Role of Magnetic Resonance Imaging in Thyroid-Associated Ophthalmopathy: Its Predictive Value for Therapeutic Outcome of Immunosuppressive Therapy

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Abstract
To investigate the efficacy of magnetic resonance imaging (MRI) in the assessment of thyroid-associated ophthalmopathy (TAO), 51 patients with TAO were evaluated by ophthalmologic examinations and MRI at 0.5 T. Thickness of extraocular muscles (EM) was measured by T1-weighted image. Signal intensities of EM and orbital connective tissue (OCT) were measured by short inversion time inversion recovery (STIR) image and expressed as a ratio by comparison to the signal intensity of cerebral substantia alba (SI, signal intensity ratio). Significant enlargement of one or more EM was observed in 86% of patients with TAO, and SI of EM (2.15 ± 0.63, mean ± SD) was significantly increased compared with control values (n = 16; 1.35 ± 0.33; t test, p < 0.01). SI of OCT tended to be greater than that in the control group, although the difference was not significant. There was a significant positive correlation between thickness of EM and severity of ophthalmopathy, assessed as an ophthalmopathy index (p < 0.05). SI of neither EM nor OCT correlated with the severity of the eye disease. To investigate whether MRI findings could predict the outcome of methylprednisolone pulse therapy, we studied 23 patients with TAO who received this treatment. SI of EM and OCT in the 12 patients giving favorable responses were significantly greater than those in the 11 patients without good response (t test, p < 0.01). On the other hand, the thickness of eye muscles did not correlate with the outcome of treatment except for that of medial rectus muscle. There was a significant correlation between SI of EM and that of OCT (r = 0.78, p < 0.01), suggesting possible similar pathologic processes in these tissues in TAO. In conclusion, MRI may be a useful noninvasive tool for determining indications for steroid pulse therapy and predicting the outcome of this treatment. We therefore recommend measuring the SI of EM by STIR imaging, as well as muscle thickness, in the evaluation of TAO.