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T.Q. Duong, H. Cheng, G. Nair, P.M. Thule, M.T. Pardue; Layer–Specific BOLD Functional MRI of the Retina . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5872.
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© ARVO (1962-2015); The Authors (2016-present)
This study reports the development of BOLD functional MRI techniques to image oxygenation and functional changes in different tissue layers of the rat retina associated with physiological challenges. The retina is composed of three major cell layers (ganglion, bipolar and photoreceptor) and is nourished by two separate blood supplies (retinal and choroidal) located on either side of the retina. Retinal and choroid vasculatures are regulated differently. Choroid blood flow is many times greater than retinal blood flow and is generally unresponsive to changes in inhaled CO2 and O2 concentrations compared to retinal blood flow.
MMRI was performed on 10 rats using a 4.7T/40cm scanner with a small single–loop coil. High–resolution anatomical T1–weighted imaging was acquired TR=100ms, TE=4ms, slice thickness=0.8mm, FOV=8x8mm, matrix=128x128. BOLD fMRI was acquired using spin–echo EPI with diffusion weighting to suppress the vitreous signal, with similar parameters except TR=1s, TE=20ms, FOV=1.1x1.1cm. Physiological challenges used 100%O2 (hyperoxia) or 5%CO2 (hypercapnia, with 21%O2). Cross correlation analysis was used to derive BOLD percent–change maps. BOLD changes were tabulated for 3 "equal thickness" strips on the rat retina.
Robust lamina–specific BOLD fMRI responses associated with physiological stimuli were also observed, corresponding to the three anatomical layers. Hyperoxia–induced BOLD percent change in the outer strip (12±2%) was significantly larger than that in the inner strip (7±1.5%). This is because hyperoxia induced vasoconstriction of the retinal blood vessels but has little effect on choroidal blood flow, leading to larger increase in venous oxygenation in the outer strip. In contrast, hypercapnia–induced BOLD change in the outer strip (1.6±1%) was smaller than that in the inner strip (10±2%). This is because hypercapnia has little effect on choroidal blood flow, but potently induces vasodilation of inner retinal vessels. The middle avascular layer showed considerably weaker BOLD signal changes (1–2%) under both hyperoxia and hypercapnia. These observations are in agreements with published Laser Doppler data.
To our knowledge, this is the first report of lamina–specific BOLD fMRI responses. Future studies will involve measurements of blood flow and the use of visual stimuli. The ability to non–invasively map oxygenation, blood flow and retinal function with layer–specificity should enhance our understanding of normal– and patho–physiology of the retina.
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