with measures from CAIAR for venule width (ρ=0.57-0.66) and arteriole tortuosity (ρ=0.71-0.81). Measurements from four graders agreed moderately well (intra-class correlations were 0.49 and 0.69 for venule width and arteriole tortuosity, respectively). Increased severity of ROP (no pre-plus/plus, pre-plus, plus) was associated with larger width (linear trend p=0.02 in two graders) and tortuosity (linear trend p<0.03 in all graders). Tortuosity measurements by CAIAR and graders were statistically different between treated and untreated eyes (p<0.002). CONCLUSIONS: We found moderate correlation between expert graders' assessment of vessel tortuosity and width and CAIAR using narrow-field images.

Computerized analysis of retinal vessel width and tortuosity in premature infants.

PURPOSE: To determine, with novel software, the feasibility of measuring the tortuosity and width of retinal veins and arteries from digital retinal images of infants at risk of retinopathy of prematurity (ROP). METHODS: The Computer-Aided Image Analysis of the Retina (CAIAR) program was developed to enable semiautomatic detection of retinal vasculature and measurement of vessel tortuosity and width from digital images. CAIAR was tested for accuracy and reproducibility of tortuosity and width measurements by using computer-generated vessel-like lines of known frequency, amplitude, and width. CAIAR was then tested by using clinical digital retinal images for correlation of vessel tortuosity and width readings compared with expert ophthalmologist grading. RESULTS: When applied to 16 computer-generated sinusoidal vessels, the tortuosity measured by CAIAR correlated very well with the known values. Width measures also increased as expected. When the CAIAR readings were compared with five expert ophthalmologists' grading of 75 vessels on 10 retinal images, moderate correlation was found in 10 of the 14 tortuosity output calculations (Spearman rho = 0.618-0.673). Width was less well correlated (rho = 0.415). CONCLUSIONS: The measures of tortuosity and width in CAIAR were validated using sequential model vessel analysis. On comparison of CAIAR output with assessments made by expert ophthalmologists, CAIAR correlates moderately with tortuosity grades, but less well with width grades. CAIAR offers the opportunity to develop an automated image analysis system for detecting the vascular changes at the posterior pole, which are becoming increasingly important in diagnosing treatable ROP.

Nasotemporal asymmetry of retinopathy of prematurity.

OBJECTIVE: To quantify an apparent nasotemporal asymmetry in the location of retinopathy of prematurity with respect to the optic disc. METHODS: Twenty-four-bit color images were captured using a contact digital fundus camera during routine screening. Semiautomated measurements were undertaken to determine the distance between the optic disc and retinopathy located in the nasal and temporal regions of the retina. RESULTS: Forty-nine image pairs (17 right eye, 32 left eye) were captured from 10 infants during a period of 32 to 40 weeks postmenstrual age. For right eyes, averaged across age, the mean (SD) distance between the optic disc and temporal retinopathy was 426 (26) pixels and that between the optic disc and nasal retinopathy was 330 (26) pixels. Corresponding measurements for the left eye were 428 (30) and 332 (24) pixels. This observed asymmetry was found to be statistically significant in both left and right eyes (Mann-Whitney U test, P<.01). While the distance between the optic disc and retinopathy increased with age by 10 to 17 pixels per week, the extent of the asymmetry did not vary systematically with age. CONCLUSION: The location of retinopathy of prematurity is asymmetric along the horizontal meridian with respect to the optic disc-an observation germane to retinal vascular development, the pathogenesis of retinopathy of prematurity, and current disease classification by circular (symmetric) zones.