Background: MRI is highly sensitive for monitoring of disease activity and treatment efficacy in MS. Patients treated with disease modifying therapy (DMT), who experience MRI activity, including contrast-enhancing lesions (CEL) or new/enlarged T2 lesions, should be evaluated for a switch to more effective treatment. Due to recent evidence of gadolinium (Gd) accumulation in the brain after repeated administration of Gd-based contrast agents, FDA recommended to limit its use. Aim: To investigate the proportion of cases in which MRI activity would be detectable only using contrast-enhanced T1-weighted sequences.Secondary aims were to assess the presence of clinical or demographic variables associated with reactivation of pre-existing lesions and to analyse therapeutic consequences of different types of MRI lesions. Methods: We retrospectively evaluated brain MRI scans, performed between 2014 and 2018, in patients treated with DMT for at least 6 months. Results: We analysed 906 scans in 255 patients. New/enlarged T2 lesions were detected in 13.7% of cases, CEL in 3.5%, CEL without new T2 lesions (old lesions reactivated) in 1.1%. No variables were associated with old lesions reactivated. CEL with T2 equivalent were at higher risk of DMT switch, compared with new/enlarged T2 lesions without corresponding CEL (OR 4.0, 95% CI 1.5–10.4, p = 0.005). Conclusions: Reactivation of pre-existing lesions is limited to a tiny fraction of MRI studies. Gd + T1-weighted images could be omitted, in patients treated with DMT for at least 6 months, without relevant loss of information.
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