A quick Google search shows me that I could afford to buy a used lower-end MRI machine... (not that I would) They don't seem that expensive (though there's a factor of 10 difference between the cheapest lower-resolution used machines and the high end)

The ease of availability of same-day MRI services in London suggests to me there's quite a bit of slack here at least.

EDIT: Though one solution to preparedness for temporary capacity issues without as much over-provisioning everywhere: Mobile suites. My local hospital currently has a mobile MRI suite parked outside to clear their backlog.