In my experience Hun generally the issue with having anything on the nail which obscures the colour of the nail bed is that medical peeps like to be able to see the colour of the nail bed to ensure circulation and blood oxygen levels are as they should be. This is the only reason people having operations are requested to remove nail polish.
Having said that, never in history has an emergency operation been halted while someone nips off to get some acetone to remove nail polish, so there are ways around it. Having had a head, neck and spinal MRI myself I had no request to remove this and in fact had polish on both fingers and toes.
I would advise clients having an operation not to have anything which requires professional removal just in case, but an MRI is usually just an out patient procedure so IMHO not an issue.
My friend had a c-section earlier in the year because her baby just would not budge despite her being induced and trying forceps and allsorts. And when they did the spinal block the anaesthist (sp?) brought some acetone for her to soak off an acrylic nail. She's a beauty therapist too so she was like there's no way I'm soaking this off and just bit it off instead. She isn't actually trained in acrylic but just uses it on herself so the adhesion probably wasn't great anyway. But the anaesthist was freaking out because she bit it off lol! All the while she's shouting "I'm a beauty therapist I know what I'm doing!" Haha!