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S. Rausch, I. A. Sigal, R. E. Norman, C. G. Olesen, I. Tertinegg, K. Morgan, S. Portnoy, J. G. Sled, J. G. Flanagan, C. R. Ethier; Measurement of Scleral Thickness Distribution in Human Eyes Using Micro-MRI. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3306.
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Previous studies (IOVS, 2005, 46:4189) suggest that scleral thickness has a substantial influence on optic nerve head (ONH) biomechanics, which is believed to play a role in the pathogenesis of glaucomatous optic neuropathy. Here we measure the three-dimensional (3-D) scleral thickness distribution of whole human eyes.
Enucleated ostensibly normal human post mortem eyes (n=3) were fixed and bathed for at least two weeks in 2mM gadoteridol (Prohance, Bracco Diagnostics), a T1 shortening agent. Micro-MRI was performed using a 7 T scanner (Varian Instruments, Palo Alto, CA) and a T1-weighted spin-echo 3-D protocol that gave 80 µm isotropic resolution over the entire eye. Scans were manually segmented, simplified and smoothed using Amira 3.1.1 software (Mercury Computer Systems, Inc.). Scleral thickness was measured at every surface node (~800,000 points) of the resulting 3D models. The sclera was divided by 6 parallels and 8 meridians, forming 56 regions. Global and regional mean scleral thicknesses were computed.
Global scleral thicknesses of the 3 eyes were (mean ±SD): 543±229 µm, 687±246 µm, and 576±193 µm. The ratios of largest over smallest mean regional thicknesses within an eye were: 4.38, 3.74, and 3.21, respectively. Mean regional scleral thickness ranged from 218 µm at the equatorial inferior temporal region of eye 1 to 1069 µm at the peripapillary nasal superior region of eye 2. Thickness distribution was not axisymmetric, e.g. eye 2 was more than three times thicker at the equator in the inferior temporal region (913 µm) than in the superior nasal region (286 µm).
Micro-MRI has been shown to provide detailed maps of scleral thickness while overcoming many of the artifacts of previous techniques. There were substantial variations in regional scleral thickness between eyes, but also within an eye. The effects of these variations in ONH biomechanics are still unclear and should be further investigated.
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