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Omer P. Kocaoglu, Ravi S. Jonnal, Sangyeol Lee, Wang Qiang, Ashley E. Herde, Donald T. Miller; Variation Of Cone Outer Segment Length And Reflectance Measured Over Days With Adaptive Optics Optical Coherence Tomography. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3198.
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The processes of phototransduction and disc renewal cause significant physiological and structural changes in the cone photoreceptor. Some of these changes are known to have optical correlates, allowing them to be probed using non-invasive optical techniques. The purpose of this study is to determine whether cones naturally undergo reflectance and length changes in their outer segment (OS) over the course of days. The experiment takes advantage of the 3D micron-scale resolution of adaptive optics optical coherence tomography (AO-OCT).
The Indiana AO-OCT used an Integral Femtolaser (bandpassed: c= 809 nm, Δ= 81 nm) and a high-speed Basler linescan detector (167 A-scans/s). The woofer-tweeter AO system dynamically compensated for ocular aberrations and provided focus. The system acquired 0.6°x0.6° volume videos (four volumes/s) at retinal eccentricities from 0.5° to 6° (temporal). Videos were acquired every two to three days for ten days on subjects free of ocular disease and with normal visual acuity. Volume registration removed motion artifacts, and individual cones were identified and tracked over the ten days. For each tracked cone, its outer segment (OS) length and reflectance of the connecting cilium (CC) and posterior OS tip were measured.
Many cones were successfully tracked over the ten days despite the small field of view and the +/-15 µm root mean square retina motion prior to registration. Registration reduced the motion to +/-1 µm, which is a fraction of a cone width. OS length was found unstable, varying by several microns over the ten days. A representative cone had lengths of 28.9, 26.4, 26.4, 30.6, and 28.1 µm for days 1,3,6,8, and 10. Reflectance of CC and OS tips (normalized to the total cone reflectance) was also unstable. A typical cone had CC reflectance of 54±4, 47±4, 48±6, 50±5, and 56±1% and OS tip reflectance of 46±4, 53±4, 52±6, 50±5, and 44±1%. For some cones, the OS tip reflection disappeared for a day and reappeared the day after.
3D imaging of the same cones over days reveals variation in OS length and reflectance. Variation may be partially attributed to disc renewal and shedding.
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