Here is how I understand this: A T1 mapping MRI is a method of doing an MRI which will show up abnormalities of the brain, beyond the older forms of MRI which show inflammation, or lesions, which are indicative of MS. If there are abnormalities of the brain other than typical MS inflammation or lesions, then the patient needs to continue to undergo further diagnostic testing.
As best I know, all but primary progressive forms of MS show some improvement with one of the drugs available to treat inflammation. Some folks respond better than others to one or other of the meds, but they all have shown to treat the inflammation of the brain, to some degree. Not a cure, of course, but at least lessen the severity.
I think you're confusing the test -- the T1 mapping MRI -- with a form of MS. It's not, of course. If a T1 MRI shows brain tissue degeneration not associated with inflammation, or lesions specific to MS, then it's quite possible the patient doesn't have MS, but some other brain disease or disorder.
...I am not a doctor, nor health professional, and don't pretend to be one, here.....