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B. Cense, Y. Zhang, R. S. Jonnal, W. Gao, D. T. Miller; Evaluation of Polarization Sensitive Imaging With Adaptive Optics and Optical Coherence Tomography. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1139.
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The combination of adaptive optics (AO) and spectral-domain optical coherence tomography (SD-OCT) allows for high-resolution 3D imaging of the microscopic human retina. We have previously demonstrated an AO SD-OCT camera that imaged the 3D morphology of individual cone photoreceptors achieving a voxel resolution of 3x3x6 µm at a high speed acquisition rate of 75,000 A-scans/s. Individuating larger cells, however, such as ganglion and retinal pigmented epithelia (RPE) cells has proven substantially more difficult. The relatively low contrast of these larger cells appears to be a major limitation. As a non-invasive means to enhance contrast, we investigate the benefit of polarization sensitive imaging in AO SD-OCT. This includes determination of phase retardation (birefringence) and fast axis, polarization properties that may help in differentiating neighboring low-contrast cells in various layers.
A 2048 pixel linescan detector in a spectral-domain OCT configuration with a Wollaston prism splitting orthogonal polarization states acquired up to 30,000 A-scans/s. The illuminating light was polarization modulated between two states that were orthogonal in a Poincare sphere representation. The relative measurement method that is incorporated this way is not affected by corneal birefringence. AO consisted of a Shack-Hartmann wavefront sensor and a 36 actuator AOptix mirror. Volume scans up to 2° by 2° were acquired through a 6.6 mm pupil and of retinal tissue near the fovea with AO compensation.
AO-OCT cameras contain a substantial number of optical components, which can potentially induce diattenuation in the instrument and thereby reduce instrument accuracy. Diattenuating (20%) pellicle beam splitters, commonly used in AO cameras, were the only components to limit accuracy of the polarization-sensitive measurements. With the adaptive optics system focusing at the retinal nerve fiber layer to increase the signal to noise of this layer, we measured the phase retardation in a small patch of thin (~25 µm) nerve fiber layer tissue at 7° eccentricity superior to the fovea. The double pass phase retardation per unit depth varied between 0.27°/µm and 0.44°/µm, similar to values found near the optic nerve head of healthy volunteers. A spatial variation in nerve fiber layer birefringence was observed, possibly related to axon density.
Polarization-sensitive detection permits access to additional information about the retina tissue. Its combination with AO-OCT permits volumetric phase-retardation and optic-axis measurements on a microscopic scale.
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