Normative Measurements of the Chinese Extraocular Musculature by High-Field Magnetic Resonance Imaging

To establish the normal measurements of diameter and cross-sectional area of the Chinese extraocular muscles (EOMs) and optic nerve-sheath complex (ON) as seen on high-field (3-Tesla; 3T) magnetic resonance imaging (MRI). Coronal 3T MRI of the orbit (TR, 700 ms; TE, 15 ms; FOV, 160 mm; matrix, 256 x 256; slice thickness, 1 mm; and slice gap, 0) was performed on 80 normal Chinese volunteers. T1-weighted images were analyzed with computer-assisted segmentation. The diameter and the cross-sectional area of EOMs and the ON were measured from the image plane at the posterior aspect of the globe and at 7 mm behind the globe. The maximum diameter of EOM was measured from their respective reconstructed images. The normal measurement (mean +/- SD) of diameter at the posterior aspect of the globe: medial rectus (MR), 3.6 +/- 0.4 mm; inferior rectus (IR), 3.7 +/- 0.8 mm; lateral rectus (LR), 2.3 +/- 0.5 mm; superior oblique (SO), 2.4 +/- 0.6 mm; and the ON, 5.4 +/- 0.7 mm. The normal measurement of diameter 7 mm behind the globe: MR, 3.2 +/- 0.6 mm; IR, 3.9 +/- 0.8 mm; LR, 4.0 +/- 0.8 mm; SO, 2.2 +/- 0.7 mm; and ON, 4.2 +/- 0.5 mm. The normal measurement of cross-sectional area at the posterior aspect of the globe: superior muscle group (SMG), 28.2 +/- 6.5 mm(2); MR, 25.5 +/- 3.2 mm(2); IR, 26.9 +/- 7.2 mm(2); LR, 17.3 +/- 3.9 mm(2); SO, 10.5 +/- 3.5 mm(2); and ON, 27.6 +/- 6.1 mm(2). The normal measurement of cross-sectional area 7 mm behind the globe: SMG, 24.1 +/- 6.6 mm(2); MR, 19.1 +/- 4.6 mm(2); IR, 29.5 +/- 6.6 mm(2); LR, 32.3 +/- 6.5 mm(2); SO, 9.7 +/- 3.9 mm(2); and ON, 17.1 +/- 2.9 mm(2). The normal measurement of maximum diameter: SMG, 4.8 +/- 1.1 mm; MR, 5.1 +/- 0.9 mm; IR, 5.4 +/- 1.0 mm; and LR, 4.5 +/- 0.9 mm. The Chinese 3T MRI EOMs and ON normative data may be useful in determining pathologic enlargement of the EOMs and ON in various orbital conditions. (ClinicalTrials.gov number, NCT00348413.).