I'm no Ph.D. but here goes.............When an MRI is done, the first scans are usually done without anything to enhance the lesions. These could be 1 week, 1 month, or 10 years old, there is no way of knowing the age of existing lesions. After these scans are done, the radiologist(or whoever is doing your scan) will give you an IV injection of Gadolinium - A chemical compound which can be administered to a person undergoing an MRI procedure, to enhance the distinction between new vs old lesions. It also increases the scan's sensitivity, which enables the imaging of lesions that would be missed.
Gadolinium enhances active lesions more than existing lesions which will determine if a person has an ongoing demylination (experiencing an exacerbation) my neurologist tells me there is a small window of time when this will happen. Meaning, enhanced lesions are current demylination.
These are the basics, there are other options with an MRI, short of having a Radiologists knowledge, understanding all the details can get pretty mucky?
A note: Gadolinium is a radioactive isotope. It has a very short half-life, meaning it breaks down really fast. I'm told the risk of injecting such a thing is extremely minimal.
Hope this is a help!