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David Lara, Carl Paterson; High-Resolution Confocal Full Polarimeter For The Living Human Retina. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3090.
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We report a new polarization sensitive imaging instrument capable of resolving individual human cones as well as RNFL fibre bundles and collecting all available polarization signals at cellular level. The system targets vision research and clinical vision applications such as Glaucoma and AMD. We will introduce it to the medical environment using objective methods of image analysis. There is strong evidence that the living human retina has polarization signatures that could be linked to the presence of Glaucoma. In a polarization sensitive ophthalmoscope the amount of light that can be used is limited for the safety of the subject, and the return is typically a small fraction of the light used for illumination, of the order of 10E-6. Furthermore, the acquisition rates have to be sufficiently fast to avoid eye-movement artefacts. The light-budget available to produce a polarization image with a scanning laser ophthalmoscope is typically in the order of 10 nW, and pixel acquisition sampling rates are of several MHz.
We have developed a cSLO with full polarization control and capable of detecting the full Stokes vector of the returning light in a single snapshot. The system can detect full Stokes vector images in 20 ms (50 Hz), at an average pixel rate of 32 MHz. Complete Mueller matrix images (256x256 pixels) can be acquired at 5 Hz. The system uses a newly designed divison-of-amplitude broadband uncoated prism that can collect 91% of the light available for the measurements. This allows for all 4 Stokes signals to be detected simultaneously, hence they are registered exactly. Temporal modulation of the polarization of the illumination allows for full Mueller matrix measurements. Any full polarization state can be chosen for the illumination. An accurate polarization calibration method and a new robust registration algorithm enable reliable estimation of the polarization properties of the retina. The system can resolve individual cone photoreceptors of ~4um in diameter. The digital pixel sampling corresponds to approximately 1um on the retina.
We have obtained Mueller polarization images of human volunteers at cellular level. Registration and calibration can be performed without loss of image frames with micro-saccades, which makes the polarization analysis robust and reliable. Polarization signals from individual photoreceptors have been measured and calibrated.
To the best of our knowledge we are reporting the most advanced cSLO with full polarization sensitivity to this date. With this instrument will be able to study the polarisation signatures of ocular tissue with the highest resolution and accuracy reported to date. This is a novel device for vision research and clinical vision applications.
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