Magnetic resonance imaging with diffusion-weighted imaging in the evaluation of thyroid-associated orbitopathy: getting below the tip of the iceberg.

OBJECTIVES: To compare extraocular muscles (EOMs) T2, post-contrast T1 (T1Gad) signal intensity ratios (SIRs) and normalized-apparent diffusion coefficient (n-ADC) values in patients with thyroid-associated orbitopathy (TAO) at different phases of activity and severity and correlate MRI modifications to clinical evolution during follow-up.

METHODS: A total of 74 TAO patients were classified as active or inactive on the basis of the clinical activity score (CAS). Severity of EOM impairment was evaluated by assigning a functional score to each rectus. T2, T1Gad SIRs and n-ADC of EOMs were compared in patients with active inflammation, those with inactive disease and 26 healthy controls, and correlated with clinical scores. MRI parameter variation was correlated with clinical modifications during follow-up.

RESULTS: All MRI parameters in TAO EOMs were significantly higher than in healthy subjects and correlated with muscle dysfunction and CAS. EOMs of active patients showed higher T2 and T1Gad SIRs than those with inactive disease. The T2 SIR and n-ADC of normally functioning TAO EOMs were higher than those of healthy controls. SIRs decreased in clinically improved and clinically stable EOMs after therapy.

CONCLUSIONS: T2 SIR, T1Gad SIR and n-ADC are objective measures of activity and severity of EOMs in TAO patients. MRI shows clinically silent muscle involvement and modifications.

KEY POINTS: • MRI and DWI measures are objective, quantitative parameters of TAO activity and severity • MRI and DWI measures significantly correlate with clinical scores in TAO patients • MRI and DWI can identify clinically silent inflammation of deep orbital structures • MRI and DWI can depict subclinical modifications during follow-up • MRI and DWI may aid clinicians in choosing the most appropriate treatment.