Normative measurements of orbital structures using MRI

Purpose: The purpose of this work was to establish criteria for the diameters of normal extraocular muscles and superior ophthalmic vein and width of the optic nerve-sheath complex, to determine normal globe position as seen on MRI, and to investigate the effects of age and sex on these structures. Method: Diameters of the extraocular muscles and superior ophthalmic vein, width of the optic nerve-sheath complex, and distance from the interzygomatic line to the posterior margin of the globe were calculated for 200 normal orbits of 100 patients on T1-weighted MR scans. Results: Normal ranges for diameters of the extraocular muscles were as follows (mean ± 2 SD): medial rectus, 3.2-4.9 mm; lateral rectus. 2.6-4.8 mm; inferior rectus, 3.7-6.0 mm; superior group, 3.1-5,6 mm; superior oblique, 2.4-4.1 mm. The normal position of the posterior pole of the globe was 8.9 mm behind the interzygomatic line (range 5.0-12.7 mm). The mean diameters of the extraocular muscles in male patients were significantly larger than those in female patients (p < 0.001). Conclusion: These data may be of value in quantitatively evaluating MRI of the orbit.