I haven't had an MRI in years (by choice), but I imagine that many of my lesions are fading or faded by now. But never fear, the Disease is still doing it's dirty work on me.....slowly but surely.
IMO, lesions are there to show that damage has been done to your nerves. When they fade or disappear, it doesn't mean your disease is not active, but that that particular lesion is the damage already done. And just as a scar heals so does it.