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G. Bhagavatheeshwaran, H. Cheng, T. Nagaoka, T.Q. Duong; Manganese–Enhanced MRI Reveals Multiple Tissue and Vascular Layers in the Rat Retina at 25x25 µm Resolution . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3316.
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© ARVO (1962-2015); The Authors (2016-present)
Manganese (Mn) is both an MRI contrast agent and a calcium analog. MRI signal enhancement in tissues by Mn is dependent on cell type, cell density, synaptic transport of Mn, regional tissue permeability to Mn, baseline and evoked neural activity. We hypothesize that Mn–enhanced MRI can be used to reveal multiple tissue layers in the rat retina. To corroborate layer assignments, MRI using Gd–DTPA (an intravascular contrast agent) was performed where the two choroid and retinal vascular layers located at either end of the retina could be visualized.
Mulitple repeated MRI measurements were made on 3 rats intravitreally injected with MnCl2 (5µl of 120mM) 4 hrs prior to MRI. T1–weighted MRI was acquired using a 4.7T Bruker scanner, custom–made detector, TR=100ms, TE=4ms, slice thickness=0.8mm, FOV=6.4x6.4mm, matrix=256x256 (25x25µm) and 16 averages. Additionally, Gd–DTPA (0.4mL/300g, 0.5M) was administered intravenously followed by T1–weighted MRI. Toluidine blue histology was performed.
At least four retinal layers of alternating bright and dark layers were robustly observed in Mn injected rats. Their thicknesses were 87±2µm (closest to the vitreous, very dark), 46±9µm (bright), 30±8µm (dark) and 48±17µm (very bright). After Gd–DTPA, an additional layer (90µm) appeared in the outer part of the retina closest to the sclera. With histological cross–correlation we made the following assignments. The strip closest to the vitreous which was heavily exposed to Mn and appeared very dark (T2* effect) likely corresponded to the ganglion layer. The outer Mn–enhanced strip likely corresponded to the photoreceptor layer, which is avascular, as validated by the absence of Gd–DTPA enhancement. The middle Mn–enhanced strip likely corresponded to the bipolar layer. The total thickness of the neural retina was 212±20µm (Mn alone), consistent with published data. Following Gd–DTPA, the total thickness was 324µm which included the choroid vascular layer, consistent with our previous MRI study on rat retinas albiet at lower spatial resolution (307±29µm, n=10).
This study demonstrates a novel application of Mn–enhanced MRI to delineate multiple tissue and vascular layers in the retina at 25x25µm resolution. Improvement in spatial resolution and contrast are expected. Additional validations of layer assignments are under investigation. These results demonstrated that MRI has the potential to approximate histological resolution and contrast, offering a unique means to investigate structure, physiology and function of the retina in normal and diseased states.
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