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Y. Zhang, Q. Peng, T. Q. Duong; Anatomical and Functional MRI of the Human Retinas. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4415.
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© ARVO (1962-2015); The Authors (2016-present)
Although MRI has comparatively lower spatial resolution compared to optical imaging techniques, it offers depth-resolved, and often quantitative, physiological parameters. In anesthetized rodents, anatomical, blood flow and blood oxygenation level dependent (BOLD) MRI has been used to image the thin retinas. Similar applications in the human retinas remain challenging because high spatial resolution on human MRI scanners may be limiting and eye movement in awake subjects could be problematic. This study reports our initial experiences in anatomical MRI and BOLD MRI of O2 challenge of the human retinas.
MRI was performed on a 3T MRI scanner using a custom-made eye coil. Multiple (2-3) measurements per subject were made on 3 normal volunteers. Volunteers were instructed to fix and not to blink during actual data acquisition (~200 ms per time point) by way of sound cues. Anatomical MRI at 0.1x0.2x2 mm with inversion suppression of vitreous signals took 2 minutes. BOLD MRI (3 second per time point) at 1.6x2x4 mm with inversion suppression of vitreous signals was performed associated with O2 challenge in which the subject breathed 3 epochs of 30 s O2 interleaved with 30 s air. For consistency, the retina strip of 10 mm on either side of the optic nerve head was analyzed for layer thicknesses and BOLD signal changes.
Anatomical MRI detected 3 (bright-dark-bright) layers. The laminar thicknesses from the vitreous-retina boundary were respectively: 217±29, 227±11 and 232±8µm (mean±SD, N=8 measurements). The total retinal thickness including the choroid was 592±31µm, which was within thicknesses reported in the literatures but on the high side, likely because of partial volume effect. BOLD MRI of O2 challenge on the retina was reliably detected. The BOLD signal change was 2.2±0.5% (N=8 measurements, P<0.05).
We demonstrate, for the first, the feasibility of achieving anatomical laminar resolution using MRI and O2 challenge responses using BOLD MRI in the in vivo human retinas. Future studies will aim to improve resolution and contrast, and quantify blood flow using MRI. Depth-resolved MRI has the potential to offer clinically relevant physiological information and could complement optically based imaging techniques.
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