Retinal capillary oximetry with visible light optical coherence tomography

Abstract

<jats:p>Assessing oxygen saturation (sO<jats:sub>2</jats:sub>) remains challenging but is nonetheless necessary for understanding retinal metabolism. We and others previously achieved oximetry on major retinal vessels and measured the total retinal oxygen metabolic rate in rats using visible-light optical coherence tomography. Here we extend oximetry measurements to capillaries and investigate all three retinal vascular plexuses by amplifying and extracting the spectroscopic signal from each capillary segment under the guidance of optical coherence tomography (OCT) angiography. Using this approach, we measured capillary sO<jats:sub>2</jats:sub>in the retinal circulation in rats, demonstrated reproducibility of the results, validated the measurements in superficial capillaries with known perfusion pathways, and determined sO<jats:sub>2</jats:sub>responses to hypoxia and hyperoxia in the different retinal capillary beds. OCT capillary oximetry has the potential to provide new insights into the retinal circulation in the normal eye as well as in retinal vascular diseases.</jats:p>

Imaging oxygenation of retinal capillaries with depth resolution

“…In this model vascular bed ( Fig. 1), Pi et al (6) show a physiologically plausible decline in oxygen saturation with increasing capillary order and perfusion distance, consistent with delivery to the surrounding nerve fiber layer and ganglion cell layer. Gaining confidence from this sanity check, they next apply their algorithm to investigate the response of the superficial, intermediate, and deep capillary plexuses to hyperoxia and hypoxia.…”

“…While reduced blood flow can be inferred angiographically, clinical OCT oximetry could more directly assess the degree of hypoxia. As suggested by Pi et al (6), these observations could be resolved in depth, indicating which layers of the retinal vasculature are first affected and possibly even predicting complicating features such as diabetic macular edema. Similar paradigms can be envisioned for retinal vein and artery occlusions, also important causes of vision loss (19,20).…”

Optical density based quantification of total haemoglobin concentrations with spectroscopic optical coherence tomography

“…This procedure reduces the errors ascribed to the two-step fitting. Our method builds upon the methodology employed for mapping stO in rodents 18 , 35 . The stO estimation directly relates the Hb and HbO content in blood to the OD.…”

“…We developed a procedure to quantify [tHb] by numerical optimization of the optical density (OD) considering the absorption flattening effect in whole-blood. Our method leverages on the depth, and wavelength dependence of the OD 18 , 35 , enabling [tHb] estimations in a single step, in contrast to traditional Lambert-Beer fitting where the attenuation spectrum is fit from the signal depth decay and [tHb] derived from the retrieved attenuation coefficient. The OD allows simultaneous evaluation of a depth interval for the entire wavelength range, thereby reducing numerical errors induced during the two-step fitting and the variability of estimated concentrations.…”

Advances in Retinal Oximetry

“…More recently, two studies have been published in 2020 demonstrating the ability to use vis-OCT angiography to measure microvascular oximetry down to the level of retinal capillaries. Pi et al 80 utilized vis-OCT angiography to assess oxygen saturation in the retinal capillary beds of rats in response to changes in the inspired concentration of oxygen. Song et al 81 performed microvascular oximetry of the capillary network in the foveal region of human volunteer subjects, finding that oxygen saturation was between the retinal arterioles and venules as expected.…”