Quantitative volumetric analysis of retinal tissue oxygen tension

Abstract

Purpose : Retinal function is sustained by oxygen delivered from dual circulatory systems, the contributions of which create complex oxygen gradients through the retinal depth. Localized regions of inadequate oxygenation occur in several retinal diseases, including diabetic retinopathy and vascular occlusions. Previous studies assessed retinal oxygenation in limited spatial dimensions, which precluded the detection of disseminated pathologies. Therefore, techniques that volumetrically assess retinal oxygenation are necessary to understand retinal physiology in health and disease. We report a method for the 3D imaging of retinal tissue oxygen tension (tPO₂) and the 2D mapping of outer retinal oxygen consumption (QO₂) in rats.

Methods : Phosphorescence lifetime imaging was performed in male Long Evans pigmented rats under systemic normoxia (N=6) and hypoxia (N=3), one day after intravitreal injection of an oxyphor. A vertical laser line was horizontally scanned across the retina and a series of depth-resolved phosphorescence optical section images were acquired. An automated customized software algorithm constructed phase-delayed phosphorescence volumes, determined the phosphorescence lifetime at each voxel, and generated a 3D retinal tPO₂ volume. The volumetric data was fitted using an oxygen diffusion model to map QO₂ in 2D. The effects of systemic condition (normoxia, hypoxia) and retinal depth on mean tPO₂ (M_tPO2) and the spatial variation of tPO₂ (SV_tPO2), measured as SD, were determined by mixed linear model. Mean QO₂ (M_QO2) and spatial variation of QO₂ (SV_QO2) were compared between normoxia and hypoxia by unpaired t-tests.

Results : 3D retinal tPO₂ volumes were approximately 500x500x200μm (horizontal x vertical x depth). M_tPO2 measured at the chorioretinal interface was significantly correlated with systemic arterial PO₂ (P=0.007; N=9). There were significant effects of both systemic condition and retinal depth on M_tPO2 and SV_tPO2, such that both were lower under hypoxia than normoxia and higher in the outer retina than inner retina (P<0.001). No statistically significant difference was found in M_QO2 or SV_QO2 between normoxia and hypoxia (P≥0.16).

Conclusions : For the first time, 3D imaging of retinal tPO₂ and 2D mapping of outer retinal QO₂ were demonstrated, with potential for the assessment of disseminated physiological alterations in animal models of retinal diseases.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.