Background: Current magnetic resonance imaging (MRI) techniques for measuring extraocular muscle (EOM) volume enlargement are not ideally suited for routine follow-up of Graves' ophthalmopathy (GO) because the difficulty of segmenting the muscles at the tendon insertion complicates and lengthens the study protocol.
Purpose: To measure the EOM sampled volume (SV) and assess its correlation with proptosis.
Material and methods: A total of 37 patients with newly diagnosed GO underwent 3-T MRI scanning with iterative decomposition of water and fat (IDEAL) sequences with and without contrast enhancement. In each patient, the three largest contiguous coronal cross-sectional areas (CSA) on the EOM slices were segmented using a polygon selection tool and then summed to compute the EOM-SV. Proptosis was evaluated with the Hertel index (HI). The relationships between the HI value and EOM-SV and between HI and EOM-CSA were compared and assessed with Pearson's correlation coefficient and the univariate regression coefficient. Inter-observer and intra-observer variability were calculated.
Results: HI showed a stronger correlation with EOM-SV (P < 0.001; r = 0.712, r2 = 0.507) than with EOM-CSA (P < 0.001; r = 0.645 and r2 = 0.329). The intraclass correlation coefficient indicated that the inter-observer agreement was high (0.998). The standard deviation between repeated measurements was 1.9-5.3%.
Conclusion: IDEAL sequences allow for the measurement EOM-SV both on non-contrast and contrast-enhanced scans. EOM-SV predicts proptosis more accurately than does EOM-CSA. The measurement of EOM-SV is practical and reproducible. EOM-SV changes of 3.5-8.3% can be assumed to reflect true volume changes.
Keywords: Graves ophthalmopathy; contrast enhancement; extraocular muscle volume; magnetic resonance (MR); orbit; segmentation.