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Qiang Wang, Barry Cense, Omer P. Kocaoglu, Jason Besecker, Weihua Gao, Ravi S. Jonnal, Sangyeol Lee, Donald T. Miller; Birefringence of Individual Retinal Nerve Fiber Bundles Measured Using Polarization-sensitive OCT With Adaptive Optics. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5277.
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Changes in birefringence of the retinal nerve fiber layer (RNFL) have been suggested to precede clinically detectable vision loss such as in glaucoma. Considerable attention has therefore been given to measuring RNFL birefringence, as for example with scanning laser polarimetery and polarization-sensitive optical coherence tomography (PS-OCT). Such techniques, however, are limited to measuring the bulk birefringence of the RNFL, and therefore may be biased by the appreciable fraction of non-birefringent tissue in the RNFL, which can vary significantly across the retina. The purpose of this experiment is to use PS-OCT with adaptive optics (AO) to locally measure the birefringence of individual retinal nerve fiber bundles (RNFBs), in this way offering a potentially more accurate assessment of nerve fiber birefringence.
We acquired PS-AO-OCT1 volumes of 30x30 retinal patches on two subjects free of ocular disease and at multiple retinal locations between the fovea and optic disc. The woofer-tweeter AO dynamically corrected the ocular aberrations and optimized focus at the RNFL. Post-processing in Matlab generated intensity and double path phase retardation (DPPR) images, the latter proportional to birefringence.
Individual RNFBs were identified in the intensity images, as for example shown in figure (a). DPPR images showed phase retardation variations that align to the corrugated pattern of RNFBs in the intensity images (example figure (b)). DPPR per unit depth for RNFBs 1, 2 and 3 in figure are 0.59, 0.52 and 0.570/µm, respectively. The average birefringence across the entire DPPR image, including both bundles and gap, is 0.390/µm.
Birefringence of individual RNFBs can be measured with AO-PS-OCT and is found larger than the bulk birefringence (bundles plus gaps).1. B. Cense, et al., Opt. Express, 17, 21634-21651, 2009.
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