Noninvasive and simultaneous imaging of layer-specific retinal functional adaptation by manganese-enhanced MRI

PURPOSE. To test the hypothesis that high-resolution (23.4 mu m intraretinal resolution) manganese-enhanced magnetic resonance imaging (MEMRI) can be used to noninvasively and simultaneously record from distinct layers of the rat retina cellular demand for ions associated with functional adaptation. METHODS. In control rats, high-resolution images were collected with or without systemic injection of MnCl2, during light or dark adaptation; inner and outer retinal signal intensities were compared. In separate experiments, 1 month after systemic administration of MnCl2 to awake dark-adapted control rats, possible toxic effects of Mn2+ on ocular health were assessed with the use of the following metrics: retinal layer thickness, intraocular pressure, and blood retinal barrier integrity. RESULT. In nonmanganese-injected rats, the signal intensity difference between light and dark states for inner and outer retina was not significantly different (P > 0.05). In contrast, after manganese administration, the change in outer retinal signal intensity under light/dark conditions was significantly greater than that of inner retina. At I month after MnCl2 injection, comparisons with controls revealed no evidence for deleterious ocular health effects as assessed by whole and inner retinal thickness, intraocular pressure, and blood retinal barrier integrity. CONCLUSIONS. The present MEMRI examination was a safe (i.e., nontoxic) and relatively straightforward procedure that appeared to robustly reflect layer-specific retinal ion demand that correlates with normal retinal physiology responses associated with light and dark visual processing. Comprehensive MEMRI measures of retinal ion demand may be envisioned in a range of animal models for the study of normal development and aging.