A Method for Three-dimensional Mapping of Retinal Tissue Oxygen Tension in Rat

Abstract

 

Purpose

 

The amount of oxygen available for energy production by the retina is determined by retinal tissue oxygen tension (tPO₂). Retinal ischemia is often multifocal and may lead to localized regions of tissue hypoxia in diseases such as diabetic retinopathy and retinopathy of prematurity. Therefore, methods for mapping retinal tPO₂ are needed for advancing knowledge of tissue oxygenation under these conditions. The purpose of the study was to demonstrate three-dimensional mapping of retinal tPO₂ in rats under systemic normoxia and hypoxia.

 

Methods

 

Using our established optical section phosphorescence lifetime imaging system, depth-resolved retinal tPO₂ images were acquired from closely spaced locations on the retina. These retinal tPO₂ images were combined to reconstruct a three-dimensional retinal tPO₂ volume, consisting of 40 enface retinal tPO₂ maps, separated in depth and spanning from the vitreous to the choroid. Imaging was performed in 1 rat under normoxia and 1 rat under systemic hypoxia, respectively. The severity of hypoxia was evaluated with the use of z-score maps, generated for each retinal tPO₂ map based on the mean and standard deviation (SD) obtained from the normoxic enface retinal tPO₂ maps at corresponding retinal depths. Inner (tPO_{2_IR}) and outer (tPO_{2_OR}) retinal tPO₂ volumes were compiled, consisting of enface maps from 1 to 20 and from 21 to 40, respectively.

 

Results

 

Mean retinal tPO₂ calculated from enface maps progressively increased with retinal depth, with the maximum value obtained near the choroid. Mean, SD, distribution peak, and 10th and 90th percentiles of both tPO_{2_IR} and tPO_{2_OR} volumes were lower under hypoxia as compared to normoxia (Table 1). Anoxic regions (PO₂ ≤ 1 mmHg) were present in the retina under hypoxia, encompassing 7% and 1% of tPO_{2_IR} and tPO_{2_OR} volumes, respectively. Under systemic normoxia, z-scores < - 2 were present in 1% of tPO_{2_IR} and tPO_{2_OR} volumes. In contrast, under systemic hypoxia, z-scores < - 2 were observed in 40% and 25% of tPO_{2_IR} and tPO_{2_OR} volumes, respectively.

 

Conclusions

 

Three-dimensional mapping of retinal tPO₂ was reported for the first time to our knowledge, demonstrating reduced tPO₂ under systemic hypoxia. The presence of anoxia under hypoxic condition indicates regions of the retina had a severe loss of energy production. Retinal tPO₂ mapping holds promise for investigating multifocal ischemia and tissue hypoxia due to retinal diseases.

 

 

Keywords: 552 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • 688 retina • 635 oxygen