MRI Metal Detector Blog

Ferromagnetic detection is a vital part of the pre-screening for persons about to enter the MRI magnet room, but it’s only one part of the overall sequence.

First, before we jump into the issue of where in the sequence ferromagnetic detection is best deployed, it’s important to break pre-MRI screening into its two constituent parts: clinical screening and physical screening.

Clinical Screening:

Before being brought to the MRI magnet, everyone (and this means patients, visitors and staff) needs to be screened for contraindications. Most often we think of pacemakers, but other contraindications include nerve stimulators, insulin pumps, prosthetics, halo vests, and a number of other objects. The screening is typically accomplished through the use of forms to help the subject identify any clinical risks for the MRI provider. The screening form is then to be reviewed between the patient and the MRI Technologist.

Once clinically cleared of contraindications for the MRI exam, then the subject should proceed to the next step…

Physical Screening:

Contrasted with the widespread uniformity of the clinical screening, the physical screening takes very different forms at different provider. However, all have the same objective, namely, to remove ferromagnetic materials from the subject and keep them away from the MRI scanner. Even small quantities of ferromagnetic material can cause artifacts in the MRI scan when near the imaging volume. Small ferromagnetic items, such as bobby pins and nail clippers, have caused serious harm when propelled by the magnetic force of an MRI magnet. And obviously, large items such as oxygen cylinders and floor polishers can have catastrophic consequences if brought to the MRI room.

Some MRI providers have outpatients simply empty their pockets, others provide gowns or scrubs for MR patients to change into, and all should use ferromagnetic screening to help verify patients’ compliance with screening instructions.

When performed in the above order, providers avoid gowning patients only to find out that the patient can’t receive the MR exam. Additionally, when clinical screening is accurately completed first, the Technologist has done everything within his or her human capabilities to mitigate the contraindication risks associated with exposure to magnetic fields. Although it is impossible to completely eliminate the chances of accidents, by following the recommended industry-standard procedures of  conscientious clinical and physical screenings followed by properly-performed ferromagnetic detection, the safety of your MRI center has been significantly enhanced.

Some of the most sensitive ferromagnetic detectors currently available use passive magnetic fields to improve sensitivity. These GS (Greater Sensitivity) detectors use a localized DC field (i.e. stronger versions of a similar type of the permanent magnet that holds your notes on your refrigerator door). While the magnetic field strength very close to the GS detector can exceed the 5-gauss threshold, that limit is for persons who haven’t been successfully cleared for MRI contraindications (a step which was just completed if the pre-MRI screening was conducted in the proper order).

While patients and caregivers should be concerned about exposing unscreened persons to the extraordinarily powerful magnetic fields around the MRI, momentary exposure of post-screened persons to the passive “fridge-door” magnetic fields of a GS ferromagnetic detector is very, very small on the relative risk-o-meter. And this minute risk comes with an enormous potential safety upside…

No ferromagnetic detection system on the market from any manufacturer is intended (or approved) for finding objects internal to the body of the patient. However as an incidental finding, ferromagnetic detectors have alarmed on the ferromagnetic content of implants (including pacemakers) that were disavowed by the patient in the clinical screening process. While ferromagnetic detection should never be used in lieu of conscientious clinical screening, they have helped to identify critical contraindications that may have otherwise jeopardized the safety of the MR patient — had they not been found by the ferromagnetic detector.

And the relative risk of being exposed to 5, 10 or even 100 gauss as a part of a physical pre-screen (particularly when already cleared of clinical contraindications) is microscopic, when compared to either the risk of the planned exposure to 15,000 / 30,000 gauss, or the potential benefit of identifying a contraindication that the patient themselves didn’t communicate.

The take-home messages from this are these:

• MRI providers should provide as thorough and comprehensive clinical screening as humanly possible for everyone approaching the MRI.
• Once the clinical screening is complete, the provider’s standard physical screening (emptying pockets, changing into scrubs, etc…) should be conducted as appropriate to the MR patient / visitor.
• And following the clinical and physical screenings, patient / visitor compliance should be verified with a ferromagnetic detector.
• If these industry-standard procedures are correctly followed, there should remain only minute (accepted) risks associated with exposure to any magnetic field, either the enormous field of the MR or the comparatively tiny field present in GS detectors.

Clearly, providers should feel free to use whatever ferromagnetic detection they wish – from their choice of manufacturer – in order to conform with ACR, VA and Joint Commission guidance, whether it be an instrument which relies on only the trace-magnetism of the Earth’s own magnetic field, or one in which the detection sensitivity has been enhanced through the use of a locally-provided, passive DC magnetic field as found in GS ferromagnetic detectors.

My recommendation is always to use a detector with the greatest possible sensitivity. Because, while they are wonderful instruments that can make a substantial improvement in a provider’s MR safety protocols, ferromagnetic detectors are dumb. They can’t differentiate ‘good’ ferromagnetic material from ‘bad’. These sorts of value judgments should be made by a trained MR technologist and not by a machine.

In my opinion, ferromagnetic detectors should be used to help find every piece of ferromagnetic material that they can, so that the Technologist knows what is about to enter their magnet room (and can make re-screening decisions as appropriate). The greater the sensitivity of the detector, the more informed those Technologist decisions will be.

Pass-through ferromagnetic detection systems, such as the newly released Mednovus Sentinel® GS 2.0 portals, also have user-adjustable sensitivity settings, so that the system can be ‘dialed back’ as needed for special circumstances, further supporting the concept of having the instrument with the greatest sensitivity, and tuning it to meet your specific needs.

As evidenced by repeated, and increasing MRI projectile accidents, there is enormous room for improvement from the prior standards. Effective pre-screening of MRI patients, including the use of ferromagnetic detection at the appropriate point, can make an significant difference in the safety of the MR exam. Providers should turn to the current best practice guidance and compare their pre-MRI screening processes, making any indicated changes to help assure the safety of their patients, visitors, and staff.