Many people just learning about MRI safety and hazards ask very similar questions. One of most frequent is, “why do I have to remove all metal before an MRI,” or it’s corollary, “can I get an MRI with some metal on (or in) me?” To answer these questions, let’s start at the very beginning…
27.5 million people… That’s how many people the healthcare market research company IMV estimates received MR exams in the US in 2007. Other estimates put the number over 30 million, but regardless of whichever number you use, we’re talking about a lot of people (27.5 million is roughly equivalent to those living inside all of the 13 most populous cities in the US)!
And when I say that these millions of people don’t want the 175% raise they’ve received, I’m not referring to their salary. No, the raise that these millions of people got — and most definitely don’t want — is … Click To Learn What 175% Raise They Got…
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.
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…
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.Tobias Gilk, President & MRI Safety Director Mednovus, Inc. Tobias.Gilk@Mednovus.com www.MEDNOVUS.com
In short, no. Hand-held magnets do not do the same job that ferromagnetic detectors do.
In many MRI facilities, foreign materials brought by people to the MR suite are tested for magnetic field hazards with high strength hand-held magnets. Ones designed specifically for MRI screening are far stronger than the ones holding up my daughter’s artwork on my refrigerator. Some of these ‘test’ magnets can be 1 Tesla at the surface (10,000 gauss)!
These extremely powerful hand-held magnets can help users differentiate between superficial materials that are, and are not, ferromagnetic, but the extraordinary strength of these magnets introduces a number of additional cautions which limit their use.
First, the key word in the paragraph above is ‘superficial.’ The magnetic field of all permanent magnets drops off precipitously (field strength drops with the cube of distance… double the distance and the magnetic field is cut to 1/8th the original value), so permanent magnets will be useful for distinguishing ferromagnetic materials only at or near the surface of an object. Ferromagnetic components below the surface may go undetected by a hand-held magnet, but rest assured that the MRI will find them if those objects make it into the scanner room!
Second, the potential forces exerted on a ferromagnetic body with magnetic field strengths of near 1 Tesla mean that shallow ferromagnetic material within the body of the patient could be moved, perhaps dangerously, by these very strong magnetic forces. But if the purpose of screening is to prevent accidents instead of preemptively causing them, hand-held magnets are poorly suited for patient screening.
Third, if screening medical equipment instead of patients, even some pieces of equipment designed for use in MRI scanner rooms have maximum allowable static and dynamic magnetic field values. Sticking a 1 Tesla magnet all over an anesthesia machine may wind up having some unintended consequences with regard to operation.
Lastly, 1-Tesla magnets stick hard to things. While the hand held magnets aren’t weighty, their magnetic force can require a bit of elbow-grease to get them separated from the cart or medical gas cylinder to which they got stuck. No, it’s not like it becomes epoxied on, but wielding one of these high strength permanent magnets is not a trivial affair.
Each ferromagnetic detection product has its own limitations, so I’m not attempting to state that FMD systems are the perfect solution to the hand-held magnet problem. Hand-held magnets can be useful, in a limited range of uses.
When it comes to the recommendations of the ACR Guidance Document for Safe MR Practices, or the Joint Commission Sentinel Event Alert (#38) on MRI Accidents and Injuries, or the U.S. Veterans Administration’s new MRI Design Guide, the experts all seem to have recognized the benefits of ferromagnetic detection and made a clear distinction between the new technology and the old custom of using permanent magnets to test for safety.Tobias Gilk, President & MRI Safety Director Mednovus, Inc. Tobias.Gilk@Mednovus.com www.MEDNOVUS.com