Automatic identification of the temporal retinal nerve fiber raphe from macular cube data

Abstract

Abstract:We evaluated several approaches for automatic location of the temporal nerve fiber raphe from standard macular cubes acquired on a Heidelberg Spectralis OCT. Macular cubes with B-scan separation of 96-122 µm were acquired from 15 healthy participants, and "high density" cubes with scan separation of 11 µm were acquired from the same eyes. These latter scans were assigned to experienced graders for subjective location of the raphe, providing the ground truth by which to compare methods operating on the lower density data. A variety of OCT scan parameters and image processing strategies were trialed. Vertically oriented scans, purposeful misalignment of the pupil to avoid reflective artifacts, and the use of intensity as opposed to thickness of the nerve fiber layer were all critical to minimize error. The best performing approach "cFan" involved projection of a fan of lines from each of several locations across the foveal pit; in each fan the line of least average intensity was identified. The centroid of the crossing points of these lines provided the raphe orientation with an average error of 1.5° (max = 4.1°) relative to the human graders. The disc-fovea-raphe angle was 172.4 ± 2.3° (range = 168.5-176.2°), which agrees well with other published estimates.

Geometry of the Retinal Nerve Fibers From Emmetropia Through to High Myopia at Both the Temporal Raphe and Optic Nerve

“…OCT data was acquired with a Heidelberg SPECTRALIS with the Glaucoma Module (Heidelberg Engineering GmBH, Heidelberg, Germany). Following our previously published method, 13 the temporal raphe was assessed in the right eye from 2 partially overlapping, vertical, high density cube scans (11 lm separation, 108 width 3 158 height). Fixation was guided by the internal spot within the device, and the acquisition region was finetuned relative to the spot by moving the position of the acquisition window in the Spectralis software (Heidelberg).…”

“…9 Recent developments allow direct visualization of the temporal raphe, 10,11 including widely available clinical instrumentation such as spectral domain optical coherence tomography (OCT), 12 with imaging settings used routinely in the clinic. 13 In some individuals, there is significant departure from an assumed horizontal raphe. [10][11][12]14 Clinically, the raphe position is relevant to the combined assessment of functional (e.g., visual field) and structural (e.g., OCT) data.…”

“…18 Hence, the anatomy of the temporal raphe may be relevant to improving the assessment of glaucoma. 11,13,14,[18][19][20][21][22] Furthermore, the low density of RNFL bundles in this area may enable direct visualization of changes to bundle density. Glaucomatous deficits occur characteristically in this region, which corresponds to the nasal step of the visual field.…”

The value of visual field testing in the era of advanced imaging: clinical and psychophysical perspectives

“…Methods of measuring retinal ganglion cell counts in humans in vivo have also been equivocal, showing large variation particularly in normal observers. For example, the position of the retinal nerve fibre layer raphe varies across individuals, which, in conjunction with relatively sparse visual field test grids, confound concordance of measurements . Individualised structure‐function mapping has been shown to be useful in improving structure‐function correlations but there still exists a range of variables, such as atypical disc appearances that are not accounted for by simple transposition of the OCT map .…”

“…For example, the position of the retinal nerve fibre layer raphe varies across individuals, which, in conjunction with relatively sparse visual field test grids, confound concordance of measurements. 176 Individualised structurefunction mapping has been shown to be useful in improving structure-function correlations but there still exists a range of variables, such as atypical disc appearances that are not accounted for by simple transposition of the OCT map. 126,177 It is possible that a combination of customised testing paradigms is required for optimal structure and function measurements for disease detection and monitoring for individual patients.…”