Abstract
Purpose: :
To demonstrate the functionality of a new retinal oximeter, the ROx-3. The ROx-3 selectively collects light scattered by red blood cells in retinal arteries and veins. It uses on-axis illumination, and it analyzes the data using the blue-green oximetry (BGO) technique. The optical density (OD) spectrum of hemoglobin has a local minimum that shifts linearly from around 467.5 to 501.5 nm as the oxygen saturation (SO2) increases from 0 to 100%.
Methods: :
The ROx-3 sequentially scans 457.9, 476.5, 488, 496.5, and 514.5 nm in the eye, building up a multi-wavelength retinal image. A confocal pinhole with bisecting wire blocks the specular reflection, passing scattered light from adjacent tissue. Near vessels, this light contains the most information about the vessel’s SO2. An enucleated swine eye was studied and the vessel OD determined at each wavelength.
Results: :
The OD spectrum was parabolic as expected with a minimum, lambdam, of 468.152 ± 0.309 nm, corresponding to an SO2 of 0 ±2.5%, as would be expected for several-hour-old blood with sufficient time to completely de-oxygenate. The coefficient of determination (R2) of the fit was 0.988-0.999, implying an uncertainty in SO2 of under ±3% saturation. The data implies the mean path length into the 200 µm vessel was just 72 µm, supporting the assumption that light did not travel through the entire vessel but predominantly scattered onto the surrounding fundus, before scattering again out of the eye. This enables the use of a simplified calculation of the OD as -log(V/F) where V and F are the average vessel and avascular fundus intensities, respectively. The noise in the images was random and the error of the ODs decreased as N-1/2 when the number of pixels averaged (N) increased.
Conclusions: :
This is the first demonstration of BGO on retinal vessels using on-axis illumination with a non-invasive device. The ROx-3 can successfully image swine retinal vessels with low and well-behaved noise. Further research is needed to calibrate the linear relation between lambdam and S02 for this system.
Keywords: oxygen • retina