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Michelle Y. Peng, Sarah Ying, David S. Zee, Peter B. Barker, Howard Ying; Three-dimensional Magnetic Resonance Analysis Of Extraocular Muscles Demonstrates Gaze-specific Contractile Shape Changes. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4854.
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To evaluate whether contractile changes in extraocular muscle (EOM) shape correlate with three-dimensional eye position .
Seven 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 assessment of muscle shape (area, slope, integral, geometric center, and eccentricity).
Individual EOMs displayed characteristic shapes in all subjects: the lateral rectus (LR) was largest in volume, followed in order by the medial rectus (MR), inferior rectus (IR), superior rectus (SR), and superior oblique (SO). Muscle bellies increased in maximal cross-sectional area, and were translated posteriorly upon gaze in its direction of action. Cross-axis thickening occurred within LR in down gaze (25%), MR in up- or down gaze (15%), SR in right- or left gaze (15%), and IR in right- or left gaze (35%). Alterations in slope and eccentricity corresponded with changes in cross-sectional area.
This is the first three-dimensional analysis of position-dependent shape changes in EOMs. Our analysis demonstrates that EOMs exhibit contractile thickening and posterior displacement with gaze in the direction of primary muscle action. Whether quantitative anatomic differences correspond to saccade velocity, accuracy, or acceleration differences are under active investigation. Cross-axis thickening and compartmentalization within horizontal rectus muscles to produce cross-axis forces may account for some of our findings.
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