Layer-Specific Blood Flow MRI of the Mouse Retina

INTRODUCTION The retina is nourished by two separate blood supplies, the retinal and choroidal vessels. The retinal vessels are located in the inner retina closest to the vitreous. The choroidal vessels are located outside the outer retina, sandwiched between the retinal pigment epithelium and the sclera. The photoreceptor layer in between the two vascular layers does not contain any vessels (7). Importantly, the two blood supplies are substantially different in regards to basal blood flow (BF) and their response to stimulations. Basal choroidal BF is much higher than basal retinal BF and is much less responsive to many factors which regulate BF (1,2). While disruption of retinal vasculature is known to occur in some retinal diseases (3), the lack of non-invasive, depth-resolved imaging techniques has limited layer-specific investigation of the physiologic and functional changes of retinal diseases in vivo. We have previously reported BF MRI of the rat retina at 90x90x1500 μm, but at this resolution the two vascular layers cannot be separated (4). In this study, we developed the continuous arterial spin labeling technique to image BF of the mouse retina at considerably higher resolution (42x42 μm) to resolve the two vascular layers and the avascular photoreceptor layer in between in the retina. We also examined the effects of two common anesthetics, isoflurane or ketamine/xylazine, on BF in the retina.