MRIs are taken for various reasons. Braces don't always need to come off for and MRI. An MRI for the head may require braces removal where an MRI for the chest or back likely would not. I am certain that all of my colleges would want to do anything possible to ensure you accomplish the best image possible.
Sometimes it is helpful for the MD to speak to the orthodontist in making this decision to help minimize cost and going through having them removed and replaced.
Braces should be removed prior to an MRI scan
It is common that we are requested to remove braces before a patient undergoes an MRI because there is interference with the metal in the braces. This makes for a poor quality scan that is difficult to read.
The bummer is that to put braces back on after wards is going to cost someone (you or the doctor) because of the time and materials involved. Also it will take a few months for the to re-adjust to the braces before your treatment can progress. So you are likely to delay your treatment. It is best if we know about the possibility of an MRI before braces are placed. Sometimes we can do things like use ceramic braces or delay parts of your braces until after the MRI is taken.