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Q.Y. J. Smithwick, A.E. Elsner, R.A. Schwarz, J.B. Stewart, M.C. Cheney, A. Weber; Contrast and Resolution Comparisions between the GDx, MP1, and a prototype Scanning Laser Digital Camera . Invest. Ophthalmol. Vis. Sci. 2004;45(13):1135.
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Purpose:To compare the contrast in retinal images acquired using a scanning laser ophthalmoscope (GDx), a fundus camera (MP1), and a prototype scanning laser digital camera (SLDC). To compare the resolvable resolution in the eye using the 3 instruments. Methods:Retinal images of a nonmydriatic normal human eye were acquired using all 3 devices with near–IR illumination. Michelson contrasts were computed in 5 regions of interest across macular and peripapillary vessels and the optic disc rim, each using 5 averaged adjacent intensity profiles and compared among the 3 devices. The three devices' optical resolutions were determined using a USAF target in a model eye, consisting of an f=25mm lens, iris, and target holder. Results:In MP1 retinal images, the optic disc is bright, vessels aren't striped, and smaller vessels couldn't be seen. In images acquired using the GDx and SLDC, the optic disc is dark, vessels appear striped and distinct in contrast, although more distinct in the SLO images. For example, contrasts of a peripapillary vessel (175µm) using the GDx, MP1, and SLDC are 0.142, 0.0277, and 0.0966, respectively. The GDx, MP1, and SLDC optical resolution imaging a USAF target are 3.17, 3.17, and 1.59 – 2.24 line pairs per mm, respectively. When imaging the retina, the finest vessels (35µm), especially near the macula, are easily seen in the GDx images, barely perceptible in the SLDC, but not detectable by the MP1. In a 15x15° field, a GDx image has 256x256 pixels, an MP1 has 212x212 pixels, while a SLDC has 480x480 pixels. Conclusions:The SLDC retinal image contrast exceeds that of the MP1 and is comparable to the GDx, although the costs and complexity of the SLDC are far lower. The optical resolution of the SLDC is lower than the GDx and the MP1, the latter two having the same optical resolution. Although the SLDC has a lower optical resolution than the MP1, the SLDC is capable of imaging smaller vessels; i.e. higher resolvable resolution for the eye. Intraocular scattering reduces contrast and resolvable resolution of the devices. Retinal feature contrast is more important in determining the image information content than image pixel count or device optical resolution. A high degree of scattering in MP1 retinal images, caused by flood illumination, is readily seen in the optic nerve head contrast reversal, consistent with scattered NIR images. The SLO and SLDC ’s use of scanning and confocal apertures greatly reduce intraocular scattered light and contrast reduction. With high contrast and moderate resolution, the SLDC offers a low cost, easy–to–use retinal imager for telemedicine.
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