High-resolution 3D MR microangiography of the rat ocular circulation

Purpose: To develop high-spatial-resolution magnetic resonance (MR) microangiography techniques to image the rat ocular circulation. Materials and Methods: Animal experiments were performed with institutional Animal Care Committee approval. MR microangiography (resolution, 84 x 84 x 84 μm or 42 x 42 x 84 μm) of the rat eye (eight rats) was performed by using a custom-made small circular surface coil with an 11.7-T MR unit before and after monocrystalline iron oxide nanoparticle (MION) injection. MR microangiography measurements were made during air, oxygen, and carbogen inhalation. From three-dimensional MR microangiography, the retina was virtually flattened to enable en face views of various retinal depths, including the retinal and choroidal vascular layers. Signal intensity changes within the retinal or choroidal arteries and veins associated with gas challenges were analyzed. Statistical analysis was performed by using paired t tests, with P < .05 considered to indicate a significant difference. Bonferroni correction was used to adjust for multiple comparisons. Results: The central retinal artery, long posterior ciliary arteries, and choroidal vasculature could be distinguished on MR microangiograms of the eye. With MR microangiography, retinal arteries and veins could be distinguished on the basis of blood oxygen level-dependent contrast. Carbogen inhalation-enhanced MR microangiography signal intensity in both the retina (P = .001) and choroid (P = .027) compared with oxygen inhalation. Carbogen inhalation showed significantly higher signal intensity changes in the retinal arteries (P = .001, compared with oxygen inhalation), but not in the veins (P = .549). With MION administration, MR microangiography depicted retinal arterial vasoconstriction when the animals were breathing oxygen (P = .02, compared with animals breathing air). Conclusion: MR microangiography of the eye allows depth-resolved imaging of small angiographic details of the ocular circulation. This approach may prove useful in studying microvascular pathologic findings and neurovascular dysfunction in the eye and retina.