Ferroguard® Background and Technical Information
1. Ferroguard® Ferromagnetic Detection System an introduction to the technology
2. How does the Ferromagnetic Detection System work?
4. How reliable is The Ferromagnetic Detection System at detecting ferromagnetic objects?
5. What is the smallest sized object that the system will detect?
1. Ferroguard® Ferromagnetic Detection System an introduction to the technology
The Ferroguard® Ferromagnetic Detection System is a cost effective and easy to operate alarm system designed to detect the passage of ferromagnetic objects before entry into a medical MRI suite. The alarm system warns personnel of the presence of ferromagnetic objects such as iron and steel that pass between the two units.
The unit’s function is to warn people entering an MRI room if they are carrying an object that may be attracted into the MRI magnet and cause damage. Such objects are referred to as projectile hazards as they can quite literally fly at high speed towards a magnet. Heavy objects that are highly magnetic can be pulled into the magnet with enough force to do substantial damage to the machine, and be hazardous to any people who are in the way. Gas cylinders, IV poles, cleaner’s buckets, and trolleys are the most common causes of major projectile hazards. Lighter objects that are weakly magnetic can also be pulled into the magnet and be an inconvenience as they have to be removed before imaging can commence. Examples of these include keys, coins, paperclips and many others.
The system may be located outside of the entrance of an MRI room, at the entrance of a controlled zone or in a special screening zone to warn if a projectile hazard is present. The basic model emits an audible alarm and has a simple LED indicator. The Beacon indicator has a magnitude display and an advance warning, eye level indicator. If a person sets the alarm off as they walk through the Ferromagnetic Detection System, they should turn around, go back out divest themselves of the object that caused the alarm, and pass back through again.
CAUTION: This system is not meant to replace the normal procedures for MRI safety in regards to prevention of projectile effect precautions. The most advanced technology can never be 100% fool proof. No safety detector can be 100% effective against the introduction of all ferromagnetic objects of all sizes into the MRI room, but the correct installation and operation of this system combined with good training and other procedures can decrease the likelihood of a significant projectile accident occurring. Ferroguard® should not BE USED TO replace current PRE-MRI screening procedures and GOOD PRACTISE. The staff and patients safety is best served by the combination of conscientious screening protocols, thorough staff training AND the installation of a ferromagnetic detection system used in the correct manner.
Advice and procedures for good MRI safety practise should be sought from your local health and safety representative. In addition good practise advice can be found on a number of websites such as www.mrisafety.com or the American College of Radiology (ACR) publication on Guidelines for Magnetic Resonance Safety or in the UK the MHRA Guidelines for MRI safety.
The key features of the Ferroguard® system are:
- It is easy to install.
- It is very simple to operate.
- It is a passive system and so has no effect on electronic devices such as medical equipment or pacemakers.
- It will not alarm on non-ferromagnetic MRI safe metallic objects such as aluminium gas cylinders.
2. How does the Ferromagnetic Detection System work?
Ferroguard® works by detecting changes in the ambient magnetic field caused by ferromagnetic objects passing by. The ambient magnetic field is the earth’s own magnetic field plus the field caused by man-made objects, such as an MRI magnet. These ambient magnetic fields are steady and do not change with time, although they can vary from one place to another, for example the further you are from an MRI magnet, the lower the ambient field. However, at any fixed point the ambient field is steady. Ferroguard® monitors the ambient magnetic field with sensors in the units either side of a thoroughfare. Where there are ferromagnetic objects, i.e. things made from steel or iron, these cause a local disturbance in the ambient field around the object. If that object is passed near to Ferroguard®, then the disturbance in the ambient magnetic field is detected and an alarm is activated. Only ferromagnetic objects can be detected because only ferromagnetic objects disturb the ambient magnetic field. Because it is only ferromagnetic objects that can be pulled into an MRI magnet and present a hazard, Ferroguard® only alarms on these potentially dangerous objects. Non-ferrous metal objects such as aluminium, copper and some stainless steels can pass without causing alarm, but these are safe from being attracted into a magnet.
Yes there can be but Ferroguard® has been designed with patented twin sensor technology to minimize the occurrence of annoying false alarms. The Ferromagnetic Detection System audible alarm will only sound if two conditions are met. Firstly that there is something in-between the two sensor units and secondly, in addition to this, there is a magnetic disturbance that is large enough to be detected. Sometimes, however, there are magnetic disturbances caused by people not entering a MRI room but who are moving large ferromagnetic items nearby to the sensors, e.g. a porter moving a steel trolley in a nearby corridor, or somebody opening a steel filing cabinet that is close to the units. By themselves this will not set the alarm off, but if there is somebody who is “magnetically clean” passing the sensors at the same time then the two conditions mentioned above are satisfied and the alarm will be activated. This can be a cause of false alarms for all ferromagnetic detectors. It is a safe false alarm because it causes the person entering the MRI room to check themselves and try again. However minimising the occurrence of false alarms is vital to successful operation of a ferromagnetic detection system. Only Ferroguard® has the patented ferromagnetic detector Dual Sensor Technology designed to reduce the number of audible false alarms. Combined with the optional Beacon advanced ferrous warning indicator Ferroguard® can help reduce the number of audible false alarms to a minimum. The Beacon display allows users to monitor the local ferromagnetic disturbances and help judge if an alarm is a false positive or not.
Other false alarms can be caused due to the fact that some articles of clothing have ferromagnetic materials in them and these can set the alarm off if the person wearing them tries to enter the MRI room. Strictly speaking these are not false alarms but real ones. Examples are belt buckles, zippers, under-wire in bras, and steel tangs in shoes. Some operators are content to enter MRI rooms with these types of clothing because all the objects are attached and do not present a projectile effect hazard. In this case it is better to operate Ferroguard® in a lower sensitivity mode to prevent an annoyingly high number of alarms. However, some loose items of similar signal size may then be missed by Ferroguard®. Setting the sensitivity level is a critical choice for the customer in consultation with the installation engineer and is a balance between high detection levels and reducing the number of potential false alarms.
4. How reliable is The Ferromagnetic Detection System at detecting ferromagnetic objects?
We cannot claim that Ferroguard® has 100% detection rates for ferrous objects, indeed no system could. However, the probability of a substantial ferromagnetic object getting through without setting off the alarm is very small. As metal objects can have magnetic signals ranging from zero to very high, there will be a cut-off below which something that is ferromagnetic, but very weakly so, will not be detected and could therefore be unwittingly taken into an MRI room. Being magnetically weak, these objects will not present a significant threat from the projectile effect either. The Ferromagnetic Detection System can more accurately be thought of as a “projectile threat” detector as opposed to a ferromagnetic object detector. In particular the system is not designed to detect small metal implants within the body. Standard MRI pre-screening questionnaires should be used to catch such internal metal objects.
5. What is the smallest sized object that the system will detect?
Unfortunately there is no straightforward answer to this and it is not possible to be specific. There is a wide range of variability for ferromagnetic signal size from different ferromagnetic objects which physically are similar in size. In addition the two modes of operation for Ferroguard® combined with the user sensitivity setting determines the type of objects that will be detected. Having said that, we can make some general statements about what sort of things are likely to be detected in different set-ups.
| Object | Worst case – object passes half way between the two sensor poles that are separated by 5 feet (1.5m) | Best case – object passes less than one foot (0.3 m) from either sensor pole |
| Paper clips | Unlikely | Borderline |
| Zippers | Unlikely | Borderline |
| Spectacles | Unlikely | Likely |
| Pens | Unlikely | Likely |
| Coins (magnetic) | Unlikely | Likely |
| Keys (highly variable) | Borderline | Mostly |
| Under-wired bras (highly variable) | Borderline | Mostly |
| Mobile phones (highly variable) | Likely | Mostly |
| Scissors (highly variable) | Likely | Mostly |
| Note: More strongly magnetic objects than these i.e. the high threat objects such as gas cylinders, mop buckets, trolleys etc. are highly likely to be detected in both cases. | ||
The smallest object that can be detected depends on several factors, the main ones being the magnetic properties of the metal object, the distance from the sensor and its orientation. We need to discuss each of these in turn:
A. The magnetic properties of the metal object
This goes against intuition, but the magnetism of a metal object is not related to its size in any strong way. For example a very small piece of highly magnetized metal can have a larger magnetic signal than a very large metal with very low magnetization. If one took several identical metal objects, for example several identical pairs of scissors, their magnetic signals may vary considerably. This is because some may have been magnetised strongly by having being next to a magnet at one time. Others may have a modest magnetic signal because they were once shocked by dropping them on a hard surface, and others may have a low magnetic signal because nothing has happened to them.
It is important to remember that the force with which an object is attracted to an MRI magnet depends on its magnetic strength not its physical size. In other words, going back to our scissors, the ones with the high magnetic signals present more of a threat than the ones with low magnetic signal.
B. The distance from the sensor
All ferromagnetic detection systems are non-uniformly sensitive in-between the sensor poles. This has advantages as well as disadvantages. The magnetic signal from a metal object falls away very quickly with distance from it. If the object is close to one sensor, then that sensor sees the larger near-field signal. As the object moves away the signal decays rapidly until you reach the minimum in the centre, and the signal starts to increase again as the object approaches the other sensor on the opposite side. This gives rise to the U-shape profile shown in Figure 1. An object that is weakly magnetized but very close to a master or slave unit may give a larger signal than a more strongly magnetized object that is further away. Clearly, the further apart the two units are, the lower the sensitivity to object in the middle.
The advantage of this profile is that people who want to be very careful can deliberately walk close to a sensor and take advantage of the very high sensitivity there. Ferroguard® incorporates the most sensitive detectors of any of the FMDs currently available and for most installations the sensitivity is turned down from its maximum value to maintain a good balance between good detection rates and low false alarm rates.
C. The object’s orientation
The magnetic signal from a ferromagnetic object is shaped more like an American-football (or Rugby ball) than a sphere. The angle that it is carried through therefore has a bearing on the size of the measured signal.
For further technical information or if you have any technical enquiry please contact our engineers through support@metrasens.com
metrasens main page