Abstract
Purpose :
SS-OCT anterior segment scans may create images which do not accurately represent the anatomy due to beam geometry and refraction through the cornea. Dewarping is important to reduce errors in irido-corneal angle measurements for accurate diagnosis of narrow angle glaucoma. The purpose of this study is to determine the error induced by warping (and corrected by dewarping) in cube scans.
Methods :
The field of view (FOV), equal optical path length deviation called sagittal heights (SAG), and anterior surface refractive correction are needed to dewarp images in a cube scan.
A flat checkerboard test eye was scanned at three axial positions (top, middle, bottom) to find the warping due to non-uniform beam geometry. This was used to calculate the mentioned dewarping parameters for five fast B-scans (at 1 to 5 mm in slow scan direction) to interpolate for all B-scans in the cube.
5 subjects (one or both eyes, multiple scans) were imaged using the PLEX® Elite 9000 SS-OCT system (ZEISS, Dublin, CA) with a front mounted external lens to focus the OCT beam on the irido-corneal angle with a FOV of 6x6x6 mm cube scans (3072x500x500 pixels corresponding to axial x fast scan direction x slow scan direction). The anterior and posterior sclera, cornea, and iris surface segmentations were used to account for changes in the refractive indices.
An expert grader manually measured the irido-corneal angle on three images at 1.5 mm, 3 mm, 4.5 mm extracted from the cube scan before and after dewarping using the protractor tool on ImageJ. The regression and mean difference plots of the measurement as well as statistics of the angle measurement errors are reported.
Results :
Fig 1 shows an example of the iridocorneal angle image before and after dewarping. The mean and standard deviation of the absolute measurement error are 4.9° and 2.4° respectively. The regression and mean difference plots (Fig 2) show that while correlation remains high (0.91), a mean difference of -4.8° was observed.
Conclusions :
Correctly dewarped images are critical to having more accurate irido-angle measurements for anterior segment biometry. When not corrected, image warping can lead to significant over-estimation of irido-corneal angle measurements, which is important for glaucoma diagnosis.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.