A ferromagnetic detection system (FMDS) is available for use in the MRI environment. Various versions of the FMDS currently exist and may be a portal system, pillar device, or a handheld version. These devices are specially designed to only detect ferromagnetic objects. Other materials, such as aluminum and copper are nonferromagnetic and, therefore, are not detected by an FMDS. For example, an FMDS will detect a steel gas cylinder and indicate a positive alarm but it will not detect or alarm on an aluminum one. Thus, the FMDS will only alarm on potentially dangerous objects relative to issues related to magnetic field interactions. Utilizing an FMDS in the MRI environment is recommended by several influential organizations concerned with MRI safety, including the American College of Radiology (ACR) and the Joint Commission.
As the name suggests, ferromagnetic detection systems selectively detect ferromagnetic objects, ignoring nonferromagnetic items. Because only ferromagnetic objects pose a missile-related hazard, an FMDS detects threats and works by monitoring the ambient magnetic field using magnetic sensors. The ambient field is a combination of the fringe field of the magnet and the Earth’s magnetic field plus the contribution from architectural steel and any other stationary steel objects in the vicinity. A ferromagnetic object distorts the ambient field in its vicinity. If it is brought close to an FMDS, the distortion is detected as a changing magnetic field and an alert is triggered.
Importantly, an FMDS ignores static magnetic fields (i.e., magnetic fields that do not change with time). In practice, this means that the FMDS is only sensitive to changing magnetic fields, or moving ferromagnetic objects. The FMDS is insensitive to a stationary ferromagnetic object, so if an object is placed near to the FMDS it will be detected as it is put in place but thereafter ignored, until it is moved again. The reason for this is that the ambient magnetic fields are very large compared with the magnetic perturbations caused by ferromagnetic objects, and it is difficult to measure tiny changes on a large background field. The large static background is therefore removed by the FMDS by filtering it out, irrespective of whichever components make up that static magnetic field including the magnet of the MR system, the Earth’s magnetic field, or a metal cabinet next to the FMDS.
For a handheld FMDS, the object may be stationary but the FMDS is moved, so it is the relative motion that is important when using this type of device. A stationary FMDS detects moving ferromagnetic objects only.
Overall, the use of a stationary FMDS significantly enhances the safety level of MRI facilities, although non-ideal aspects of the current systems can impact the day-to-day activity of the MRI staff members, especially the MRI technologists. The individuals who are believed to cause most missile-related accidents are non-MRI workers who enter the MR system room. While MRI technologists may not be able to constantly supervise and control access to the MR system room, an FMDS can accomplish this and, thus, provides a warning to a person entering when the door is unsupervised. If non-technologists cause “alarms”, they should be trained to seek the advice of an MRI technologist before entering the MR system room.
When an FMDS alarms, its purpose is to prompt the MRI technologist to investigate. As an example, an MRI technologist pushes an MR Conditional gurney into the room and the alarm sounds, as always, because the gurney has ferromagnetic components. The correct response of the MRI technologist is to stop and to perform a final check of the gurney. Is the gurney acceptable to use with this particular MR system? Is there an oxygen cylinder or IV pole present that is MR Unsafe? Is there a ferromagnetic object under the sheets? The incorrect response is to ignore the alarm because an MR Conditional gurney will always trigger an alarm. Many accidents have occurred because of ferromagnetic items being placed on top of gurneys or underneath sheets. In this case, the alarm from the FMDS acts as a reminder to do a final check.
The Patient Screening Process Using an FMDS. Screening the patient using an FMDS is an additional step in the screening process and is the last step before the MRI examination. It is important to note that it is not a replacement for any aspect of the screening procedure but rather it is an addendum that adds a final objective check prior to performing MRI. This type of screening normally takes less than one minute to complete provided that there is no positive alarm. It takes somewhat longer to use a handheld FMDS because it has to be manually scanned over the entire area of the patient’s body. Ideally, there will be a line at the bottom of the screening questionnaire that records the result of FMDS screening and any observations or actions as a result.
If a patient passes the FMDS screening without a detection occurring (i.e., no positive alarm), this should be documented on the screening form and the patient may then proceed with the MRI examination. If an alarm occurs, then the patient must be investigated for the presence of a ferromagnetic object and it should be removed (if possible). Once this has been done, the patient should be re-screened using the FMDS. If a ferrous object cannot be found, the FMDS screening should be repeated in case the original result was a false alarm. For genuine alarms that cannot be resolved, the MRI technologist must then suspect the possibility that the ferromagnetic object is internal, being either an implant or a foreign body. The patient’s history should then be thoroughly checked before proceeding to MRI.
Using a Wall-Mounted or Piller FMDS Versus a Handheld FMDS. The use of a wall-mounted FMDS provides head-to-toe, whole-body screening that is easy to accomplish and fast to perform for cooperative ambulatory patients. For non-ambulatory patients the only means of screening with a wall-mounted FMDS is to use a ferrous-free gurney or wheelchair and perform a “drive-by” in two directions parallel to the FMDS, pushed by a ferrous-free MRI technologist. However, this process will not provide the close range required to detect the smallest objects, but is nonetheless useful for the detection of larger personal items.
The use of a handheld FMDS is somewhat similar to using a standard handheld metal detector (e.g., the type used at airports), insofar as it must be swept or scanned over the surface of the body at close range, usually within 5-cm of the surface. The sensing area is quite small (approximately 5-cm x 9-cm) so care must be taken to ensure that screening occurs with no gaps while maintaining a relatively short, stand-off distance. Due to this being a manual process, the quality and reliability of the screening depends on the person performing the scan using the handheld FMDS.
Staff members may be reluctant to screen an intimate patient area, such as the groin. Notably, the only available handheld FMDS at this time has a strong permanent magnet within it to boost the magnetization of ferrous objects. Because of this, this type of handheld FMDS should not be used close the eyes or near cardiac pacemakers or other similar implanted devices in case the magnetic field poses possible problems. A handheld FMDS can be used for a non-ambulatory patient on an MR Conditional gurney or wheelchair, as opposed to ferrous-free ones. With a gurney, the patient needs to turn over from one side to the other to get full coverage. With a wheelchair, it is more difficult to get full coverage unless the patient can stand for a short while.
Using an FMDS to Screen for Metallic Implants, Devices, and Foreign Bodies. The question surrounding the detection of ferromagnetic implants, devices, and foreign bodies is a topic with growing international interest, as reported by Shellock and Karacozoff (2013). Currently the use of a patient screening FMDS is not approved by a governmental entity or organization and, thus, it is not intended to be used for the specific purpose of detecting implanted objects. However, because human flesh is effectively transparent to ferromagnetic detection, the distinction between ex vivo and in vivo ferromagnetic objects is merely one of range. To date, studies on implant detection have been conducted as well as on foreign bodies. Although research is at an early stage, the initial results indicate that the use of a patient screening FMDS is capable of detecting many in vivo ferromagnetic objects and this has important implications for patient safety in the MRI environment.
[Excerpted from Mark N. Keene, Ph.D. Using Ferromagnetic Detection Systems in the MRI Environment, MRI Bioeffects, Safety, and Patient Management F.G. Shellock and J.V. Crues, Editors. Biomedical Research Publishing Group, Los Angeles, CA, 2014]
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