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