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Siyu Chen, Ji Yi, Hao F Zhang; Dynamically measuring retinal oxygen saturation at microvascular level using visible-light OCT angiograph. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):3308.
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© ARVO (1962-2015); The Authors (2016-present)
Although hemoglobin oxygen saturation (sO2) is a vital physiological indicator for the functionality of retina tissues, accurate measurement of ocular microvasculature sO2 remains challenging. Here we propose to use optical coherence tomography based angiography working within visible-light spectral range (Vis-OCT) to measure absolute retina sO2.
We developed a Vis-OCT system working from 520 nm to 630 nm to image rodent retina in vivo. Long Evans rats were anesthetized using Ketamine/Xylazine cocktail. Once the animal stabilized, it was ventilated normal air (21% O2) and a reference image was taken. Then we supplied the following inhalation gas mixtures in random order: (1) pure oxygen; (2) carbon dioxide oxygen mixture (21% O2, 5% CO2); and (3) low oxygen air (10% O2). For each inhalation gas, the animal was allowed to stabilize for 3 minutes, and normal air was supplied for at least 3 minutes between changes. We took one additional measurement after the sequence with normal air ventilation. During the experiment, a pulse oximeter was attached to the hind limb to monitor the peripheral capillary oxygen saturation (spO2). The acquired OCT images were processed using phase-sensitive decorrelation algorithm. A series of short time Fourier transform allowed the extraction of the sO2 encoded spectral information. We calculated the first derivative of the wavelength-dependent angiography intensity (A’) around 574 nm. The value was then calibrated into absolute sO2 percentage using spO2 readings.
A linear regression showed strong positive correlation between A’ and spO2 readings (R2 = 0.98). When inhaling normal air, the sO2 was 80.6±7.4% for arteries and 65.2±4.9% for veins (n = 8). During inhalation of gas 1 and 2, both artery and veins showed significant increase in sO2 (A: p < 0.05; V: p < 0.01). The increase was more dramatic in veins and when gas 1 was inhaled. We also observed a significant decrease in sO2 when gas 3 was supplied (p < 0.01). When normal air was supplied again, sO2 return to its original value for both arteries (p = 0.65) and veins (p=0.10).
Our results demonstrated that Vis-OCT angiography can accurately measure retinal sO2 and its variations, which can be invaluable for the early diagnosis of several retinal metabolic diseases.
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