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B. Cense, Y. Zhang, R.S. Jonnal, J. Rha, W. Gao, D.T. Miller; Polarization–Sensitive Retinal Imaging With Adaptive Optics And Spectral–Domain Optical Coherence Tomography . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3508.
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© ARVO (1962-2015); The Authors (2016-present)
Fibrous tissue, such as the retinal nerve fiber layer (RNFL), has birefringent properties that are of interest for glaucoma research. It has also been reported that the retina pigment epithelium has depolarizing properties which might be helpful for studying age related macular edema. Polarization–sensitive optical coherence tomography (PS–OCT) has been used to investigate these phenomena. A major obstacle, however, is the high signal–to–noise necessary to achieve reliable polarization measurements. While recent use of spectral–domain OCT (SD–OCT) has significantly reduced this barrier owing to its higher sensitivity (more than 25 dB above conventional time–domain OCT), additional substantial gain might be realized by incorporating adaptive optics (AO) to correct the ocular aberrations. We evaluate this possibility of an AO PS–SD–OCT retina camera.
An AO PS–SD–OCT system was developed having an axial resolution in retinal tissue of <6 µm and a sensitivity up to 94 dB. Polarization of the illuminating light was modulated between two states that were orthogonal in a Poincare sphere representation. A 1000 pixel linescan detector acquired up to 60,000 A–scans/sec. A Wollaston prism was used to separate the two orthogonal states, which permitted simultaneous capture by the linescan detector. AO consisted of a Shack–Hartmann wavefront sensor and a 36 actuator AOptix mirror. B–scans up to 2° in length were acquired through a 6 mm pupil and of retinal tissue near the fovea and optic disc with and without AO compensation.
The AO system corrected the most significant ocular aberrations and increased the sensitivity of the OCT instrument up to ∼10 dB within the depth of focus. Using a theoretical OCT noise model, we determined that the observed 10 dB gain should improve accuracy of the birefringence measurement by ∼30 times for the typical RNFL thickness (50 µm) and birefringence (1.2e–4 at 840 nm) immediately temporal and nasal of the optic nerve head. With AO, birefringent measurements of the RNFL were successfully acquired at retina locations that were previously difficult to quantify owing to the RNFL being thin and less birefringent. Aberration correction and a large pupil also provided the benefit of increased lateral resolution and decreased speckle size that permitted birefringent measurements at a microscopic scale not previously reported.
The addition of AO to PS–SD–OCT noticeably improves instrument sensitivity and resolution. This permits more accurate retinal birefringence measurements over a smaller microscopic scale.
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