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

55 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

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