To explore the value of magnetic resonance fat quantification technology (MR mDixon-quant) in evaluating thyroid-associated ophthalmopathy (TAO) activity.

Case control study was performed.Fifty patients (100 eyes) with TAO which met Mourits criterion were enrolled from September 2016 to March 2018 in Ningxia Hui Autonomous Region People's Hospital.The TAO patients were grouped by clinical activity score (CAS). Twenty nine patients (58 eyes) with CAS≥3 served as TAO active group, twenty one patients (42 eyes) with CAS<3 served as TAO non-active group.Twenty three healthy subjects (46 eyes) were collected as normal control group at the same time.All subjects underwent orbital MR mDixon-quant and MR imaging.Orbital fat content (FF values) and exophthalmus was calculated on MR work-station.FF values and degree of exophthalmus were analyzed by analysis of variance.The correlation of FF values and CAS in TAO patients was conducted by Spearman rank correlation analysis.The cut-off values of FF in predicting TAO activity was determined by receiver operating characteristics (ROC) curve.The Kappa consistency test was chosen to assess the consistency of CAS and FF values.Intraclass correlation coefficient (ICC) were used to analyze the intra-observer and inter-observer repeatability of the FF values and the degree of exophthalmus.This study was approved by the Ethics Committee of Ningxia Hui Autonomous Region People's Hospital (No.20160718). All the candidates signed informed consent.

The FF values were (85.190±4.346)%, (88.715±5.686)% and (82.345±5.445)% in TAO active group, TAO inactive group and normal control group, respectively, with a significant difference among the 3 groups (F=17.072, P<0.001). The FF values of TAO active group and TAO inactive group were significantly higher than that of control group, and the FF value of TAO active group was significantly lower than that of TAO inactive group (all at P<0.01). The degrees of exophthalmos were (20.221±1.714), (20.855±2.103) and (15.363±1.667)mm in TAO active group, TAO inactive group and normal control group, respectively, with a significant difference among the three groups (F=126.298, P<0.01). The degrees of exophthalmos in TAO active group and TAO inactive group were significantly higher than that in normal control group (both at P<0.01), there was no statistical difference in degrees of exophthalmos between TAO active group and TAO inactive group (P<0.05). Spearman rank correlation revealed that FF value was negative correlated with CAS (r_s=-0.443, P<0.01). The cut-off value of FF value was 87.180%, the sensitivity, specificity, positive predictive value, negative predictive value and accuracy were 74.14%(43/58), 66.67%(28/42), 75.44%(43/57), 65.12%(28/43) and 71.00%(71/100), respectively.There were consistencies of FF values with the CAS (Kappa=0.431, P<0.001). The intra-observe and inter-observer reproducibility were fairly good for FF value (ICC=0.953, 0.920) and degree of exophthalmus (ICC=0.935, 0.917).

MR mDixon-quant can quantitative measure orbital fat with good measurement repeatability.Fat quantification technology by MR is valuable in evaluating TAO activity.

Thyroid-associated ophthalmopathy; mDixon-quant; Fat quantification; Activity