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B. Cense, R. S. Jonnal, J. Brown, W. Gao, D. T. Miller; Functional Imaging of Cone Photoreceptors Using Ultrahigh Resolution Optical Coherence Tomography and Adaptive Optics. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4511. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Visible flash stimulation of the retina has been reported to trigger fast temporal variations in the reflectance pattern ("scintillation") of individual cone photoreceptors, as for example observed with an adaptive optics (AO) flood illumination camera (1). Cone "scintillation" has been hypothesized to originate from interference between reflecting surfaces that straddle the photoreceptor outer segment in conjunction with optical path length (OPL) changes that occur in the outer segment. The scintillation duration and pattern suggests an OPL change of 2 µm. To test this hypothesis, changes in OPL were directly monitored before and after flash stimulation using an ultra-high resolution optical coherence tomography (OCT) camera equipped with AO. Unlike prior cameras used to report scintillation, AO-OCT permits resolving the cone outer segments in all three dimensions with micron resolution.
The ultra-high resolution OCT system consisted of a Superlum BroadLighter (T-840-HP, c = 840 nm, Δ = 115 nm), a custom achromatizing lens for compensating the ocular chromatic aberrations across the BroadLighter spectrum, and a 2048 pixel linescan detector in a spectral-domain OCT configuration. The OCT system acquired up to 30,000 A-scans/s. The AO consisted of a Shack-Hartmann wavefront sensor, a 37-element AOptix deformable mirror and a 144-element BMC deformable mirror in a woofer-tweeter configuration. Volume scans up to 3° by 3° were acquired through a 6.6 mm pupil, with dynamic AO compensation, and of retinal tissue near the fovea. Bleaching of cone photoreceptors was realized with a single 400 µs flash from a xenon flash lamp.
Change in the outer segment OPL, as measured with the AO-OCT camera, was found smaller than the coherence length (~2.5 µm) of the OCT light source. The position of reflective layers could be detected more accurately than the coherence length, however, through statistical analysis of the acquired B-scans. Such analysis revealed a small, but significant change in OPL.
Optical path length differences occurring during cone "scintillation" were observed with ultra-high resolution OCT equipped with AO.References: 1. R. S. Jonnal et al., Optics Express 15, 19 (2007).
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