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Hao F Zhang, Ji Yi, Wenzhong Liu, Siyu Chen, Amani A Fawzi, Nader Sheibani, Robert A Linsenmeier; Investigating outer retinal oxygen supply using visible-light OCT. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5022.
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To noninvasively investigate how oxygen consumption in the outer retina is jointly supplied by the retinal and choroidal circulations.
We developed a visible-light OCT (Vis-OCT) to measure both retinal hemoglobin oxygen saturation (sO2) and retinal retinal flow. The Vis-OCT used a supercontinuum laser as the illumination light source centered at 585 nm. The axial resolution in the retina was 1.3 µm and the A-line rate was 75 kHz. To measure retinal sO2, we developed a sophisticated inverse algorithm to fit the optical absorption spectrum of the whole blood within the Vis-OCT spectral range. To measure blood flow, we performed double-circular-trajectory scans around the optic disk to obtain the absolute blood velocity. After obtaining the retinal sO2 and blood velocity, we further measured retinal vessel diameter and calculated the retinal oxygen metabolism rate (MRO2). To test the capability of Vis-OCT, we imaged wild-type Long-Evans rats inhaling normal air. To introduce oxygen supply deficiency in the choroidal circulation, we mixed pure oxygen with nitrogen and gradually increased the volume fraction of nitrogen in the mixed gas.
The Vis-OCT was able to measure the sO2 value in every single major retinal vessel around the optical disk as shown in Figure 1. When breathing normal air, the averaged sO2 in arterial and venous blood in Long-Evans rats was measured to be 95% and 72%, respectively. The total retinal blood flow was 7.6 µl/min. Over a 14-day period, the variations in sO2 and blood flow measurements were both less than 5% and the variation in MRO2 measurement was less than 8%. We found that when the choroidal circulation supplied less oxygen to the outer retina, the oxygen deficiency was compensated for by increased oxygen extraction from the retinal circulation. The measured increasing oxygen extraction rate from the retinal circulation agrees well with oxygen microelectrode measurements in rodents from terminal studies.
Vis-OCT is a sensitive tool to measure retinal MRO2 with a high repeatability. It opens up a new window to investigate several significant blinding diseases, such as diabetic retinopathy and glaucoma, which strongly associate with retinal oxygen metabolic disorders. Vis-OCT also holds promise in clinically identifying biomarkers of these diseases in their early stages.
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