The retina is approximately 300 mu m thick and is metabolically very active. With limited energy reserve, it depends solely on a continuous supply of oxygen and nutrients to maintain its functional and structural integrity. The retina is nourished by two blood supplies, namely, the retinal and choroidal circulation, that feed the inner (i.e., ganglion cells) and the outer (i.e., photoreceptors) retina, respectively. Oxygen transport into the highly structured retinal cell layers relies heavily on diffusion. As such, the oxygen tension midway between the inner and outer retina is close to hypoxic level under normal conditions and, thus, the retina is very susceptible to ischemic injury. Oxygenation and perfusion deficits have been implicated in numerous retinal diseases. A non-invasive method to map changes in retinal and choroidal oxygenation and perfusion would be valuable. Non-invasive magnetic resonance imaging (MRI) has been widely used for investigating anatomy, physiology and function in both animals and humans. While tissue oxygenation and perfusion imaging is routine for studying brain functions, its application to the eye is technically difficult due to the thin retina and its proximity to the air-filled cranial cavity. Recent data from our lab demonstrate that blood-oxygenation-level-dependent (BOLD) functional MRI can dynamically map visual-evoked changes in oxygenation in the retina. The main goals of this proposal are: 1) to develop and validate high-resolution (58x58x1000 mu m3) MRI modalities to map choroidal and retinal oxygenation and perfusion in the retina, and 2) to investigate the visual-evoked layer-specific responses to differential activations of photoreceptor versus ganglion cell activity by measuring changes in the choroidal versus retinal oxygenation and perfusion. Studies will be performed on cats using a 4.7 Tesla scanner.
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