Geometrical Markers for Glaucoma Diagnosis Enabled by Three-Dimensional Minimum Distance Mapping of Volumetric Optical Coherence Tomography

Abstract

Purpose: : To demonstrate the potential of optical coherence tomography (OCT) based volumetric tissue analysis via retinal layer minimum distance mapping (MDM) for improved glaucoma diagnosis and to examine an associated novel geometrical marker for quantification of neural loss at the optic nerve head (ONH).

Methods: : Cup to disk ratio (CDR) evaluation is the commonly used procedure for glaucoma diagnosis. OCT gives access to the 3D topology at the ONH and allows extraction of the associated absolute 3D neural thickness. From 3D OCT data a segmentation procedure extracts the retinal pigment epithelium (RPE) as well as the nerve fibre layer (NFL) boundaries, inner limiting membrane (ILM) and NFL-Ganglion cell layer interface. The 3D information formed by these layers is used to calculate not only the already known layer thickness maps but also novel MDMs representing the true Euclidean minimum distance between layers. From the MDM between RPE and ILM (retinal MDM) the minimum circumpapillary band (MCB) around the scleral rim (SR) can be determined and the MCB area normalized by the SR area is examined for quantification of neural loss.

Results: : Comparison of the novel colored MDMs with thickness maps allows better visual judgment of retinal topologies. Especially the retinal MDM shows a clearly visible ring of constriction around the SR for advanced glaucoma which was previously not visible on retinal thickness maps. This constriction was not found in normals and in contrast the intermediate stages already show some narrowing, again previously not visible on thickness maps. Whereas the SR area varies strongly between subjects (1.1 - 2.1 mm^2) the MCB area was found divergent between normals (> 1 mm^2) and glaucomas (< 1 mm^2). The relative geometrical marker based on the quantified areas for normals (~20%), intermediate glaucoma (~ -10%) and glaucoma (~ -60%) permits objective check of neural loss associated with glaucoma for the first time.

Conclusions: : True 3D examination of layer distances which is possible on 3D OCT topology data enables determination of MDMs and calculation of the MCB and SR area. Neural loss at the ONH contributed by thinning of the MCB could be quantified even for exceptional cup or rim forms and our results on the associated geometrical marker are found to be pathognomonic.

Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • image processing • imaging/image analysis: non-clinical