Contrast vs. non-contrast MRI...

I had an MRI done two years ago when I first started having arm tremors. When I relax my right arm, it tends to twitch uncontrollably. It doesn't interfere with the use of my arm, just when I relax it, and especially when I'm tired. I found out that the my neuro ordered a non-contrast MRI on my brain and spine (the neuro I was referred to is an arrogant, semi-retired fee collector, by the way), and the results came back negative. My question is this: what is the difference between contrast and non-contrast MRIs, and do the no-contrast MRIs miss things the others would detect.
I was dx with fibro two years ago, but my symptoms started 7 or 8 years ago. My rheumy says that he has never seen the degree of myoclonus that I have and wants me to see another neuro, but I just can't afford it at this time; I'm still paying off all the neuro tests that were done on me two years ago.

[RobinHood70]

Yes, contrast MRIs can pick up things that non-contrast can't (http://en.wikipedia.org/wiki/Mri#Image_contrast_and_contrast_enhancement).  I don't know how that applies to your myclonus, however, as I'm not a radiologist or doctor.