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Amir H. Kashani, Erlinda Kirkman, Gabriel Martin, Mark S. Humayun; Measurements Of Intravascular Oxygen Saturation Gradients And Arteriovenous Gradients: A Study Using Hyperspectral Imaging Computed Tomographic Spectroscopy. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1247.
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To non-invasively measure intravascular oxygen saturation gradients within retinal vessels and between retinal vessels in normal and ischemic eyes using a hyperspectral imaging camera.
Four rabbits (3-4kg) were dilated using 2.5% phenylephrine and 0.5% tropicamide ophthalmic solutions. Hyperspectral images were acquired with a custom made hyperspectral camera attached to a Zeiss FF450 fundus camera. Each image contains 60 spectral bands and was obtained in <3msec. An unsupervised algorithm was used reconstruct images and calculate oxygen saturation (oximetry) based on individual pixel optical densities from 26 spectral bands (Reichert Technologies). The results are displayed as a pseudocolored map.
In many cases, we noticed minimal or no significant arteriovenous (AV) oxygen gradients for Sa02 ranging from 70-100% (Fig A,B). Ventilation with 100% oxygen dramatically increased both arterial and venous oxygen content but did not result in an increased AV oxygen gradient (Fig C,D). Under ischemic conditions (IOP >100mm Hg), vessel oxygen content decreased significantly to < 70%. Decreasing IOP resulted in reperfusion and prominent AV oxygen gradients in freshly perfused arteries and veins (Fig E,F). After complete reperfusion, the AV difference was minimal indicating that the rabbit retina may have physiologically low AV difference. In addition, striking gradients of oxygen saturation were noted within individual vessels that have not been previously described.
Our hyperspectral images show detailed variations in intravascular oxygen saturation that are consistent with physiological changes in blood oxygen saturation and have not been described previously. Based on these results and details of intravascular oxygen saturation evident from our images we propose a model of the rabbit retinal circulatory physiology. In the future, similar imaging paradigms may be useful in evaluation of retinal vascular disease.
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