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John E. Legarreta, Thomas R. Friberg, Zoran Vatavuk, Goran Bencic, Carlos A. Medina-Mendez, Peter M. Brennen; Imaging of Peripheral Retina with Optos Ultra-Widefield Imaging: Evaluation of Aperture Size on Image Quality. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1163.
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Previously we have used the Optos P200A system to image the retina in cases where examination of the peripheral fundus by biomicroscopy or indirect ophthalmoscopy was very difficult if not impossible because of dense cataracts, small pupils or substantial vitreous opacities. Recently, we imaged an eye where the cornea appeared completely opaque secondary to a failed corneal graft and extensive calcium deposits. We were able to obtain a clinically useful image, and we wished to explore further the limits of such imaging by simulating severe corneal opacities with special contact lenses.
Using the Optos P200A system, we evaluated healthy volunteers who placed an opaque contact lens in one eye. The opaque contact lenses were altered to create small apertures of varying sizes to severely limit the amount of light entering the eyes to be imaged. In addition, the apertures varied in location (central, peripheral). The primary outcome measure was to find the minimal aperture size through which a useful fundus image could be obtained, as assessed by an independent retinal specialist.
Clinically useful retinal details were revealed in healthy eyes using this technique. Several needles of varying gauges (30, 25, 20 and 18) were used to create holes in the contact lenses, which approximately corresponded to 0.31mm, 0.51mm, 0.91mm and 1.27mm holes respectively. In our study, an aperture size of 1.27 mm diameter was the lower limit for the detection of retinal details. Images were reviewed by an independent retinal specialist who had access to image editing software and was allowed to manipulate images in order to visualize retinal structures. Given the average corneal area of 144 mm2, and the smallest aperture area size of 1.4mm2, approximately 1% the corneal area was required in order to obtain an image with retinal details. The Optos system has a narrow illuminating beam (0.3mm) and a long wavelength laser beam (633 nm) which both facilitates in reducing backscatter and allowing for imaging in less than ideal conditions. Post-processing of images is essential when the opacities are severe, and they may appear completely useless at first glance. After careful image adjustments with software, clinically relevant retinal details could be visualized.
Ultra-wide angle imaging is an important clinical application and can be used in clinical assessment of eyes with dense media opacities. The clinical implication suggests that examination of the peripheral fundus may be possible in individuals with small pupils and other media opacities with the Optos system, especially when clinical examination proves difficult, if not impossible.
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