Lesions can grow larger and some can even disappear (from what I've seen on my own MRI's). I came across a pretty good web site--but it's loaded with technical medical information. If you can sift through some of that, there's some useful information about MRI's and lesions.
Here's the web site:
http://spinwarp.ucsd.edu/NeuroWeb/Text/br-840.htm
Deb