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Hao F Zhang, Siyu Chen, Xiao Shu, Peter L Nesper, Amani A Fawzi, Wenzhong Liu; Visible-light Optical Coherence Tomography Oximetry in Healthy Individuals. Invest. Ophthalmol. Vis. Sci. 2017;58(8):3100. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Though retinal oxygen metabolism is a key indicator of pathogenesis among major blinding diseases, it is not part of the current diagnostic standards due to lack of measurement tools. We seek to fill the vacancy by investigating the feasibility of visible-light optical coherence tomography (vis-OCT) on human subjects.
We imaged healthy subjects using our vis-OCT prototype. The 6.4°×6.4° field-of-view (FOV) covers an area proximate to the optic nerve head (ONH), where branches of major retinal arteries and veins can be identified. The vis-OCT intensities within the vessels were fitted against the known optical attenuation spectra of oxy-/deoxy-hemoglobin to yield sO2. The illumination power was 220 µW. We investigated a statistical model confirming that noises in OCT follows Rice distribution. Using this knowledge, we developed a statistical fitting-based approach to retrieve true OCT intensity, improving the accuracy of subsequent sO2 measurements. To investigate the spatial distribution of sO2, we manually selected regions-of-interest (ROIs) at different branching level and compared their respective sO2.
Vis-OCT provided better axial resolution (0.97 µm) compared to near-infrared OCT with similar bandwidth. Fig. 1 shows a vis-OCT B-scan containing 13 distinctively identifiable layers. In the en face images in Fig. 2a and 2c, superficial retinal arterioles and venules as small as 25 µm in diameter could be identified. Numerical simulation showed that the statistical fitting-based approach reduced the uncertainty in sO2 calculation by 3 percentage points. Albeit small in value, this improvement is crucial when variations in retina oxygenation is small. We plotted sO2 against the branch orders in Fig. 2b and 2d. We found the conclusion held true universally among available datasets that sO2 decreases as arteries bifurcate and travel downstream, whcih is consistent with the Murray’s law on oxygen gradient and unloading.
We demonstrated that vis-OCT can measure retinal sO2 in humans. The illumination power used was well under ANSI laser safety standards. In addition, the statistical fitting-based approach improved the accuracy of vis-OCT oximetry application, allowing us to appreciate the spatial variation retinal oxygenation.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
Fig. 1 Vis-OCT B-scan showing 13 layers in a human retina.
Fig. 2 Spatial variation of retinal circulation sO2 near the ONH.
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