Hi Eleanor -
I'm sure much more MS educated people will be coming along and giving you some great advice, but I thought I would jump in here too.
The website above is a basic understanding of what is needed to dx MS. There also is a TON of information on the Interent if you google multiple sclerosis. Some sites actually break down how to get dx'd with different types of symptoms backed up with the proper test results. I am not dx'd as of yet either, although my MRI came back with several large lesions. I had been having tremors and numbness for about 2 - 3 years and started progressively getting worse, so I went to see my pcp in October 2006. My pcp referred me to a neuro and she started the journey of tests, mri, eeg, blood work. Since I already had an MRI in November, I will not get a follow up MRI until first week in April (6 mos). This is only to see if more lesions have formed or if any of the lesions show up active on this MRI since they were not active on the last MRI. I think a follow up MRI is something that is done IF the first MRI was abnormal AND you are showing symptoms that everything else but MS has been ruled out. Possibly if your symptoms get really bad, your doctor may ask for another MRI in a couple months and not wait the full 6 months, but that will be under his discretion and have to have something solid to back up the MRI for insurance reasons.
This is just my 2 cents worth. Hope it helps some. Please keep everyone posted on how you are doing.