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.

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

  1. Pingback: ferromagnetic metals

  2. Kevin Smith

    About 3 months ago i was chushed by a Mac truck ,right after the surgery i remember waking up to nurses talking they were strapping me to an MRI table, I went back to sleep. I then again woke to a noise, but it was not a nurse. They had powered up the MIR and I could see the indicator light in the ring to show its spinning. after a moment went by I began to fall back to sleep, but stoped by a strange feeling in my hips where I have steel implants. Red warning lights started flashing , nurses were screaming and grabing me pulling. In all the dismay i started to panic not knowing what the problem was, I realised that i was no longer laying on the table i was floating in mid air! I became hotter and hotter to the point it was to much. Finally a doctor powered down the MRI causeing me to drop violently onto the table. After that I passed out from shock and delauded.

  3. Tobias Gilk Post author


    I’m sorry about your injury and I hope that your recovery progresses smoothly. With regards to your story, I suspect you have elements of your treatment jumbled… not unusual following a traumatic injury.

    If there was something spinning around you inside the scanner, that suggests that what you were being scanned in was a CT scanner, not an MRI scanner (they can look very similar from the outside). Also, any implant would likely be stainless steel (or an even less magnetic material). The magnetic field of the MRI (had it been an MRI) would not / could not levitate you based on a hip replacement (even if it were raw iron). There are no red warning lights that I’m aware of that are a part of any MRI scanner. And lastly, while it is possible to easily stop an MRI scan, it is not easy to turn off the magnetic field of an MRI scanner (in virtually all scanners used for clinical care in hospitals). The magnetic field doesn’t turn on-and-off… it’s on 24/7.

    I’m not suggesting that the things you are remembering didn’t happen, only that the what you remember may be out of sequence and your mind seeks to fill in the missing fragments and construct a narrative that helps make sense of the pieces you remember.

    I wish you well.

  4. Karen

    Hi, I have Titanium rods,screws and bolts holding my back together due to cancer in my spine.
    I need regular scans to check the tumour has not spread as it is inoperable due to it’s location.
    I am unable to have any more MRI scans due to all the metal but i can have CT scans.
    Are MRI scans more thorough than CT scans?

  5. Nikki

    I came across this article online while looking up… what happens If a person has metal on inside an MRI. I had a mri done on my head and neck today. The first part of the mri was fine. The usual uncomfortable feeling of being enclosed. The second mri for my neck I noticed a tingling almost immediately in my left hand. Soon in intervals my right elbow started feeling hot then my body temperature I could feel rising. I noticed that I began sweating and my elbow getting hotter and hotter. I shifted my elbow just enough not to disturb the imaging however the further into the scan my body temperature was almost unbareable. I felt a conduction across my right elbow across my stomach into my clasped hands and and especially fingertips of my left hand. Then it was over. I complained the source…. a metal snap button on the hospital gown. Now I wonder will there be short or long term side effects?

  6. Nikki

    Also… I would say a bare 1st degree burn on my elbow I treated with burn solution 5 hours later mild irration at the site.

  7. Tobias Gilk Post author


    Sorry for the slowness in replying.
    MRI scanners work on entirely different principles than do CT scans. It’s not a question of which is more thorough, but which imaging methodology is best for a specific part of the body or given set of conditions.

    One of the disadvantages of MRI is that it is disrupted by metal implants. This can distort, or even ‘erase’ the area right around the implant. If the doctors are looking for indications right around the hardware in your spine, MRI would then present a problem.

    I hope this helps.


  8. Tobias Gilk Post author


    You mentioned metal being with you in your first post… but you didn’t say what metal that might have been (or where).

    There is the potential for metal materials to heat up during an MRI exam, but this depends on their size, shape, and location within the MRI scanner during the exam.

    Completely independent of any metal heating, it is entirely normal for your body to warm during an MRI exam. The RF energies used for the imaging do convert to heat inside your body, so an overall warming can happen (depending on the MRI machine and the exam).

    Elbow burns, on the other hand, are a particular known risk that can / should be prevented. If your elbow was allowed to touch the side-wall of the MRI (or had something only as thick as a sheet or shirt sleeve to keep it from touching), then your elbow may have received some concentrated RF energies. This is not dangerous in the way that an over-exposure to X-rays might be, but it can (and sounds as if it did) produce something roughly equivalent to a sunburn.

    There should, however, be no lasting effects from your MRI exam or from the irritating burn on your elbow (once it heals).

    I hope this helps.


  9. Helen

    I had an MRI on my knee today. I took off everything metal before the scan but when I left the building I realised I hadn’t taken my hip belt off during the MRI which contained some coins in it.

    I’m assuming the MRI will now be distorted as a consequence? Will I suffer any ill effects?

  10. AJ

    Hey, wonder if I could get some advice. I have the remains of a bullet in my neck (tongue) – about 1 cm cubed in size – should I be concerned about having an MRI? I suspect the metal is lead as I was shot with a shot gun but I cannot be sure. My worry is that the MRI could move the foreign body and cause further injury. I have no need to have an MRI at present but my concern is if I should ever be involved in an accident where I understand it is often routine to perform a scan. I currently wear an ID chain around my neck requesting no MRI – just in case!

  11. Tobias Gilk Post author

    Distortions from metal in the MRI vary depending on the type of metal and the specific type of scan that’s being performed. In all cases, however, the distortions’ reach diminishes as you move away from the location of the metal. While it is always wise to remove all metal, in your case I would expect that if there was a problem, the technologist / radiographer administering the exam would have seen it on the screen and not let the exam continue without investigating.

    The fact that there had been metal in the scanner with you should not have any negative effects on you (if it didn’t fly at you during the exam, or produce a burn). If you have any concerns about the presence of the metal somehow negatively affecting the image, you should be able to contact the place where you had the MRI done and verify with them.

    I hope this helps.

  12. Tobias Gilk Post author

    The concerns about retained metal have to do with whether it will move… if it does, what structures is it near that the movement could impact… and how much will this metal negatively affect the image of that area…

    1 cm is pretty large fragment, and the neck is full of critical structures. If it is a ferromagnetic material and not lead, this could result in significant risks in an MRI. If it were to be lead, then the risks associated with movement (from the magnetic fields of the MRI) would drop to near zero.

    Unless / until you can verify what the foreign object is made of, or have a physician perform a risk:benefit evaluation of performing an MRI on you, in my opinion I think it is prudent to be cautious and avoid any unnecessary exposure to the powerful magnetic fields of an MRI scanner.

    I hope this helps.

  13. Dave

    Hi Tobias,
    thanks for you informative web page. I am asking a question because my girlfriend and I just took part as volunteers in a research study that involved MRI scans on healthy subjects.
    While my girlfriend was undergoing the MRI scan, the operator noticed that there was a zone in the back of her skull that was not correctly imaged (it looked like a hole, a little bit fuzzy at the extremities, on the skull or the skin). She asked me if she had any metal objects on her head, and I told her no.

    The scan continued without problem, apart from a little anxiety attack that required my girlfriend to take a break to go to the restrooms, but we are now wondering what this “hole” could have been. And above all, I am wondering if she might have had some object in her hair (a forgotten hair-clip or something) that might have caused her some damage without her noticing it. What do you think?

  14. Norman

    Hope you can answer this.
    I had surgical wire put inside me during an emergency surgery.
    Since I’ve had several mri s. Each time I’m sore in my midsection where I had surgery.
    I have even bled .
    So my question is, should I have an Mr I ?
    If so,what safety measures should be taken if any.
    Thanks. .

  15. Tobias Gilk Post author


    As you stated that the MRI operator had explained, that ‘hole’ is most likely attributed to something metallic that was on / in / around the region that didn’t show up. It could have been as simple as a barrette from the previous subject that had fallen out the hair of the prior subject and your girlfriend was (unknowingly) laying on.

    Because MRI scans vary (significantly) in their sensitivity to metal, it could also have been something you would have no idea contains metal. Many cosmetics, hair colorants, and other personal care products can have small amounts of metal in them, and these could have been the culprit. It is also the case that many of us have little bits of retained foreign bodies from slips, falls, and other bumps (mostly from childhood). It is also possible that your girlfriend has a tiny sliver of something metal embedded in her scalp.

    In my opinion, none of what I’ve described above represents a significant risk to her from having that MRI. Were it me, I would not have any concerns about health consequences from there having been the artifact that you describe.

    I hope this helps.

  16. Tobias Gilk Post author


    Thank you for your note. Unfortunately, without knowing more about the wire (what it is made of, where it was placed, what is its length and position), I can’t help to identify / quantify any MRI risks.

    My initial reaction is that most metal medical devices (including wire sutures) placed inside patients are of non-ferromagnetic (or fairly weakly ferromagnetic) materials. If this is the case with your retained wire, then the magnetic field would exert little force on it.

    I’m sorry I can’t be more help with the information at hand.


    Tobias Gilk

  17. Peter

    Hi, I would like to know , why or when some form of “cover plate”
    is used in CT scan’s to cover both sides of your skull, 2 plates,
    1 each side, I had to keep ’em still, or is it “normal handling” in CT scan’s,
    …I was in accident and my head / neck was scanned to see
    that everything is in order…and the doctors also used “cover plates”
    to cover certain areas of my skull while doing x-ray on me,
    is it “normal process”, sorry if I didn’t sound clear enough on this,
    thanks for all info

  18. Tobias Gilk Post author


    Without knowing the specifics of your exam, it’s difficult to say for certain, but in my opinion it sounds to me as if the hospital was using immobilizing block to keep your head from moving during the exam.

    I hope this helps.

  19. Adriana

    I had an MRI on my left knee last week. I was called inside and was guided to leave my purse in a locker and then I was guided to the MRI machine. I thought they were going to give me a gown but they did not. So I mentioned my bra has metal on it and the radiologist said it was ok but I was not sure so I asked again just to be sure, because I got an MRI before and in that time they told me to undress completely. In the middle of the MRI I realized my pants have a zipper which is metallic so I got scared and also I was getting dizzy so I pressed the buzzer to talk to the radiologist and he asked me is the zipper pulling up and I said I don’t think so and he said metal objects may distort the MR images but it is no pulling is ok. At the end of the test I asked so it is not a health concern for wearing a metal inside and MRI and he did not replied. Well since that day I’ve been so worry. Therefore I research and I found your web page. My zipper could have been ferromagnetic and that could have caused a risk on my test and even if was not ferromagnetic the RF interacts with electrically conductive materials that could have posed another risk. So I want to ask you based on your experience: What are the consequences of my test? Am I going to have issues later?? Will I suffer any ill effects? I am going to have the right knee MRI tomorrow, my insurance Aetna does not allow to take simultaneous MRI, so I am so worry that If something happened during the first MRI more magnetic exposure would make it worse. Is there a maximum exposition or maximum number of MRIs that you can get a year. I am really worry so I’ll appreciate your advise.

    Thank you


  20. Tobias Gilk Post author


    Let me begin with your largest question, first. To my knowledge there is no concern with any long-term health effects from repeated non-contrast MRI scans. Unlike X-rays (or CT scans, which use X-rays), MRI scanners do not use ionizing radiation (which can cause cellular damage, which can lead to a risk of cancer). MRI scans use magnetism and radio frequency energies, neither of which are known to cause the cellular damage that ionizing radiation causes.

    Now, with regard to having had an underwire bra, or the zipper on your pants, inside the MRI scanner with you while your knee was scanned, in my opinion you have no cause for worry about your health. Yes, some metal within an MRI scanner can distort an MRI image, but the distortion is generally limited to the area very near the metal. If it had been a problem, my expectation would have been that the technologist running the scan would have recognized it as a problem and worked with you to fix it. And even if the image of your knee had been somewhat distorted, the effect that metal would have had on the image would have no (of which I’m aware) effect on your health.

    It is good that you are conscientious about your safety and the best practices for MRI. If I were in your situation, I would personally have no concerns about my long term health as a result of having multiple non-contrast MRI exams, even if I had worn my street clothes for them.

    I hope this helps.

  21. Allyse

    I found your article fascinating! I came across that when I was trying to find out if something I saw on CSI: Cyber could actually happen: the reversing of the polarity of an MRI machine. Is that even possible? The only articles about reversed polarity that I came upon dealt with cochlear implants. I’m sure there are a lot of embellishments on television shows, but it did bring up an interesting question and you seem like an expert, so I thought I would ask :-) thank you in advance!

  22. Tobias Gilk Post author


    It is, in fact, possible to reduce the polarity (direction) of the magnetic field for a superconducting MRI scanner. It’s not a question of flipping a switch, however. To do it, you essentially have to de-energize the MRI scanner, and then re-energize it by connecting the positive “jumper cable” to the negative terminal of the MRI, and vice versa. This process is likely to take a day or two, and require special tools and specially-trained technicians.

    I hope this helps.

  23. mom

    Thirty years ago I had an accident where a sewing needle became lodged in my knee. I had surgery at the time to remove it which was unsuccessful. It remains lodged in the bone.
    I very recently had a Dr recommend an MRI for an unrelated reason and had an X-ray to confirm the presence of the needle – yes, it’s still there. My Dr concludes that as long as the needle remains in my knee, I am unable to ever have an MRI.

    Because the MRI is not highly necessary at this time, a surgery to remove the object doesn’t seem warranted – given the risks of any surgery – simply to afford me the ability to have access to the MRI in the future.
    I just wanted to confirm that in my case, having a sewing needle lodged in my bone, would preclude me from having an MRI.


  24. Tobias Gilk Post author

    The risks associated with having a metal foreign body inside you during an MRI are that (a) it will move from the attractive and rotational forces of the magnetic field and damage tissues / structures nearby when it does, and / or (b) that it will heat up from the RF energies.

    First, RF… In order to heat up to clinically-significant levels the metal object needs to be the right ‘antennae’ length. This length varies depending on different conditions, but generally speaking heating is not of a significant concern if the item is less than 10 cm in length (there are caveats to this, such as specialty imaging, or imaging at field strengths greater than the typical clinical MRI scanners, greater than 3.0 Tesla). And even if it is the right length (or near to it) RF heating is only a concern if the metallic object is within (partially or fully) the volume of space that is getting the RF energies. If, for example, you were having an MRI of your brain, the needle in your knee would likely get a trivial amount of RF energy from the scan.

    The forces that act to twist or move the needle will be a function of the magnetic nature of its material (which you probably can’t know), and the size of the needle (length and mass). These forces will be attenuated by how effectively it is anchored.

    So you say that the needle is lodged in the bone. If it is fixed there, then it can’t move freely (your doctor may be able to assess the degree to which it is fixed from an X-ray of the area). If it can’t move, then it can’t damage the surrounding tissues (not any more than has already happened over the past 30 years, anyway). Even if it could move, it may be in an area where there aren’t particularly sensitive structures nearby (again, this is a question for a physician), in which case the movement may produce minimal damage (and the value of an MRI might outweigh the potential risk).

    Ultimately, this becomes a question of the ratio of potential risk to benefit, which I think a radiologist who is familiar with MRI safety (look for one who has an MR Safety Certified – or MRSC™ – credential as an MRMD, if you can find one) would best be qualified to answer.

    In my – non-medical / non-physician – opinion, no, a needle in your knee is not an absolute contraindication that should automatically preclude you from having an MRI if you need one. It does introduce a potential additional risk, so I wouldn’t be cavalier about getting one. But if the circumstances indicate that you would benefit from an MRI, I think it may be possible to manage the additional risks that you have.

    I hope this helps.

  25. Donna

    I had an MRI (full spine) yesterday and as soon as it was finished I got up and my vision was blurred short and long distance. I also have what I would best describe as pressure in the back of my head and my head feels heavy. I’m not my usual self. I was under the assumption that there were no side effects. I did not have any contrast but did feel parts of my back heat up towards the end of the 25 min scan. Can you please advise of why this is happening to me?

  26. Tobias Gilk Post author


    Being inside / around an MRI, it’s not unusual to experience some dizziness or disorientation. Those feelings, however, usually fade quickly as you leave the MRI room. The warming you described as happening during the exam is also not unusual. I’m not aware, however, of persistent feelings / symptoms that you should be / could be feeling well after your MRI exam. This has me wondering if there isn’t something coincidental happening.

    In any case, if you feel as though there is something not right with your health, I do recommend that you get seen by your physician. Whether or not the cause as anything to do with your MRI exam, you don’t want to stay feeling poorly.

    I hope this helps.

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