Retinal nerve fiber layer reflectometry must consider directional reflectance

Abstract

Recent studies reveal that measurements of retinal nerve fiber layer (RNFL) reflectance provide more sensitive detection of glaucomatous damage than RNFL thickness, but most do not consider directional reflectance of the RNFL, an important source of variability. This study quantitatively compared RNFL directional reflectance, represented by an angular spread function (ASF), measured at different scattering angles, different wavelengths and different distances from the optic nerve head (ONH) and for bundles with different thicknesses (T). An ASF was characterized by its amplitude (A) and width (W). Internal reflectance of a bundle was expressed as A/T. The study found that A varied significantly with scattering angle and wavelength and that A/T was different among bundles but constant along the same bundle, indicating that the internal structure of axons may vary among bundles but does not change with distance. This study also found that W was larger near the ONH and at longer wavelengths, but did not depend on scattering angle or T. Because a 4.3° change in incident angle can change reflected intensity by a factor of 2.7, accounting for directional reflectance should improve the accuracy and reproducibility of RNFL reflectance measurements.

Novel Technique for Quantifying Retinal Nerve Fiber Bundle Abnormality in the Temporal Raphe

“…It will be useful to investigate how the directional reflectivity of the nerve fiber bundles affect OCT enface imaging of the temporal raphe. 23 …”

Changes in Retinal Nerve Fiber Layer Reflectance Intensity as a Predictor of Functional Progression in Glaucoma

“…For example, improvements could involve removing the effects of vessel shadows, and/or taking into account the directionality of retinal reflectance. 7 , 8 , 29 Directionality is particularly important for cylindrical structures, such as axons and their aligned cytoskeletal components, 7 which our normalization procedures, therefore, do not address. Indeed, the test–retest variability of our measure shown in Figure 2 still is more than 10 times higher than the variability of RNFLT measurement.…”

“…It is well known that the reflectance of the retinal nerve fiber layer (RNFL) bundles is highly directional and spectrally dependent. 6 – 8 Loss of axons from the RNFL, especially when loss is concentrated within specific axon bundles, leads to the appearance of RNFL bundle defects, thought to be one of the earliest clinical signs of glaucoma. 9 , 10 However, it also is known that RNFL reflectance depends on the integrity of the axonal cytoskeletal ultrastructure.…”

Increased Inner Retinal Layer Reflectivity in Eyes With Acute CRVO Correlates With Worse Visual Outcomes at 12 Months

“…Also, analysis of optical intensity is subject to significant variation based on media opacity and incident light. 25,26 Finally, nonlinear postprocessing applied by commercial OCT software further prevents intersubject signal comparisons. The current study used a normalization ratio to allow for consistent optical intensity measurements through a ratio (the OIR) and used raw OCT data to bypass postprocessing images.…”