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T.Q. Duong, Q. Shen, H. Cheng, T.F. Chang, R. Shonat; Magnetic Resonance Imaging of Anatomical Layers in the Cat Retina . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1049.
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Purpose:The goal of this study was to explore the optimization of T2, diffusion, and contrast–enhanced T1 magnetic resonance imaging to resolve anatomical (photoreceptor, bipolar cell, ganglion cell layer) and vascular layers in the retina by sensitizing to the heterogeneous local biophysical and biochemical environment in different retinal layers. Since the retinal and choroidal vasculatures largely define the boundaries of the retina, Gd–DTPA contrast–enhanced T1–weighted MRI was also used to visualize these vascular layers. Methods: Multiple experiments were performed on 5 cats (0.7–1.4kg) under isoflurane and mechanical ventilation. ETCO2 and rectal temperature were monitored. T2–, T1– and diffusion–weighted images (DWI) were acquired with matrix=256×256, FOV=2.56×2.56cm (100x100µm), and a slice thickness=1.5mm. In some studies, 50x100µm in–plane resolution was used. Results: Multiple "layers" were observed in the retina, as indicated by the alternating bright, dark and bright strips on the T2–weighted images. These laminar structures disappeared in and around the optic nerve head which appeared slightly protruded. Higher resolution T2 and DWI showed similar laminar structures and thickness. Further, layer contrast on DWIx was markedly different from those of DWIy and DWIz, indicative of diffusion anisotropy. The T2, ADC// and ADC of different layers and the vitreous were tabulated. The middle strip showed the smallest and the vitreous showed the largest T2 and ADC. The retinal thickness, including the retinal/choroid complex, was 400–550 µm, consistent with those reported using invasive techniques. Contrast–enhanced imaging was used to further corroborate the classification of different retinal strips. T1–weighted images of the retina after Gd–DTPA showed marked enhancement of the anterior segment of the eye due to the high permeability of the ciliary body. Subtraction of post– and pre–contrast T1–weighted images showed marked signal enhancement on either side of the retina, with the outer strip being more enhanced and appeared thicker than the inner strip, consistent with known vascular structures and the relatively high choroid blood flow. The middle strip and the vitreous were not significantly enhanced. Conclusions: This study demonstrates the visual resolution of three distinct strips in the cat retina using MRI. The potential for layer–specific imaging of the retina to yield physiological (e.g., blood flow and tissue oxygenation) and functional information would enhance the diagnostic utility of MRI in the retina.
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