Abstract
Purpose: :
High resolution imaging of the microscopic retina has garnered interest as a highly sensitive and noninvasive approach for measuring cell structure and function, both critical for assessing cell health. Of particular importance is the ability to image the thick retina in all three spatial dimensions and time. The purpose of this study is to demonstrate the feasibility of using ultrahigh-resolution adaptive optics optical coherence tomography (UHR-AO-OCT) for imaging a single type of retinal cell - cone photoreceptors - in all four dimensions.
Methods: :
We used an UHR-AO-OCT system with a Superlum BroadLighter (T-840-HP, c = 840 nm, Δ = 115 nm) light source that is described in detail elsewhere (Cense et al. Opt. Express Vol. 17, 4095-4111, 2009). Key technological improvement not previously reported was integration of a Basler Sprint camera for substantially higher image acquisition (125,000 A-scans/s). 0.5°x0.5° volume videos were acquired at a 2 Hz volume rate on healthy subjects (papillomacular region). AO dynamically compensated for the ocular aberrations and positioned focus at the photoreceptor layer. Most volumes were acquired at 3° and 6° retinal eccentricities. En face projections of the outer retina were used for cone identification. Cones were tracked from frame to frame in the en face projection videos as well as B-scan videos.
Results: :
The 3D morphology of many cones could be tracked over time at both 3° and 6° retinal eccentricities. Identification of cones was primarily impeded by eye motion, which generated noticeable image warp and shift within and between volumes, respectively. The length (B-scans) and width (en face projections) of individual cone outer segments were found highly consistent across volumes. Repeatability of outer segment lengths of 3 cones randomly selected at each eccentricity were 23.9±0, 27.2±0.5, 26.5±0.9 µm at 3° and 25.9±0.5, 24.1±0.5, 25.2±0.7 µm at 6°.
Conclusions: :
Imaging individual cone photoreceptors of normal subjects in 4D is possible using UHR-AO-OCT. Quick acquisition of volumes is critical for minimizing motion artifacts at the level of cones.
Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • photoreceptors • retina