Have you read the health pages topic: "How MRI's Show Lesions in MS?"
There should be something in there to help...
I understand that 'active' lesions will light up (enhance) with the contrast thats injected for the MRI, inactive or the fully scared over lesions will not. When this happens its showing demyelination is happening, myelin is the fatty protein that is around the axons of the nerve cells, basically evidence that the blood brain barrier is compromised.
There isn't many conditions that will do this, MS is one but there are some others, but if your young MS is the more common, I believe.
I'm with JJ. My understanding is that all lesions will show up without contrast. When the contrast agent is added, those lesions that are currently inflamed will light up. Older lesions will not appear any differently in the presence of the contrast agent. For this reason MS MRI's are most frequently done both with and without contrast. It helps establish "dissemination in time" which is part of the MacDonald DX criteria.