Hi, the key term here is "nonspecific finding", which means that what is seen on your MRI can point to more than one particular cause.
It might be MS, but apparently, they'd ordinarily expect to see more than one such bright line (linear hyperintensity on T2) in MS --- while you have only one such bright line. An alternative association mentioned has to do with having migraines.
This study says that, in older people, it can be from normal aging without symptoms. Back in 2013, only 47% of what looked like MS turned out to actually be MS. "Compared to the neuropathologic reference standard, radiological assessment for periventricular WMHs showed a good sensitivity (83%) but only low specificity (47%) "
... that's when the bright line is in the periventricular area of the brain, which is where yours is at. So that's a good alternative explanation, depending on a person's age.
I know that this question is not recent, but maybe the reply might help somebody else who happens upon this post.