Automated Segmentation of Neural Canal Opening and Optic Cup in 3D Spectral Optical Coherence Tomography Volumes of the Optic Nerve Head

Glaucoma is a chronic neurodegenerative disease of the optic nerve that, if left untreated, may result in increasing visual field (VF) loss and blindness. It is characterized by the degeneration of the retinal ganglion cell axons.1,2 It has been suggested that the initial axonal insult occurs in the optic nerve head (ONH), which leads to optic disc cupping and VF loss through retinal ganglion cell apoptosis.1 The risk for VF loss from glaucoma can be minimized by early diagnosis and careful monitoring of disease progression. However, the latter requires a reproducible measurement of the disease state. Currently available methods include VF, planimetry based on stereo disc photographs, ONH tomography (Heidelberg Retina Tomograph; Heidelberg Engineering, Heidelberg, Germany), and peripapillary nerve fiber layer thickness analysis (using polarimetry or optical coherence tomography [OCT]).3–7 Many of these modalities involve a subjective component, by either the patient or the examiner, which can decrease reproducibility. Spectral domain optical coherence tomography (SD-OCT) is a relatively new modality that provides a cross-sectional, microscale depiction of the optical reflectance properties of the biological tissues.8 The high-resolution SD-OCT scans of the ONH provide a potential to identify glaucomatous changes using 3D information. However, it is unclear which ONH parameters derived from SD-OCT can be best used to quantify glaucomatous changes. We have recently described a voxel classification approach for automatically segmenting the clinically familiar glaucomatous parameters—the ONH rim and cup—directly from the SD-OCT volumes9 by extending our previous approach for automated planimetry on stereo fundus photographs.10,11 However, this approach has the ultimate limitation in that the algorithm essentially mimics the subjective assessment of 2D parameters by human experts. It is not based on objective anatomic landmarks within the 3D volumes, and we have found that the optic disc margin does not overlap with a single constant anatomic structure in volumetric OCT, consistent with the recent comparisons of clinical and SD-OCT optic disc margin anatomy by Strouthidis et al.12,13 They found that the varying combinations of the termination of Bruch's membrane (BM), border tissue, or the anterior scleral canal opening may manifest as the 2D disc margin seen on photographs, depending on the border tissue architecture and anatomy. Given the wealth of volumetric information available with SD-OCT, it is likely that better parameters can be obtained for measuring glaucomatous change that move beyond what is possible using stereo fundus photography alone. A central requirement for the detection of the ONH structural change is a longitudinally stable zero reference plane. As described by Strouthidis et al.,12,13 the neural canal opening (NCO)—the termination of the RPE/BM complex—can serve as a basis for a stable reference plane from which various optic nerve morphometric parameters can be derived, based on the assumption that the NCO is not likely to change substantially with glaucomatous progression.12,13 Thus, the core hypothesis motivating our present study is that the segmentation of a stable 3D structure, the NCO, from SD-OCT will enable more reproducible and objective glaucomatous parameters than are currently possible from manual planimetry alone (even using a consensus of glaucoma experts). This is fundamentally different from our previous segmentation method,9 which, by construction (using training data), attempts to obtain parameters as close as possible to what would be obtained using manual planimetry of stereo fundus photographs. Although the present study does not directly test this hypothesis, it sets the necessary underlying framework for the ability to automatically detect the NCO and to better understand its relationship with the widely accepted optic disc margin from manual planimetry. More specifically, the purpose of the present study was to describe an algorithm for automatic delineation of the NCO from SD-OCT volumes and optic disc metrics derived from the NCO (and cup boundaries) and to compare these with the reference standard (RS; disc and cup margins) obtained from human expert planimetry of stereo photographs of the same eye.