Steroids and MRI's

  I'm curious about something my neuro-ophamologist said... I developed Optic Neuritis and went to a neurologist who put me on Oral Steroids...by the time I made it to my neuro-ophamologist, I had already taken several days worth.  He immediately changed me to IV steroids and ordered a MRI with contrast.  He said he would have treated me differently had I gone to him first, but that it will be harder to find a cause for the Optic Neuritis since I had already started steroids.  My question is, do oral steroids change the picture of an MRI? I know they look for white matter- I was wondering if steroids *clear* it up, thus making diagnosing more difficult.  Thanks!  Great forum!  Lots of neat info!
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Of the posible sites of MS involvement, lesions in the optic nerves  (due to their
small size ), are  somewhat more difficult to visualize on MRI.
The main finding on MRI in an acute optic neuritis would be swelling of
the optic nerve due to edema, this swelling of the nerve is reduced when
steroids are given so the lesion is harder to see.