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

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

  1. Tobias Gilk Post author

    I would suggest that you and your physician check with a radiologist before scheduling an MRI. It is an open question as to whether an MRI would be able to indicate the presence of a needle next to a large metal implant. The implant (even if it’s titanium) will distort the magnetic field (and hence, the image) of the area immediately around it. Your physician and a radiologist, in concert, may be able to figure out what imaging tools (if any) are the best to help identify where this foreign body is. I hope this helps.

  2. Rhonda Odom

    My doctor left a screw (part of a screw) in my hand while removing a titanium rod from my hand – within 3 months, he ordered an MRI that has a “blooming artifact” that is distorting the MRI – should a CT Scan have been done instead?

  3. Corey

    I went to have an MRI of my knee and told them I have a piece of metal in my hand from a hammer that exploded about 12 years ago while knocking out pop rivets. I was told if I had the MRI that the magnet could twist and pull the metal damaging nerves and possibly tearing an artery. They said if I have the MRI it could cause internal bleeding they would be unable to stop and that it would kill me. Need less to say I am not having an MRI until this metal is cut out.

  4. Tobias Gilk Post author

    Rhonda,

    Hindsight is usually 20/20. It may not have been known that there was a retained part of the screw, or that it would negatively interfere with the MRI image. Different MRI sequences have vastly different sensitivities to interference from metal objects, so it’s also possible that the screw remnants interfered substantially with some images, and less so on others.

    MR imaging and CT produce images from very different methods, which means each type of scanner is inherently better than the other for some types of imaging. Your doctors should know which is preferable for the condition they were hoping to diagnose, so I presume that MRI was recommended because it was believed to be superior (or perhaps the only option) do diagnose what they wanted to diagnose. CT may have been an option, but it may not have been.

    I hope this helps.

    Tobias

  5. Tobias Gilk Post author

    While I think that there is justifiable concern when a patient has a retained piece of metal in their body, I think that ‘unstoppable internal bleeding and death’ are a pretty dramatic overstatement of the potential risks.

    I think it’s safe to assume that a piece of a hammer would likely be ferromagnetic, and therefore would be subject to the pulling and twisting forces of a magnetic field. But depending on the MRI, there’s the potential that you could keep your hand over your head and have it far enough away from the scanner that (given the size of the fragment) the magnetic forces would be appreciably lower than if your hand was right at the mouth of the MRI (where the ‘pulling’ forces tend to be the greatest).

    Additionally, while there are nerves, vessels, and organs throughout our bodies, in many cases our body’s own natural defenses to foreign bodies will encapsulate the foreign body in scar tissue to help immobilize it. I think it would be a reasonable step – if your doctor wants you to have an MRI – to have X-rays taken of the hand to try and identify if there are nerves or vessels uncomfortably close to the metal fragment (if this hasn’t already been done).

    It is reasonable to assume that a ferromagnetic foreign body will experience pulling and twisting, and at its mildest this could result in only discomfort, or bruising which – if it were me – might be perfectly acceptable outcomes to get a recommended MRI of my knee. Of course there is at least the possibility of more significant injury.

    If a careful risk assessment hasn’t been done (looking at the size, shape, and position of the retained foreign body) involving a MRI radiologist, I should think that would be an appropriate next step.

    I hope this helps.

    Tobias

  6. Brenda Glass

    I had a disc ectomy with Fusion in my neck at 5 and 6. I have no idea what type of metal was used as the plate or screws however an ortho surgeon wants to do an MRI if my lumbar area and pelvis and both hips. He said the metal in my neck is not going to the magnetic because of the type metal used. He does not have my records They are old and I am going simply by his word that this will not harm me. Is it standard to always use non-magnetic metal in a disc ectomy with Fusion? Am I safe to have this MRI of my lower body done?

  7. Tobias Gilk Post author

    Brenda,

    There are three relative risk ‘groupings’ for patients with implants receiving MRI exams, and these come from the three electromagnetic energies used by MRI… static magnetic fields, time-varying gradient magnetic fields, and radio frequency. If your fusion implant is more than 30 cm from the highest point that the ortho surgeon wants to image, then the predominant safety factor would be static magnetic fields (attraction and torque). Depending on the specifics of the MRI system upon which you’re being scanned (and this is a question that can only be answered by the MRI provider), gradients and RF may be of zero clinical concern. Check with your provider.

    With regard to the static magnetic field risks, I don’t catalogue the metal makeup of medical implants, however, the vast majority of those that I’m aware of today are using non-ferromagnetic or weakly-ferromagnetic materials. Depending on how superficial the components of the fusion are (below the skin), it might even be possible to test this with either / both a strong hand-held magnet and/or a ferromagnetic detector.

    Ultimately the safety evaluation (and the resulting decision as to whether it is appropriate for you to proceed with an MRI exam) will be that of the radiologist working for the hospital or imaging center where you get your MRI. S/He is ultimately the one to help you resolve any questions or concerns that you might have.

    If you find that the MRI provider struggles with the risk assessment piece of your implant evaluation, you can look at getting a single scan device risk assessment that might help collect and (to the degree reasonable) quantify the risk information.

    I hope this helps.

    Tobias

  8. Rachel

    I had an MRI on my right knee to detect the extent of a cartilage injury.
    I have a needle in my left foot discovered a few years ago on an X-ray. I have a vague recollection of doing something as a child, but it’s never caused me any problems.
    The radiologist was happy to continue with the MRI on the basis that it had been in my foot for 40+ years.
    Now though, a week after the scan, my foot is slightly swollen (where the needle is) and uncomfortable to walk on; and there is a dull, continuous ache. I’m getting rather concerned …

  9. Tobias Gilk Post author

    Rachel,

    While your left foot wasn’t being imaged, it was within the magnetic field (and, ironically, may have been near the part of the MRI scanner where the magnetic field and attractive forces are the greatest, depending on the MRI scanner).

    The needle in your foot has had 40 years to ‘find’ the spot where it’s out of the way of the mesh of bones, muscle, and tendons that are throughout the foot. It has probably been relatively undisturbed for many, many years (as indicated by the fact that you had largely forgotten about it). When this happens, often the foreign object winds up in a spot where normal movement doesn’t affect it.

    But then you go in for an MRI, and the needle is now subjected to forces that it’s never (before) had to respond to. Depending on the size, shape, and orientation of the needle fragment, those new forces could have been very irritating, or even mildly damaging to the surrounding tissues. The foot is such a tightly-packed structure, that even a small amount of inflammation / swelling can be very uncomfortable.

    The force that a ferromagnetic object can exert near an MRI is a function of many variables, but one of them is the mass of the object. A needle fragment is pretty small, which limits the amount of force it can exert.

    If you have a concern that there might be a new injury as a result of the motion of this needle fragment, I would encourage you to visit your physician. Were it me, and it didn’t rise to the level of pain / making it difficult to walk, I might try anti-inflammatory medication for a day or two to see if that made a noticeable difference.

    I hope this helps.

  10. STACEY GREEN

    I had MRI done and ,I had a ring on and the tec told me to put my hands across my stomach and I felt my in sides boiling. now i have a very bad oder coming from my mouth.is it possible that my insides are cooked?

  11. Suzanne

    Hi, I’m reading up about MRI’s and metal because my partner is getting an MRI of the brain tomorrow and he has small metal retainer-like pieces which are cemented to his teeth. They have been there for about 10 years. His doctor said to ask a dentist if it was safe, who said to ask the radiologist. A call to the radiologist was very brief – they didn’t seem concerned and said it would probably be fine. This evening we tested to see if the pieces are magnetic, and they are. I’m wondering if it’s still possibly safe to have an MRI, and what could happen?

  12. Tobias Gilk Post author

    Stacey,

    A properly functioning MRI scanner is limited to general body temperature increases of 1 degree Celsius. While there are potentials for concentrated (focal) heating, in the absence of a foreign body within you that is profoundly unlikely.

    I hope this helps.

  13. Tobias Gilk Post author

    Suzanne,

    It seems to me that there are two questions wrapped up in your query.

    Safety: Magnetic attractive force is a function (in part) of the size and shape of a magnetizable object. Here we’re dealing with very small wires that have been successfully cemented into dental work for a decade. The amount of force that wires of a few grams could exert, even in a very strong magnetic field, is small because of the low mass of the wires. They might experience significantly more stress and strain during chewing than they will during an MRI.

    Image Disruption: The amount of magnetism in a metal object will also affect the MRI scan to an extent. Like attraction forces, the mass of the metal has an impact on this (and again, we’re dealing with grams), as does the type of MRI scan that will be run. Some types of MRI scans are profoundly susceptible to distortion from magnetic materials, and the area around a metal object that will be disrupted (in the image) may be many cm. Yet there are other types of MRI scans that are specifically designed to counteract the presence of metal, and minimize image disruption.

    Make sure that your partner fully discloses any and all objects (including the dental work) in his body that weren’t there when he was born to the MRI provider.

    I hope this helps.

  14. Danielle

    Thanks so much for this blog, and especially this post! It has been incredibly informative for me.
    I am scheduled to go for an MRI. In a recent surgery, my surgeon re-routed a section of my bowel so that it ends in the middle of my abdomen, a few inches above my belly button. This end is marked with a needle that was curved into a shape of a ring. It’s secured with sutures right below the skin. I am nervous about the prospect of entering an MRI machine with this metal ring. I’ve been told that I may feel some heat or it might twist, but the risk is not very high. Based on your responses to other people who’ve had needles in their body, it sounds like my risk is low. Is there any other precaution I should take? Is the risk low enough that I should just proceed with the MRI?
    Once again, thanks so much for answering these questions. I’ve been quite worried about this and was so pleased to find your blog!

  15. Phil

    Fascinating article – is there a size limit on what metal objects can be affected in an MRI? What happens to the iron in a haem moleculefor example?

  16. Tobias Gilk Post author

    Danielle,

    The risk of significant heating for small objects is quite low, and the needle may have very weak magnetic potential (or none at all) based on the materials it’s made out of. Your MRI provider / radiologist should be able to give you an accurate risk-assessment.

    I hope this helps.

  17. Tobias Gilk Post author

    Phil,

    Every metal interacts with MRI’s magnetic fields, it’s all a question of degree. Most metals aren’t physically attracted to magnets (at least not at the strengths that MRIs typically operate, even though they’re profoundly strong), but even non-magnetic metals do reshape / perturb the magnetic field distribution.

    Blood iron can affect MRI images… there are certain medical conditions in which patients accumulate iron, which can make MRI imaging impossible (though it certainly doesn’t ‘pull the iron out of a patient’s body’).

    I hope this helps.

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