Quantitative oximetry and chromophore mapping in the rat inner retina with visible light spectroscopic OCT

Abstract

 

Purpose

 

The purpose of this study is to test the hypothesis that by 1) changing the wavelength of Optical Coherence Tomography to the visible wavelength range and 2) developing and validating novel spectroscopic algorithms, reliable estimates of oxygenation, total hemoglobin, oxyhemoglobin, and deoxyhemoglobin in the retina can be achieved.

 

Methods

 

A high-speed visible-light spectral/Fourier domain OCT system was constructed for in vivo imaging of rodents, using a supercontinuum light source. An ex vivo calibration system was also devised to validate both oxygenation and hematocrit measurements. Whole blood flowing through FEP tubing was imaged and the corresponding sO2 values were estimated. Long Evans rats (Charles River Lab, MA) were used in the imaging studies. During the experiment, the rat was supplied a mixture of isoflurane in oxygen and medical air through a ventilating system. Imaging was centered on the optic nerve and required approximately 10 minutes.

 

Results

 

The ex vivo validation experiments showed good agreement between hemoglobin concentrations and oxygenations measured by our spectroscopic algorithm and those determined by a centrifuge and blood gas analyzer, respectively. Imaging results for saturation, total hemoglobin, oxyhemoglobin, and deoxyhemoglobin are shown in the Figure.

 

Conclusions

 

Here we introduce, validate, and demonstrate methods for quantifying oxygenation and hemoglobin content in the inner retinal vessels with spectroscopic OCT. When combined with flow, these methods will enable metabolic imaging of the inner retina.  

 

The figure shows quantification of chromophores in the rat retina in an en face view. The saturation map shows clear distinctions between arteries and veins (A). The map of the maximum of the product of total hemoglobin concentration and distance shows larger values in larger vessels (B). A map of the maximum of the product of oxygenated hemoglobin concentration and distance shows that veins and arteries contain oxyhemoglobin (C). By comparison, under the given experimental conditions, most of the deoxyhemoglobin is contained in veins (D). It should be noted that quantitative measurements of chromophores can be achieved by integrating the maps (B-D) in the transverse plane (x and y dimensions). All maps were displayed using an en face alpha map based on the local coefficients of determination at each transverse location, averaged over depth.