Three-dimensional mapping of chorioretinal vascular oxygen tension in the rat.

Abnormalities in retinal oxygen delivery and consumption are thought to play significant roles in common retinal diseases, among them diabetic retinopathy and vascular occlusion. Thus far, however, knowledge of fundamental mechanisms that implicate oxygen in the development of retinal abnormalities remains deficient and sparse. Noninvasive technologies that allow the assessment of oxygen tension (PO2) in chorioretinal vasculatures and retinal tissue are greatly needed to broaden knowledge of disease pathophysiology and thereby advance diagnostic and therapeutic procedures. Several techniques have been developed to assess PO2 in the retinal tissue and vasculature. Retinal tissue PO2 has been directly measured with high-depth resolution using oxygen-sensitive microelectrodes,1–12 but measurements are limited to one-dimensional linear profiles through retinal depth. Although in principle multiple line profiles may be derived, in practice this process is time consuming and may disturb the retinal microenvironment. Magnetic resonance imaging (MRI) has been used to study changes in preretinal oxygenation,13–18 but it does not provide an absolute measurement of tissue PO2, and it has lower resolution than optical techniques. Multi-wavelength reflectance spectrophotometry has provided measurements of blood oxygen saturation in retinal vasculatures. 19–27 However, the relationship between PO2 and oxygen saturation of hemoglobin is determined by the oxygen-hemoglobin dissociation curve, which may be altered because of various metabolic conditions. Additionally, measurements of oxygen saturation are limited to large retinal blood vessels and technically are not achievable in retinal capillaries. Measurements of retinal oxygen tension and consumption28 and retinal vascular PO216,29–36 have been reported based on fluorescence and phosphorescence quenching of molecular probes, respectively. However, these imaging methods lack adequate depth resolution and only detect fluorescence or phosphorescence from the entire retinal thickness and choroid. Specifically, retinal capillary PO2 measurements are not accurate with these methods because measurements are higher than retinal arterial measurements,35 a result that is not physiologically plausible. The contribution of phosphorescence from the choroid is the most likely explanation for these measurement errors. We have previously developed a novel system for optical section phosphorescence imaging. The system has been used to generate two-dimensional (2D) PO2 maps through retinal depth and to quantitatively measure PO2 separately in the choroidal and retinal vasculatures.37–39 In the present study, we report further development of our system for three-dimensional (3D) mapping of PO2 in the chorioretinal vasculatures. Our system generates en face PO2 maps at different retinal depths, allowing quantitative PO2 measurements in the choroid, retinal arteries, capillaries, and veins, as well as evaluation of vascular PO2 variations in horizontal retinal planes. En face chorioretinal vascular PO2 maps are useful for identifying potentially vulnerable retinal areas, in terms of oxygenation, in diseases such as diabetic retinopathy and macular degeneration. Furthermore, mapping of retinal vascular PO2 improves understanding of oxygen gradients along retinal vasculatures.