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D. T. Miller, B. Cense, Y. Zhang, W. Gao, J. Jiang, A. Cable; Retinal Imaging at 850 nm With Swept Source Optical Coherence Tomography and Adaptive Optics. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2769.
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Adaptive optics (AO) cameras equipped with time-domain and spectral-domain optical coherence tomography (SD-OCT) have achieved unprecedented 3D resolution imaging in the living human retina. SD-OCT has recently commanded more attention owing to its substantially higher acquisition speed without loss in sensitivity. However new OCT architectures, in particular swept source OCT, may offer additional benefits that are attractive for retinal imaging. To this end AO swept source OCT was evaluated and compared to AO SD-OCT.
The Indiana AO SD-OCT camera is described in detail elsewhere . In short, the AO consisted of a Shack-Hartmann wavefront sensor and an AOptix mirror that dynamically corrected the ocular aberrations over a 6.6 mm pupil and at up to 25 Hz. The large stroke of the AOptix mirror permitted quick focusing in the retina. The SD-OCT consisted of a SLD (=840 nm, Δ=50 nm) that delivered 300 µW into the eye, a 512 linescan camera that acquired up to 75,000 A-scans/s, and custom OCT software. Conversion of the AO SD-OCT to AO swept source OCT involved swapping the SLD with a prototype commercial swept source (Thorlabs, Inc.) (=850 nm, Δ =35 nm) that acquired 20,000 A-scans/s. Average power delivered to the eye was 300 µW. Balanced detection was realized with an optical circulator. Thorlabs’ software provided 32 Hz real time display and hard drive storage of B-scans. Volume and B-scan images up to 1° in diameter were acquired at retinal eccentricities of 2° and 7° on two subjects with focus ranging from the nerve fiber layer to the photoreceptors. Axial resolution and sensitivity measurements were performed on a model eye.
Images at 7° revealed individual cone photoreceptors and nerve fiber bundles that approached the quality of that collected with the AO SD-OCT. Multiple reflections from the outer segment tips were more difficult to distinguish owing to the reduced resolution of the swept source, and motion artifacts were more apparent due to the slower speed. Cones were also observed at 2 degrees. Small blood vessels were observed at both eccentricities. Axial resolution in air and sensitivity of the AO swept source OCT were 9.4 µm and 96 dB, respectively.
The combination of AO and swept source OCT provides high-resolution images of the retina that approach the quality of those acquired with AO SD-OCT. Disadvantages are the reduced speed and axial resolution. Advantages include a 3 dB decay over a 3 mm imaging depth, turn key operation of the swept source, and user-friendly software. Y. Zhang et al., Opt. Express 14, 4380-4394 (2006).
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