MRI magnets are always active, even when the machine is not scanning a patient.

The core of a modern MRI machine consists of a solenoid coil made of niobium-titanium or niobium-tin wires. To achieve superconductivity, these coils are submerged in approximately 1,700 liters of liquid helium, which maintains a temperature of 4.2 Kelvin or -268.9 degrees Celsius. At this extreme cold, the electrical resistance drops to zero, allowing a massive current to circulate indefinitely without an external power source.The magnetic field strength of a standard clinical MRI is typically 1.5 to 3.0 Tesla. For comparison, this is roughly 30,000 to 60,000 times stronger than the Earth's magnetic field. Because the current never stops flowing, the magnetic pull is always present, even during holidays or overnight when the hospital is quiet. This creates a permanent 'missile effect' risk where ferromagnetic objects like oxygen tanks or floor buffers can be sucked into the bore at high speeds.Intentionally turning off the magnet, known as a quench, involves heating the superconducting wire until it regains resistance. This causes the liquid helium to boil off rapidly into a gas, expanding 700 times in volume. A single quench can cost a hospital over $50,000 in lost helium and potential hardware damage. Consequently, technicians only trigger an emergency shut-off if a person is pinned by a metal object or if there is a life-threatening fire.