Your optometrist is right about MRIs but the brain MRI is the standard test for diagnosing MS, though it is sometimes hard to diagnose and people wait years before they're diagnosed.

Optometrists are fine for glasses prescriptions but they aren't qualified to diagnose and treat eye disorders. I hope you can see an ophthalmologist, who might want to do a visual fields test as well as a couple of other tests to get at the reasons for your double vision.