Normative measurements of orbital structures using MRI

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.

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.

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).

These data may be of value in quantitatively evaluating MRI of the orbit.