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Brian Vohnsen, Diego Rativa; Confocal Scanning Laser Ophthalmoscopy with Ultrasmall Spot Size. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4061. doi: https://doi.org/.
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The confocal Scanning Laser Ophthalmoscope (cSLO) allows retinal imaging with high resolution and discrimination against out-of-focus light. When combined with Adaptive Optics (AO) it has successfully been used to image cone photoreceptors away from the fovea centralis. The purpose of this study is to examine the feasibility of going beyond the current transverse resolution limitation set by the scanning beam diameter, wavelength, image pixel size, remnant aberrations and noise.
To accomplish this, an AO-cSLO has been designed using a 785 nm laser diode in an annular illumination scheme. A stop is used to block the center of the entrance beam thereby increasing the effective numerical aperture of the eye and reducing the size of the scanning light spot probing the retina cones. Aberrations are corrected in both pathways to and from the eye, and the signal is detected by a confocal APD. The subject’s pupil is dilated with accommodation paralyzed to enhance stability, and the frame rate is set at 47 Hz to reduce the impact of saccades during video acquisition.
The results show a clear reduction in the size of the probing light spot once the central part of the entrance beam has been blocked. Indeed, a 5 mm annular beam allows for a resolution that is comparable to that of an 8 mm full pupil. The imaging capability of the AO-cSLO with and without the central stop is examined both for the parafovea and fovea regions to examine the viability of the approach. The results show an increased visibility of the fovea region whereas larger parafovea cones are dimmed by the annular illumation scheme.
The results show a direct impact of the probing light spot where best image quality is obtained when the spot size at focus is matched to the diameter of the cone photoreceptors being imaged. This shows that retina photoreceptor waveguiding is of crucial importance for ultrahigh resolution retinal imaging.
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