I have a question about ms lesions and when they should or shouldn't light up under gadolinium.

MS Lesions: may "light up" if they are acute (new). This is due to the leakiness of the tiny vessels allowing the contrast material to cross into the plaque lesion. After the plaque matures the contrast no longer crosses the "blood brain barrier" Both new and older lesions will produce increased signal on both T2 and FLAIR MRI sequences.

Gadolinium: When MS lesions are gadolinium positive using T-1 software, this indicates an acute area of inflammation and a breakdown of the brain-blood barrier. If this occurs or T-2 lesions are increasing or getting larger, all are signs that the treatment program is inadequate and a more potent medication is needed.