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Phillip A. Bedggood, Andrew Metha; Oximetry Imaging Of The Retinal Microvasculature Using Adaptive Optics. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5660.
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© ARVO (1962-2015); The Authors (2016-present)
To quantify oxygen content in the smallest vessels in the living human retina. Current methods lack the resolution to resolve these vessels due to limitations imposed by optical imperfections of the eye.
A flood adaptive optics system was built to image the human eye at 570 nm and 605 nm simultaneously, at 12-15 fps. The light source is a supercontinuum laser passed through an acousto-optic tunable filter. A dichroic filter separates the light onto two cameras. A single subject was imaged in a 5° area centered on the foveal center.570 nm light is absorbed equally by de-oxygenated and oxygenated haemoglobin (Hb). This controls for variations in blood column thickness. Assuming a whole blood Hb concentration of 150 g Hb/L, the thickness of the blood column was also determined at each image point.The 605 nm data is preferentially absorbed by de-oxygenated haemoglobin, creating an oximetry signal. Application of the Beer-Lambert law ideally allows absolute quantification of blood oxygen saturation. In practice for static imaging only relative measures can be obtained, due to background light from the choroid and other absorbers. We used the optical density ratio (ODR), which gives a reliable measure of relative change within a particular subject, in areas with minimal change in background pigmentation.
Vessels in the area imaged ranged from 5 - 35 μm in thickness while blood column thickness ranged from 5 - 24 μm. This implies ellipticity in the cross-section of the larger vessels, with the wider aspect oriented in the retinal plane, as expected. Ellipticity ranged from 0-58%, with larger vessels showing higher ellipticity while capillaries tended to be highly circular.Generated ODR maps revealed variations in O2 content as a function of axial distance along the vascular tree. ODR often dropped sharply (as high as 35%) immediately after smaller branches (~15 μm pre-branch diameter), while milder differences (8-14%) were evident in larger vessels. We attribute the sudden changes following a branch to slowed blood and thinner vessel walls promoting O2 exchange.Capillary ODR was highly variable, changing by up to 45% over a distance of only a few microns. This is an artifact of a bright specular reflex at 605 nm that was not present at 570 nm. When present on larger vessels, a similar reflex may be ignored by considering pixels on the edge of the blood column, but this is not possible with capillaries.
Multi-spectral adaptive optics imaging allows reliable quantification of relative oxygen content in the smallest arterioles and venules in the living eye. Use of temporal information may circumvent specular reflections from the vessel wall, improving accuracy.
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