Comparison of Stereo Disc Photographs and Alternation Flicker Using a Novel Matching Technology for Detecting Glaucoma Progression

Introduction

Glaucomatous optic neuropathy is characterized by progressive visual field loss and optic nerve change over time. Detection of optic nerve change is essential to the monitoring of disease progression.¹ Because structural damage to the neuroretinal rim and retinal nerve fiber layer may be detected prior to visual field changes in eyes with glaucoma, the side-by-side comparison of serial sets of stereophotographs by an experienced observer represents the gold standard for determining progression. ^2--4 Furthermore, review of stereophotography has been validated for use as a reference standard in evaluating new optic nerve imaging technology.⁵

Alternation flicker is a technique in which serial optic nerve photographs are aligned and then alternated to allow detection of structural optic nerve change over time. This may be achieved with manual alignment of images (eg, with overlapping slide projectors), which is somewhat time-consuming but is effective. ^6,7 A computerized alternation flicker technique has been developed in which six corresponding points where vessels cross the optic nerve on each image are manually registered and the images are aligned along these points.⁸ This technique performs similarly to side-by-side viewing of stereo disc photographs. Although computerized alternation flicker benefits from improved superimposition compared to manually aligned images, manual registration of image points is still required, which is time-consuming and could potentially lead to bias or registration error.

We evaluated a new automated alternation flicker algorithm that automatically registers and matches serial photographs to the sub-pixel level (Fig. 1). This algorithm identifies features (eg, intersections of retinal or choroidal vessels) in each photograph and aligns the photographs using any digitally identifiable peripheral features, not limited to the crossing of vessels. The algorithm applies global transforms to the images until the best match is achieved between the two images. Color and illumination are not altered by the algorithm. Automated registration has been validated in the field of radiology and is more accurate, more repeatable, and freer from investigator bias or error than methods that depend on manual registration. ^9,10

The goal of this investigation was to compare agreement of the flicker technology and stereophotograph inspection for detection of progressive glaucomatous optic neuropathy.

Patients and Methods

Approval for this study was obtained from the Institutional Review Board at the New York Eye...