Lesions can be caused by many things. It is a very difficult thing to look into the brain without opening it up and possibly damaging it. So the use of MRI is a wonderful thing. The problem is that MRI will show that something is there but not tell you exactly what that something is. A lesion is basically a spot. They can have many causes: tumor, stroke, concussion or injury, lyme disease, HIV/AIDS, meningitis or other infection and of course MS. After finding a lesion or lesions, the doctor must then rule out other possible causes before handing down the MS diagnosis.
MS tends to be only in the white matter of the brain and so that can be helpful. If someone were to have a spot or lesion on the gray matter or another brain structure that is probably not MS.
So it just seems that the lesions that showed up on your MRI are in the right place and have the "look" of typical MS lesions.
Gretchen co-moderator MS board diagnosed with MS July 2006