Apparatus for detecting a ferromagnetic object to protect, for example, a doorway to an MRI room comprises a passive primary sensor apparatus comprising at least one first magnetic sensor, a secondary non-magnetic sensor apparatus comprising at least two sensors, a detection zone for one of the secondary sensors being distinct from the zone of the other one of the secondary sensors, a signal processing circuit arranged in communication with the primary and secondary sensor apparatus, and a warning device. The signal processing circuit identifies temporal variations in the signal from the primary sensor and correlate them with instances in which both sensors of the secondary, non-magnetic, sensor apparatus detect the presence of an object in their respective zones.
