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W. Gao, Y. Zhang, B. Cense, R.S. Jonnal, J. Rha, D.T. Miller; Measuring Retinal Contributions To The Optical Stiles–Crawford Effect With Optical Coherence Tomography . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2665.
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The directional component of the retinal reflection, i.e., the optical Stiles–Crawford effect (SCE), is well established to result from the waveguiding property of photoreceptors. Considerable uncertainty, however, remains as to which retinal reflections are waveguided and thus contribute to the SCE. To this end we have developed a retina camera based on spectral–domain optical coherence tomography (SD–OCT) that axially resolves these reflections and permits a direct investigation of the SCE origin.
A fiber–based SD–OCT system was developed having an axial resolution in retinal tissue of <6 µm and a sensitivity up to 94 dB. The illumination source was a superluminescent diode (=840 nm), and the detector was a linescan camera that acquired A–scans at up to 20,000 per second. A bite bar and forehead rest stabilized the subject’s head. Subjects were dilated. Illumination and imaging of the retina were realized through the same 2mm aperture that was systematically translated across the subject’s pupil. B–scans of the same patch of retina were analyzed as a function of aperture location. Speckle noise was reduced by averaging adjacent A–scans.
B–scans obtained on several subjects were of sufficient axial resolution and sensitivity to permit analysis of the primary retinal reflections, including those from within the photoreceptor, retinal pigment epithelium, and choroids layers. Reflections from the photoreceptor inner/outer segment junction and near the posterior to the outer segment were found highly sensitive to the aperture location with a decrease in brightness up to several times occurring with an increase in aperture eccentricity. Brightness of the other reflecting layers was insensitive to aperture location.
OCT represents a new approach for studying the optical SCE. Its axial resolution and sensitivity greatly surpass those of established methods. Using SD–OCT, the primary SCE contributors at near infrared wavelengths are a sharp reflection at the photoreceptor inner/outer segment junction and a more diffuse reflection occurring near the posterior of the outer segment.
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