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J.M. Bueno, C.J. Cookson, M.L. Kisilak, J.J. Hunter, M.C. W. Campbell; Degree of Polarization of Light Reflected from Optic Nerve Head . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4059.
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The polarization properties of light reflected from the fundus have not been fully characterized. Degree of Polarization (DOP) is expected to vary with retinal position and with age. The birefringent nature of the retinal nerve fibre layer (RNFL) near the optic nerve head (ONH) is useful in clinical diagnosis. However, non–negligible depolarization may affect the performance of retinal imaging devices designed to measure RNFL thickness via birefringence. Polarization properties are also important to improving imaging of the lamina cribrosa. Here, we measure the depolarization properties of the ONH as a function of age using a polarimetric confocal scanning laser ophthalmoscope (CSLO).
A polarimeter, incorporated into a He–Ne based CSLO, consisted of two combinations of linear polarizer plus rotating quarter–wave, one in the illumination pathway (generator) and another in the recording pathway (analyzer). Video–rate fundus images were recorded with a 15–degree field of view. Four video segments were recorded for appropriate orientations of the analyzer. For each polarization setting, a background was subtracted from each of eight images. They were then registered and averaged. The resulting averaged frames were used to compute the DOP for each pixel position, giving a spatially resolved DOP map. Measurements were carried out on one eye of each of 5 subjects with normal vision ranging in age from 19 to 64 years.
The quality of images varied with the polarization state of the analyzer unit. An ANOVA showed that DOP differed among four different square areas spaced around the ONH. In an annular area surrounding the ONH, the DOP values ranged from 0.05 to 0.75. However, this parameter did not follow the typical shape found for retardation. An ANOVA showed significant differences among subjects in the values of DOP for this annulus.
CSLO imaging polarimetry has been used to extract the DOP of the ONH. Depolarizing effects were dependant on both the imaged area and the subject. Detailed knowledge of DOP may help to improve the clinical measurement of the RNFL.
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