Layer-Specific Blood-Flow and BOLD fMRI of the Mouse Retina Associated with Hypoxic Challenge

INTRODUCTION: The retina is nourished by two separate vasculatures: the retinal and choroidal vessels. The retinal vessels are localized within the inner retina next to the vitreous, and the choroidal vessels are behind the retinal pigment epithelium. The photoreceptors and segments between these two vascular layers are avascular (1). Oxygen tension in the photoreceptor layers is in fact near 0 mmHg under normal conditions (2). As such, the photoreceptors are more susceptible to hypoxic injury. The ability to image retinal and choroidal blood flow (BF) and oxygenation associated with hypoxic challenge could have important applications in retinal diseases such as retinal ischemia, diabetic retinopathy and glaucoma. Optical imaging techniques, widely used to study the retina, cannot unambiguously resolve choroid BF (ChBF) and retinal BF (RBF) or oxygenation. In contrast, MRI has recently been shown to be capable of measuring quantitative RBF and ChBF in mice at 42x42x400 μm (3). In this study, BF MRI was further developed to include inversion-recovery suppression of the vitreous to improve sensitivity in detecting layer-specific BF in the retina. We applied this new approach to investigate lamina-specific (retinal and choroidal) BF and BOLD MRI responses to mild hypoxic challenge.