MRI And Metal

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…

As far as the MRI machine is concerned, there are two different types of metal, ferromagnetic and non-ferromagnetic. You may remember back to high school chemistry and the periodic table of elements where many of us learned (and then promptly forgot) that Fe is the symbol for iron.

Fe - Iron from the Periodic Table

Fe - Iron from the Periodic Table

“Fe”, the symbol, is derived from ferrum, the Latin word for iron. Ferromagnetic does not mean that a metal contains iron, but rather that the material has magnetic properties as iron can.

Ferromagnetic metals are iron, cobalt and nickel. These raw ingredients are common in many, many things made from metal, including (likely) the steel grommets in your shoes, to the zipper in your pants, to components in your wristwatch. Another common area to find these metals is in batteries, such as those found in your hearing aid, cell phone and iPod. There are a few non-metal ferromagnetic materials, but these are not very common.

Alright, alright, already… enough chemistry. What does this mean?

When exposed to magnetic fields, ferromagnetic materials become magnets themselves. You can prove this yourself with a fridge-door magnet and a few paper clips. You’ll probably find that paper clips right out of the box aren’t capable of magnetically ‘sticking’ to one another. If you stick one to a chunky fridge-door magnet, however, that paper clip is now magnetized and will likely be able to magnetically ‘stick’ to another paper clip. The length of the magnetic chain of paper clips you can create is a function of how strong the fridge-door magnet is and the magnetic properties of the paper clip steel.

Now, the exact same thing happens with ferromagnetic metals approaching the MRI, but a crucial difference is the distance at which the materials get attracted. With your fridge-door magnet test, the paper clip needs to be touching (or very nearly so) the magnet before the attractive effects are felt. MRI’s, by virtue of the fact that they’re both 1,000′s of time stronger and larger than your fridge-door magnet, can exert profound attractive force at a good distance away from the magnet.

The size and strength of MRI magnets is so great that people have been trapped, injured, and even killed by the force of ferromagnetic objects attracted to the MRI. From concealed roller-skate tennis shoes, to steel-reinforced furniture, to conventional hospital wheelchairs and gurneys, to steel oxygen cylinders, all of these normally harmless (outside the MRI suite) items become life-threatening when subjected to the enormous pull of the MRI’s magnet.

Not all metals are ferromagnetic. In fact, in an MRI suite a concerted effort is usually made to rid the area of ferromagnetic materials and use non-ferromagnetic replacements whenever possible. Non-ferromagnetic metals include aluminum, titanium, brass, copper, and many others. These (and other) non-ferromagnetic metals can present other problems and hazards during MRI imaging, but that’s a topic for another day.

It is almost impossible to determine whether a material is ferromagnetic just by looking at it. In fact, even sometimes when you know what an object is made of, it still isn’t enough to know whether it’s ferromagnetic or not. Stainless steel, is one of these examples.

Stainless steel is not a metal, but rather a family of recipes for metal. Some stainless steel ‘recipes’ (alloys) call for ingredients with ferromagnetic properties. Others which include ferromagnetic ingredients are specially formulated to change the structure of magnetic materials into non-magnetic versions of the material. These special ‘de-magnetized’ stainless steels can become ferromagnetic if the steel is manipulated (shaped, bent, heated, or stressed), so even magnetically ‘safe’ stainless steels can become ‘unsafe’ under certain circumstances (a change that isn’t observable to the eye).

It is remarkably difficult to distinguish magnetically ‘safe’ metals from magnetically ‘unsafe’ metals, either by simply looking at them or, sometimes, even if you know what the metal is. As a result, MRI facilities must assume all metals to be magnetically unsafe unless and until they’ve been verified to be non-magnetic.

So, how do MRI facilities distinguish magnet-unsafe metals? They can use magnets, which shouldn’t be used on patients or sensitive equipment, limiting their applicability. The safer option (and arguably more effective, to boot) is to use a ferromagnetic detector, at least on patients and sensitive equipment.

Ferromagnetic detection instruments, such as the Mednovus products, should be used to help identify magnetically-unsafe materials. This is the standard established by the American College of Radiology, the VA’s MRI Design Guide, and even recommended by the Joint Commission in Sentinel Event Alert #38.

As a patient, it is vital to take seriously the admonitions against wearing or carrying metal into the MRI suite. If you have shrapnel, penetrating metal injuries (particularly in the eye), or any surgeries, implants or prosthetics, it’s critical to have the full information on each to share with your MRI provider. Metal inside the body may not fly across the MRI room like a loose oxygen cylinder (don’t believe what you see on House), but the twisting an pulling that the magnet will exert on an internal ferromagnetic object can be just as dangerous. Active implanted devices, such as pacemakers or nerve stimulators, present particular problems because of both the magnetic attraction and potential interference with the normal function of the device.

Patients should also actively seek out MRI providers that conform with the contemporary safety recommendations, including the use of ferromagnetic detection. You can even contact Mednovus when you want to find providers near you who have this technology available.

Providers of MRI services should make sure that the pre-screening and safety services they provide are in accord with the contemporary best practices, including the use of ferromagnetic detection. With available ferromagnetic detection products equal in cost to only a few hours worth of technical revenue, there’s no financial rationale for not providing this valuable safety benefit to patients and staff. Plus, when weighed against the costs of ferromagnetic object accidents, these instruments of safety are clearly effective risk-management investments.

In all cases, metal brought to the MRI suite (either inside or outside the body of the visitor) should be scrutinized by a trained MRI staff person. This investigation should be aided through the use of ferromagnetic detectors, both to help characterize the hazards of any particular object and to help find ferromagnetic materials that weren’t caught in the prior screening process.

Tobias Gilk, President & MRI Safety Director
Mednovus, Inc.
Tobias.Gilk@Mednovus.com
www.MEDNOVUS.com

57 thoughts on “MRI And Metal

  1. Tobias Gilk Post author

    Julie,

    I would think that pretty much any plastic would be fine.

    I wish you well.

    Tobias

  2. Mary

    Is there a specialized MRI that can detect and/or minimize artifact from stainless steel? I have an ankle injury which had a stainless steel plate and screws but need an MRI for diagnosis of tendon damage but was told that no MRI can be done because of the metal artifact.

  3. Tobias Gilk Post author

    Mary, thank you for your question. Some MRI scanners do have software that allows for special “metal suppression” imaging sequences. These can improve image quality (reducing artifact / distortion from metal materials), but they can’t totally eliminate the negative effects of metal. The specific type and grade of the stainless steel implant will also be a determining factor in how much the metal suppression techniques are effective. Irrespective of the grade of stainless steel, if the implant is very close to the tendon that your physician wants the image of it may not be feasible.

    To find out if it may be possible, I would suggest contacting the radiology department of the largest hospital around, or a hospital associated with a medical school. Ask to speak with the most senior MRI technologist, and explain to him / her your situation. Ask about their capability to perform metal suppression sequences. If they don’t have the right MRI scanner (or software options for the MRI scanner), ask them who they think might, then call them.

    The technologist may tell you that the plate and screws are just too close to the tendon that the doctor wants to see, and that no contemporary MRI will be able to get useful images that close. I very much hope that this isn’t the case for you.

    Best of luck to you!

  4. Mary

    Is there a specialized MRI that can minimize the artifact effect from stainless steel? I have 6 screws and a plate in my ankle but need an MRI to diagnose tendon and ligament problems, the hardware cannot be removed for another 7 months.

  5. Tobias Gilk Post author

    In general, you want to find a lower-field-strength MRI system (the less the magnetic field, the less artifact). And like your personal computer, it’s the software that makes the magic happen. The software version and options that the provider has will make a significant difference. There are generic settings that any MRI operator can use (see http://radiopaedia.org/articles/metal-artifact-reduction-sequence, or http://www.revisemri.com/blog/2011/metal-artefact-reduction/), and there are more effective proprietary software systems, with acronym-rich names that vary depending on the system manufacturer (MARS, MAVRIC, SEMAC).

    The only way to sort through these variables will be to talk with the MRI provider, and let them evaluate the options based on your implant, the anatomy that needs imaging, and the capabilities of the hardware and the software of the MRI center.

    I hope this helps.

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