What is the difference between a non active white matter brain lesion and an active one?

Contrast enhancement: Active white matter lesions tend to contrast enhance on MRI. Inactive lesions do not. You can see contrast enhancement on a contrasted CT scan but you cant see a lot of the MS plaques that are there without MRI sequences designed to identify MS plaques (FLAIR sequences and to a lesser extent T2 weighted images will reveal MS lesions that would escape detection otherwise.