Why It’s Important To Find Metal Before MRI

A few weeks ago I posted my layperson’s summary of why there’s even an issue with metal and MRI (click here to read that post on MRI and Metal). In this posting, I hope to explain why it’s so critical to find metals, particularly ferromagnetic metals, being carried by people or inside objects.

First, let’s get the issue of non-ferromagnetic metals taken care of.

Metals that aren’t attracted to magnets are non-ferromagnetic. However, even if they aren’t attracted to the magnet, non-ferromagnetic metals do still interact with the magnetic field. They can cause local distortions which can mess up MRI scans (making it very difficult to image anatomy close to any metallic implant or object). Orthodontic braces may make certain facial / brain scans difficult. Orthopedic implants may disrupt the MR imaging of areas right around the pin / plate / screw / rod. Different materials will have different disruptive properties, so never assume that you can’t be imaged simply because you have an orthopedic implant. Check with a radiologist.

Also, MR imaging makes use of radio frequency (RF) energy. Like magnetism, RF is non-ionizing (doesn’t break down DNA and give rise to cancers as X-ray energies have been shown capable of), and like magnetism RF interacts with electrically conductive materials. If an electrically conductive element is the right shape and/or size, the material may inadvertently serve as an antenna for the RF signal and the energy may disproportionately collect in the conductor. As you may remember from high school physics, energy doesn’t just ‘go away,’ it converts. in the case of RF energy, it converts to heat. If you have the ‘ideal’ antenna length and/or configuration for a particular radio frequency, it can cause remarkable heating and that heat can cause damage.

But just as with the issue of image disruption, don’t assume that the presence of an electrical conductor inside your body is an automatic contraindication for an MRI exam. Consult your radiologist.

For these reasons, it is important to identify all electrically conductive materials on or in the patient. But even with these real risks associated with non-ferromagnetic materials, the greatest threat, both in terms of numbers of incidents and fatalities, is ferromagnetic materials.

Now, let’s move on to ferromagnetic materials. Some of this may seem familiar to you if you’ve read my prior post on MRI and Metal, but work with me here and you’ll find that we delve a little deeper into what happens that makes ferromagnetic materials such a concern.

When a ferromagnetic material enters a magnetic field, it becomes a magnet itself. A ferromagnetic material accepts an induced magnetic field. Many ferromagnetic materials give up the field almost as easily as they accept it, so they aren’t significantly magnetized. Think of them in the same way as I’m a baseball fan… when surrounded by baseball fans, I can pretend to be interested. Away from other baseball fans, I have almost zero interest in the game.

So, if a ferromagnetic material becomes a magnet when exposed to another magnet, we now have two magnets, and we all know what happens when we bring two magnets together… [SNAP]

Actually, when we bring two magnets together, two distinct things happen. The first is that the two magnets work to align themselves to one another. We know that two like magnetic fields (positive-to-positive) will repel each other, but opposite polarity fields will attract. The natural action is that the magnets will work to rotate themselves in order to align their fields positive-to-negative. Compass needles are the living illustration of this as we count on them to rotate to align with the North Pole.

In the case of a ferromagnetic object brought near an MRI, let’s compare our two magnets. One weighs perhaps 12 tons and is bolted to the floor, the other is a pair of scissors that weigh a few ounces. Which of these two things is going to rotate to align itself? Right, the scissors.

So the smaller ferromagnetic objects that we wear, carry, or have placed within our bodies, are going to be subject to intense forces that will be working to align the magnetic polarity of the object to the massive (in weight and strength) magnetic polarity of the MRI magnet. This results in torque forces that can twist, turn and even tear whatever may be trying to hold them in place.

The other mechanical force that develops between two magnets is the one we’re all very familiar with… attractive force. As we bring two magnets that have aligned themselves to one another (or, as it the case of sticking a magnet to your fridge door, the non-magnetized large ferromagnetic material develops a localized magnetic domain in order to receive the fridge-door magnet you’re sticking to it), they snap together, often with startling speed and strength.

We describe this phenomenon in MRI as the ‘missile effect’ because ferromagnetic objects, propelled by enormous amounts of magnetic energy, can launch across the room with tremendous force towards an MRI. While magnetic projectiles may look as though they’ve been launched from a cannon, unlike ‘launched’ projectiles, these magnetic missiles don’t lose their inertia just because they hit something. Their singular mission in life is to reach the strongest part of that magnetic field and, if interrupted in their flight, they will incessantly continue applying pressure to try and push their way towards the peak of the magnetic field (typically the center of the MRI).

The torque from rotating ferromagnetic materials and the force of flying ferromagnetic materials have each killed people in the MRI, and caused many injuries, and done horrific damage to MRI machines and their components. This presents two major problems…

First, metal is everywhere. It’s in our shoes. It’s in the shiny filaments in our clothes. Our belt-buckles. It’s in the stuff in our pockets. It’s often in thing that are labeled ‘sand bags’. It’s in stuffed animals and even often in hospital pillows. Metal is an unavoidable part of modern life.

Second, as I described in my prior post on metal and MRI, it’s impossible to visually distinguish between magnetic and non-magnetic metals. Even if we know something is made out of wood, for example, doesn’t mean that we can be confident that it isn’t held together with steel screws or reinforced with a steel rod. So, not only is metal ubiquitous, but ferromagnetic metals are perhaps the most widespread types of metal used in contemporary life.

Because of the torque and attraction risks of ferromagnetic materials, many tools and devices made for use in the MRI environment that require the strength and durability of metal use of aluminum, titanium, brass and other non-magnetic materials.

It is the intersection of these concerns – that all types of metal are everywhere and that we usually want to admit non-ferromagnetic metals into the MRI room – that generates the need for a detection system that distinguishes only ferromagnetic material.

The name of this blog is the MRI Metal Detector for precisely this reason… while I frequently digress and discuss many things relevant to MRI safety, at the heart this forum is about the specific risks associated with ferromagnetic metals and, equally importantly, the contemporary tools that can be effectively deployed to help reduce those risks.

To help protect patients, staff, and millions of dollars of MRI equipment, I recommend (as do the VA, the ACR and others) that every MRI provider avail themselves of ferromagnetic detection to help more effectively screen people and equipment intended to enter the MRI suite.

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

119 thoughts on “Why It’s Important To Find Metal Before MRI

  1. Tobias Gilk Post author

    Roger,

    Even if you still had a steel penny within you, with a gram or two of steel, the amount of force that it could exert would be negligible, in my opinion.

    Most lens implants have little if any metal in them. If you can get the specifics on these implants from your eye doctor, and provide this to the imaging provider, my expectation would be that they would be able to positively identify the risks (if any), and would likely be able to scan you without delay.

    I hope this helps.

  2. Colin Hansen

    Question … since 2005 I have a titanium washer in my soft tissue from hip surgery; I had a serious motorcycle accident and broke my hip in several places. My surgeon told me that during the operation, he lost a titanium washer in my soft tissue … that this would prevent me form having an MRI done in the future. Since them, x-rays have confirmed that I have what appears to be a quarter in my right pocket – this is the titanium washer lost in soft tissue. If titanium is non-ferromagnetic, why is it an issue that I not have any MRI evaluations? I ask this question as I know need to have an MRI evaluation on my right knee. Thanks for your help!

  3. Tobias Gilk Post author

    Dear Colin,

    There are two potential safety concerns with metal on or in a patient’s body during MRI imaging. There’s the potential that the powerful magnetic field of the MRI will pull on a magnetizable piece of metal, and that the metal could heat up if it acted like an antenna and absorbed and concentrated the RF energies that are used as a part of the MRI scan process.

    Titanium is not a ferromagnetic (magnetic or magnetizable) metal, so that piece of the safety concern goes away. ‘Antenna effect’ heating is particular to the size, shape, and configuration of the object, and the degree to which it matches with the frequencies being used by that particular MRI scanner. I am unaware of any reported thermal MRI injury from ‘antenna effect’ by a piece of retained metal similar to that which you describe. A US quarter is simply too small, in my opinion, to be subject to this particular effect at MRI field strengths of 3.0 Tesla or lower.

    Tissues and materials could potentially still experience significant heating if they were particularly close to a transmitting element of the RF coil (but the MRI department staffers should make sure that you’re not so positioned).

    In short, in my opinion I think your surgeon gave you advice that was significantly overly-conservative on this issue. If you want to know specifically about the safety of getting an MRI with the hip repair and washer, your most accurate answer would come from the MRI provider.

    I hope this helps.

  4. Herman J. Vela

    I have a metal plate and 8 screws as an internal splint on my left radius bone installed in Saudi Arabia in 1995. Can I have an MRI on my spine in the l4 to S1 area?

  5. Benjamin Klassen

    Lets say sonebody worked in the machining industry and had a metal splinter lodged in their hand or paerhaps a few that had been forgotten about and grown under the skin. If they were to have an MRI scan what would happen to these tiny bits of metal?

  6. Rodge

    My doctor wants me to have an MRI done. I told him I have two shrapnels in my neck from a tank shell. He says we’ll let the radiologist decide if I should or should not get an MRI. I’m really worried because I remember one time I tripped a metal-detecting alarm in a federal building. Because of this, I think the shrapnel material is most likely ferromagnetic. What do you think? Thank you.

  7. John Smith

    I recently was turned away from getting an MRI due to some small metal wires in my skull – i had some plastic surgery following a motorcycle accident in 1975 – reattached orbital floor and some bones in nose. Apparently the heating and possible arcing would be pretty dangerous – especially near the eyes.

    It’s a bit of a downer – i need the MRI of my lumbar spine to identify some possible stenosis.

  8. Maura

    I had a MRI last Friday. I had forgotten that years ago I had an X ray reveal that a surgical pin was still inside me and failed to mention it to the MRI technician.
    My MRI went okay but it did not reveal the results I had expected about my back issues. Do you think the surgical pin could have prevented seeing some concerns?

  9. Lynn Bernier

    I have a spinal cord stimulator implanted in my left upper buttock with leads to limbo-thoracic nerves. I also recently lost my hearing completely in one ear with no trauma. I a on a heavy 12 day course of prednisone, hoping that it is a virus between the eardrum & nerve causing it to swell. If that is NOT the case, it is thought that I may have a cyst behind my ear which can only be visualized through MRI. I have read your article in regards to different types of metal & how they may interact with the imaging among other things. Is there anything that can be wrapped around my torso that will allow me to have. Head scan? Thank you for your consideration in this matter. Regards, Lynn Bernier

  10. Tobias Gilk Post author

    Herman,

    Most orthopedic implants need not automatically disqualify patients from receiving any MR exams. Your best bet is to contact the hospital or MRI center where you would have the scan done, and inquire with them about their specific implant / device screening and clearance requirements.

    I hope this helps.

  11. Tobias Gilk Post author

    Eric,

    Most orthodontics are not an automatic ‘no’ for receiving an MRI. There are some unusual orthodontics with magnetically-activated parts that could represent a risk, but as I say, these are not commonplace.

    I hope this helps.

  12. Tobias Gilk Post author

    Benjamin,

    If the metal fragments are ferromagnetic, they will want to rotate, like a compass needle, to align with the north-south direction of the magnetic field, and they may also experience some pulling forces. If the fragment is tiny, then the force that either of these actions impose (twisting or pulling) will be similarly tiny. Hands are rich with nerve endings and may be very sensitive, so you may feel an irritation. Before getting an MRI, please make sure you disclose any and all retained foreign bodies with the MRI provider, as they may have specific risk assessment methods or policies that govern.

    I hope this helps.

  13. Tobias Gilk Post author

    Rodge,

    “Airport style” metal detectors don’t differentiate between ferromagnetic and non-ferromagnetic metals, so I, personally, wouldn’t take the alarm from the detector as meaning anything more than ‘there’s metal there,’ which you already knew.

    I think your doctor is giving you wise advice… discuss this with a radiologist and let him/her assist with identifying the specific risks and benefits of you having an MRI exam.

    I hope this helps.

  14. Tobias Gilk Post author

    John,

    The risks of heating tend to be at their maximum (a) towards the middle of the length of the ‘tube’, and (b) nearest the side walls. If you are having a lumbar spine exam, then your lower back would be in the middle of the scanner, and your head / face would be both some distance away from the middle of the tube, and not likely near the side wall. In my opinion, there may indeed be a way for you to safely have an MRI scan of your lumber that does not introduce any meaningful risk of heating for the orthopedic implants in your head.

    I hope this helps.

  15. Tobias Gilk Post author

    Maura,

    The metal pin may have disrupted or distorted the image in the immediate vicinity, but likely wouldn’t affect the image at a distance away from where the pin is located.

    I hope this helps.

  16. Tobias Gilk Post author

    Lynn,

    There are three energies involved in an MRI exam… the ‘always on’ static magnetic field, the time-varying magnetic field (that makes the awful noise during the exam), and radio frequency energies. Each of these three electromagnetic forces happen over a different region of the MRI scanner.

    There is nothing (practical) that you could ‘wrap’ around yourself to protect from exposure to either the static or time-varying magnetic fields. There are products that ostensibly shield from radio frequency exposure… but these may not be necessary.

    First, I would encourage you to find out if your stimulator is MR Conditional, which would mean that it has been identified as being safe to scan under specific sets of conditions. If your stimulator is, that may be the only thing you need to know for your particular scan to be able to go forward.

    If, however, your implant is not MR Conditional (or all information about the particulars of your implant are lost and you can’t determine whether or not it is MR Conditional), then it might be possible to do a risk assessment based on the distribution of those electromagnetic forces and the degree to which they would interact with your implanted stimulator. There is no way to ‘shield’ from exposure to the static magnetic field, but the position of the stimulator relative to what is being imaged (which part of you) may move the implant far enough away that risks for either / both the time-varying magnetic field and radio frequency energies are effectively nil.

    If your MRI provider has difficulty conducting this type of assessment, you may suggest that they look at a service I offer to help them with it: http://www.gilkradiologyconsultants.com/wp/contracted-mr-implant-risk-assessment-services/

    I hope this helps.

  17. Mark

    Hello;

    I may need an MRI of the auditory nerve; I have a permanent dental splint on the front six top teeth. This is definitely a conductor (I tried with a tester) and definitely NOT ferromagnetic (I tried sticking a neodimium magnet to it and nothing).

    Do you think the induced currents may be a concern?

  18. Tobias Gilk Post author

    Mark,

    Most Western orthodontic materials do not pose safety risks for most MR studies. The odds are quite good that your splint does not represent any meaningful risk to your prescribed study. It is important, however, that the site that is providing the MRI exam for you conducts the screening and any indicated risk analysis.

    My recommendation would be to contact them prior to your scheduled MRI (a few days before, preferably), and discuss the specifics of your orthodontic implant with them. I suspect it will be fine, but they’re the ones that will ultimately make that determination.

    I hope this helps.

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