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O. P. Kocaoglu, B. Cense, Q. Wang, R. S. Jonnal, W. Gao, D. T. Miller; Imaging Retinal Nerve Fiber Bundles Using Optical Coherence Tomography With Adaptive Optics. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4772.
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© ARVO (1962-2015); The Authors (2016-present)
Early detection of glaucoma is crucial for managing the progression of visual field loss. Glaucoma can be characterized by loss of axonal tissue in the retinal nerve fiber layer (RNFL). While a variety of techniques are available to characterize RNFL structure (e.g., thickness with optical coherence tomography (OCT)), none provide a description of the cross sectional footprint of individual nerve fiber bundles as they traverse the retinal surface. A description at this microscopic level may provide fundamental insight into differences between normal and glaucomatous axonal tissue. The purpose of this study is to demonstrate whether such imaging of the RNFL bundles in normal eyes is possible using OCT with adaptive optics (AO-OCT), a technology that provides unprecedented 3D resolution and sensitivity.
We used an ultra-high resolution AO-OCT system with two different light sources: a Superlum BroadLighter (T-840-HP, c = 840 nm, Δ = 115 nm) and a Femtosecond Integral (c = 825 nm, Δ = 120 nm). The AO-OCT system - without ultra-high resolution - was described in detail previously (Zhang, et al. Opt. Express 14, 4380-94, 2006). Volume scans of up to 3° by 3° were acquired on healthy subjects through a 6.0 mm pupil and with dynamic AO compensation. Scans were taken at retinal locations near the fovea and in the papillomacular area. Particular attention was given to 3° and 6° retinal eccentricities. Focus was optimized for visualizing the RNFL and maximizing its signal.
In one subject, individual nerve fiber bundles were clearly distinguishable at the two 6 degree retinal eccentricities (superior and inferior) examined, observable in both cross-section (XZ) and en face (XY) views. The bundles were approximately 40 µm in both width and thickness. In another subject and at the same eccentricity, the bundles were noticeably less clear, being distinguishable in en face, but not in cross section, perhaps because of smaller gaps between adjacent bundles. At 3° nasal to the fovea, bundles become noticeably separated and flat (40 µm wide; 10 µm thick). Close to the optic nerve head, individual nerve fiber bundles could not be distinguished.
Imaging the cross-sectional footprint of individual nerve fiber bundles is possible using AO-OCT, but bundle clarity varies with subject and retinal location. We speculate that bundle clarity is strongly influenced by the gap size between adjacent bundles, more so than the actual size of the bundles themselves.
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