The magnetic field is way less strong (0.064 Tesla compared to at least 1 in most machines), but the machine doesn't have to generate heat while producing a magnetic field
So long as they stay under their critical temperature. If that changes… well, a lot of liquid helium is about to stop being liquid.
And that's one of the reasons why a permanent-magnet MRI is so interesting -- it doesn't depend on a constant supply of power (for the helium cooler) and gaseous helium (to replace lost gas), so the system is portable, and maintenance costs are likely to be much lower.