Evaluation of a technique for estimation of extraocular muscle volume using 2D MRI.

Measurements of extraocular muscle volume may be useful as an outcome indicator in the assessment of treatment options in thyroid-associated ophthalmopathy. In this study, a technique for estimating extraocular muscle volume from two-dimensional T1 weighted MR images acquired in the coronal plane is described. For each image slice, the cross-sectional area of individual extraocular muscles was estimated using a combination of semi-automated outlining based on a local thresholding technique and manual outlining where thresholding was not possible. The technique was assessed using three sets of images: a computer-generated virtual image set with "extraocular muscles" of known volumes; a set of images from 19 biochemically and clinically euthyroid patients; and images from 7 patients with thyroid-associated ophthalmopathy. From the computer-generated images, the accuracy was determined as 7.6%. Intraobserver agreement was investigated using repeated MRI scans on a single subject and repeat outlining of muscles on five sets of images from different subjects. The mean difference between repeat measurements was 2% (SD 5.9%). We conclude that two-dimensional MR images can be used to determine changes in muscle volume of greater than 12% during treatment of thyroid-associated ophthalmopathy.