not only vitamin c but fruits containing oxalic acid if I read that right. But I'm far more interested in when such contrast agents are warranted, because I'm not aware that in Europe that contrast agent would be used that much for MRI

For your anecdata I'm in Sweden and definitely had a contrast agent (presumably gadolinium based) for a recent MRI of my gallbladder/pancreas/liver area.

see sections “Why Would I Need a Contrast MRI?” and “Which Types of MRI Require Contrast”

https://www.ashospital.net/blog/why-do-i-need-contrast-for-m...