Purpose: To determine the normal diameters of orbital structures on high resolution magnetic resonance imaging (MRI) sequences and correlate with patient demographics in an Australian cohort. Method: Retrospective review of patients who underwent 3T T1 -weighted fat suppressed contrast enhanced MRI orbits. The maximum extraocular muscle and superior ophthalmic vein (SOV) diameters on the normal orbits were recorded. Patients with bilateral disease or poor scan quality were excluded. Results: A total of 213 orbits from 138 patients were included. The mean age of participants was 58 years (20-94 years) and 65 (47.1) were male. Normal measurements (mean ± SD) were as follows: medial rectus 4.14 ± 0.52 mm; inferior rectus 4.63 ± 0.72 mm; lateral rectus (LR) 3.90 ± 0.69 mm; superior muscle group 4.49 ± 0.71 mm; superior oblique 3.03 ± 0.68 mm and the SOV 1.78 ± 0.68 mm. Males had significantly larger mean diameters of the LR (p < 0.01) and inferior rectus (p = 0.03). Significant correlation was found between age and medial rectus (r = -0.21, p < 0.05) and LR (r = 0.25, p < 0.01) diameters. Conclusion: Contrast enhanced MRI is the preferred imaging modality for evaluating pathological conditions affecting the orbit including infections, neoplasms and vascular malformations. This 3T-MRI data may help in diagnosing pathologic enlargement of the extraocular muscles and SOV on a contrast enhanced MRI sequence.
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