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.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.We retrospectively evaluated brain MRI scans, performed between 2014 and 2018, in patients treated with DMT for at least 6 months.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).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.