Liquid helium is very sneaky stuff. Sure, they try to keep it in as much as they can (it's expensive!) but it's gonna leak eventually. A quick web search indica

Liquid helium is very sneaky stuff. Sure, they try to keep it in as much as they can (it's expensive!) but it's gonna leak eventually. A quick web search indicates that MRIs in common use lose 1-6% of their helium per month.