An arrangement for association with an operable MRI apparatus located within a room that has an access opening and with the MRI apparatus providing a residual magnetic field that extends to a location of the access opening, and for detecting presence of ferrous material and discriminating out far field magnetic disturbance, the arrangement including: a detector configured to passively monitor the residual magnetic field at the location of the access opening, the detector including an array of magnetic field sensors to be arranged about the periphery of the access opening in a spaced arrangement of two spaced apart banks (52a, 52b) such that each sensor becomes associated with a different portion of the access opening, each of the sensors being configured to output a signal having a value indicative of the sensed magnetic field at the associated portion of the access opening, which magnetic field may change in response to a presence of ferrous material at the access opening, but also in response to a far field magnetic disturbance; characterized in that the arrangement further includes: a corrector configured to correct each signal value from the change in response to the far field magnetic disturbance and to provide respective corrected signal values, the corrector including: a summer configured to sum all of said signal values to provide a summed value, a divider configured to divide the summed value by a total number of the magnetic field sensors to provide an average signal value, and a subtracter configured to subtract the average value respectively from each of said signal values to provide the respective corrected signal values; and a processor configured to use the corrected signal values to determine whether a ferrous-indicating limit is exceeded and to provide a safety response that addresses the ferrous-indicating limit exceed condition. a detector configured to passively monitor the residual magnetic field at the location of the access opening, the detector including an array of magnetic field sensors to be arranged about the periphery of the access opening in a spaced arrangement of two spaced apart banks (52a, 52b) such that each sensor becomes associated with a different portion of the access opening, each of the sensors being configured to output a signal having a value indicative of the sensed magnetic field at the associated portion of the access opening, which magnetic field may change in response to a presence of ferrous material at the access opening, but also in response to a far field magnetic disturbance; characterized in that the arrangement further includes: a corrector configured to correct each signal value from the change in response to the far field magnetic disturbance and to provide respective corrected signal values, the corrector including: a summer configured to sum all of said signal values to provide a summed value, a divider configured to divide the summed value by a total number of the magnetic field sensors to provide an average signal value, and a subtracter configured to subtract the average value respectively from each of said signal values to provide the respective corrected signal values; and a processor configured to use the corrected signal values to determine whether a ferrous-indicating limit is exceeded and to provide a safety response that addresses the ferrous-indicating limit exceed condition. a corrector configured to correct each signal value from the change in response to the far field magnetic disturbance and to provide respective corrected signal values, the corrector including: a summer configured to sum all of said signal values to provide a summed value, a divider configured to divide the summed value by a total number of the magnetic field sensors to provide an average signal value, and a subtracter configured to subtract the average value respectively from each of said signal values to provide the respective corrected signal values; and a processor configured to use the corrected signal values to determine whether a ferrous-indicating limit is exceeded and to provide a safety response that addresses the ferrous-indicating limit exceed condition. a summer configured to sum all of said signal values to provide a summed value, a divider configured to divide the summed value by a total number of the magnetic field sensors to provide an average signal value, and a subtracter configured to subtract the average value respectively from each of said signal values to provide the respective corrected signal values; and a processor configured to use the corrected signal values to determine whether a ferrous-indicating limit is exceeded and to provide a safety response that addresses the ferrous-indicating limit exceed condition.