The present invention relates to apparatus for detecting the presence of ferromagnetic objects and to a protected doorway assembly for a room which include an apparatus for detecting ferromagnetic objects and in particular to a device for detecting the presence of ferromagnetic objects in the vicinity of the doorway of a room containing a magnetic resonance imagining (MRI) scanner.
In GB 2 395 276 there is taught an apparatus which is able to detect ferromagnetic objects by providing a primary sensor means comprising first and second passive magnetic sensors which detect the disturbances in the ambient magnetic field which occur as the object moves through the field. The apparatus also includes a secondary non-magnetic sensor means which detects movement of objects in the vicinity of the primary sensor means. If both the primary and secondary sensing means detect a moving object an alarm is triggered. It is proposed in that patent that the apparatus may be mounted to the wall on either side of a doorway to a room containing a magnetic resonance imaging apparatus, with the secondary sensor means being arranged to detect objects approaching or about to pass through the doorway. Because both the primary and secondary sensors must detect the object this arrangement helps to reduce false alarms.
The apparatus described above works very well to warn people who may be unintentionally about to take a ferromagnetic object into an MRI room. It is known that there have in the past been several unfortunate accidents which have occurred due to a ferrous object entering an MRI room and being magnetically propelled, at high velocity, into the MRI machine within the room. This is called the projectile effect, and is well described in GB 2 395 276.
Recently there is a trend towards providing doors within the doorway of an MRI room which open outwards, i.e. out of the room. This means that the door will pass through the zone of sensitivity of an apparatus which is located on the wall outside of the room. The door will often carry ferromagnetic items, such as door handles or lock parts, and as the door and items move through the zone the alarm may be triggered giving a false alarm. Furthermore, where the secondary sensor means comprises a light beam the door will move to block the light beam and then any ferromagnetic object within the range of the primary sensor means but not about to pass through the door will trigger a false alarm.
It has been proposed to solve this problem by placing a detection apparatus on the inside of the MRI room when an outward opening door is provided. This ensures the door does not pass through the zone of sensitivity of the secondary sensors of the apparatus. However, the applicant believes this is not a good solution. MRI rooms are subject to high levels of magnetic radiation from the MRI device, which could cause interference with the apparatus. Similarly, MRI devices are sensitive to external radiation and so the apparatus may need to be shielded to prevent it from interfering with the MRI device. It has been proposed to switch off the apparatus whenever the door is closed to ameliorate this problem but this introduces further complexity and a further source of unreliability.
Alternatively, the problem can be solved by placing the apparatus sufficiently far in front of the doorway on the outside of the room for the door not to enter the zone of sensitivity. However, this will cause the apparatus to take up a lot of space, and barriers may then be needed between the apparatus and the doorway to prevent people entering the room without passing through the zone of sensitivity. In the case of a doorway in the sidewall of a corridor it may be impractical to install the apparatus in such a manner as it would block the corridor.
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