A real-time anterior segment tracking method

Abstract

Purpose : Motion artifacts pose a challenge in optical coherence tomography angiography (OCTA). While motion tracking solutions that correct these artifacts in retinal OCTA exist, the problem of motion tracking is not yet solved for the anterior segment (AS) of the eye. This currently prevents the use of AS-OCTA for diagnosis of diseases of the cornea, iris and sclera. Here we report the performance of a prototype motion tracking method for the AS.

Methods : A telecentric add-on lens assembly with internal fixation was used to enable imaging of the AS with CIRRUS™ 6000 AngioPlex (ZEISS, Dublin, CA) with good patient alignment and fixation (fx). Using this add-on lens on CIRRUS 6000, widefield (20x14 mm) line scanning ophthalmoscope (LSO) image sets were taken of 25 eyes from 15 subjects – 6973 images in total (4798 central & 2175 peripheral fixation). Motion in these image sets was then tracked with an algorithm using real-time landmark-based rigid registration between a reference image and the other (moving) images from the same set (see Figure 1). The algorithm first detects an anchor point in an area of the reference image with high texture values. This anchor point is then located in the moving image by searching for a template (image region) centered at the reference image anchor point position. Next, landmarks from the reference image are found in the moving image by searching for landmark templates at the same distance to the anchor point as in the reference image. Finally, translation and rotation are calculated using the landmark pairs with the highest confidence values. The registration error is the mean distance between corresponding landmarks in both images. This value is calculated after visual confirmation of landmark matches and successful registration.

Results : Figure 2 shows the tracking performance for all 25 eyes. The mean registration error and rotation angle are similar for straight and peripheral fixation (both approx. 30µm and 0°). However, the standard deviation for the registration error is greater for peripheral fixation. Subject eye motion with peripheral fixation appears more pronounced than with straight fixation. The average tracking execution time was 9 ms using an i7-8850H CPU and 32 GB RAM.

Conclusions : We proposed an anterior segment tracking method that can potentially be used for OCTA acquisition. It is sufficiently fast and accurate to track AS movement for OCT scans with 30 µm A-scan spacing.

This is a 2020 Imaging in the Eye Conference abstract.