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Michelle Peng, Robert Geary, Sarah Ying, David Zee, Peter Barker, Howard Ying; Three-dimensional magnetic resonance analysis of extraocular muscles demonstrates gaze-specific contractile shape changes. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1924.
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To evaluate whether contractile changes in extraocular muscle (EOM) shape correlate with three-dimensional eye position.
Eight normal volunteers were scanned in the supine position with near visual targets placed in nine positions of gaze at 20 degrees eccentricity. Orbits were imaged as targets were viewed through a 3” open surface coil. Magnetic resonance (MR) image sequences consisted of oblique-coronal fast-spin echo T1-weighted MR in 15 coronal sections, with the most anterior slice placed just posterior to the equator (field of view 8 cm, matrix size 256x128, gap 2.0 mm, TR 500 msec, TE 11 msec, echo-train length 6, scan time 39 sec). Images were analyzed using the Medical Image Processing, Analysis, and Visualization platform (MIPAV, National Institutes of Health, Bethesda, MD) for EOM delineation, and Matlab (Mathworks, Natick, MA) for orbital volume registration and assessment of muscle shape. Local area over the curve (AOC) was defined as the sum of the areas less than the peak area but greater than the muscle area for the five slices anterior or posterior to the peak.
Assessment of EOM contractile thickening was performed using several indices: change in cross-sectional area, maximal cross-sectional area, change in area under the curve, change in slope of area curve, change in size of muscle peak, and change in AOC. Of all the indices, AOC best captured the change in contractile thickening observed during visual inspection of the area plots. For secondary gaze positions, change in AOC showed a trend towards increasing in the direction of action for most muscles (e.g. left vs. right gaze for lateral rectus). Interestingly, change in AOC increased to a greater degree for related tertiary gaze pairs, (e.g. up-left gaze vs. up-right gaze for the lateral rectus).
These results further expand upon the three-dimensional analysis of position-dependent shape changes in EOMs that were presented previously. The current study demonstrates that change in AOC can provide a quantitative assessment of contractile thickening. Using this novel metric, we find statistically significant cross-axis thickening for the rectus muscles. This finding suggests either that cross-axis gazes require additional stabilization from rectus muscle contraction or that rectus muscles generate cross-axis forces for eye movement in that direction.
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