Objective To study the clinical value of orbital magnetic resonance imaging(MRI) in diagnosis and making a judgment about the curative effect of glucocorticoid in patients with Graves' ophthalmopathy (GO).Methods A total of 106 patients with GO were divided into active GO group or inactive GO group according to whether clinical activity score(CAS) ≥3 or not,respectively.T2 relaxation times(T2RTs) (ms) and areas(mm2) of four extra-ocular muscles (EOMs) (superior,inferior,medial,and lateral rectus) were directly measured by MRI T2-Mapping.Correlation analysis of T2 RTs and areas with eyeball motility,diplopia,exophthalmus,visual acuity and intraocular pressure (IOP) were also performed.The T2RTs and areas of EOMs before and after intravenous methylprednisolone(iv MP) pulse therapy were recorded,respectively.Results The mean T2RTs (88.9 ± 13.8)and mean areas(58.1 ± 23.8) of EOMs in active GO were significantly greater than those in inactive GO (80.6 ± 10.0and 46.2 ± 18.6,respectively),bing both significantly greater than those in volunteers with normal eyes(76.2 ±4.7and 30.3 ± 6.1,respectively) (P＜0.01).Maximal T2RTs and maximal areas of EOMs showed significant positive correlation(P＜0.01),and both were positively correlated weakly with CAS,reduced movement of eyeball,diplopia,exophthalmus and loss of visual acuity (P＜0.01).On the contrary,maximal T2RTs and maximal areas of EOMs showed negative correlation or no correlation with spontaneous retrobulbar pain,painful eye movement,redness of eyelids and abnormal IOP.Even if 1 ≤ CAS＜3,ophthalmic symptoms and signs also showed a positive responses to iv MP in patients with moderate to severe GO as long as prolonged T2RTs occurred at least in one of EOMs.The maximal T2RTs,maximal areas,mean T2RTs and mean areas of EOMs were significantly decreased at the end of iv MP treatment,meanwhile ophthalmic symptoms and signs were obviously improved.Conclusions Prolonged T2RTs,especially accompanied by enlarged area on the identical EOMs is an remarkable symbols of acute inflamed muscles.The findings suggested that the use of prolonged T2RTs can improve diagnostic sensitivity and predict the curative effect of iv MP in patients with moderate to severe GO and CAS ≥ 1.