Most hospital MRIs use superconducting magnets that carry a persistent, loss-free current locked inside coils chilled by liquid helium. That current—and therefore the main magnetic field—continues 24 h × 365 days, even when no scan is running; only the gradient coils and RF amplifiers cycle on/off during each image. Permanent-magnet scanners are likewise “on” by nature, while low-field resistive systems could be powered down but usually aren’t because warm-up and field-stabilization take tens of minutes.
The only routine ways to kill the field are a planned ramp-down (hours) or an emergency quench (seconds) that vents helium and sacrifices the magnet. Hence MRI suites are designed, staffed, and policed under the assumption that the magnet is always on.