Quantitative magnetic resonance analysis
A single observer (J.M.) defined additional ROIs on the TE = 8.6 ms multi-echo image of the first coronal slice posterior to the globe, for the four rectus muscles bilaterally using the ITK-SNAP software [10]. Only voxels wholly within the muscle border were included to avoid partial volume errors arising from pixels containing both muscle and orbital fat. The small size of the superior oblique EOM prevented reliable ROI placement and this EOM was therefore omitted from the analysis. The mean ROI signal intensities were calculated for each TE image and fitted to a mono-exponential decay function to obtain the intercept, T2 decay constant and a DC offset using the Levenberg-Marquardt algorithm implemented in Wolfram Mathematica (Champaign, IL, USA).