Quantitative analysis of anterior segment optical coherence tomography images: the Zhongshan Angle Assessment Program

To describe a new image analysis method and software for anterior chamber images obtained by the anterior-segment optical coherence tomography (AS-OCT) and to assess its intraobserver and interobserver measurement reproducibility.

Twenty 8-bit greyscale 600x300 AS-OCT images with apparent wide angles and 20 images with apparent narrow angles were consecutively selected from a database. Two glaucoma fellowship-trained ophthalmologists used proprietary image analysis software to analyse the images twice. Algorithms defined the borders and curvatures of anterior chamber (AC) structures and measured AC parameters using scleral spur location as the only observer input. The intraobserver and interobserver reproducibility of scleral spur location and angle parameters was calculated in terms of limits of agreement (LOA; mean of differences+/-1.96 SD of differences) and coefficient of variation (CV; SD of differences/overall mean).

The analysis software successfully measured all parameters in all images. When the same image was assessed twice by the same grader, the mean differences ranged from 0 to 0.010 mm in linear measurements and 0.001 to 0.006 mm2 in angle area measurements. LOA tended to be greater in the wider angles. The upper and lower limit values of LOAs were approximately 1/5 to 1/4 of the overall mean. Measurements between two graders had a higher variance. Reproducibility in terms of CV was better in wide angles when compared with narrow angles. The reproducibility of scleral spur placement between observers was poorer in narrow angles (p = 0.001). About 50% of the interobserver variance in angle-area measurements was attributable to the variance of scleral spur placement, while this proportion was only 10-20% in linear measurements.

Determination of angle parameters using semiautomated software leads to variability in measurement. Variability increases when more than one observer identifies the scleral spur. Variability differs in narrow and open angles, and so including both types is essential when evaluating angle-assessment software. A fully automated analysis and higher image resolution would likely improve quantification of Visante AS-OCT images.