Why must metal objects be removed before an MRI scan? We explore the dangers of strong magnetic fields on metal and ways to reduce them.
Everyone has probably seen the sign at the entrance to the radiology department that says, “Please remove all metal objects such as earrings, necklaces, and watches that may interfere with the MRI scan before entering the MRI room.”
But why is it necessary to remove these metal objects? To understand the reason, we will first explain what MRI is, how it works, and then examine the risks associated with the presence of metal objects during MRI scans in relation to magnetic fields. MRI (Magnetic Resonance Imaging) refers to an advanced medical device that uses a powerful magnetic field to obtain cross-sectional images of the body, or the images produced by such a device. In this article, we will focus on the former meaning.
The principle of MRI is as follows. The atomic nuclei in bodily tissues normally rotate, but within the strong magnetic field of an MRI machine, they undergo precession. When high-frequency waves are applied, the atomic nuclei enter a high-energy state, and when the high-frequency waves are stopped, they return to their original state while releasing energy. This energy is collected by an antenna and converted into an image by a computer.
So why can’t you wear metal jewelry during an MRI scan? There are three main reasons.
The first reason is “the rearrangement of metal substances due to strong magnetic fields.” Under the influence of the strong magnetic field of an MRI, metal substances are aligned in the same direction as the magnetic field. This can cause not only metal jewelry but also metal substances implanted in the body to tear surrounding body tissues.
In fact, there have been cases where a patient with a metal aneurysm clip implanted in the brain suffered brain tissue damage during an MRI scan. The second reason is the “magnetic force caused by the strong magnetic field.” The strong magnetic field of an MRI affects not only the interior of the equipment but also its surroundings. If you approach an MRI with a metal object, the magnetic field will pull the metal object toward the MRI like a bullet.
There have been cases where objects such as guns, scissors, and patient beds were sucked into the equipment, and an accident occurred where an oxygen tank brought by a doctor was sucked toward a boy inside the MRI and struck his head. Additionally, the magnetic field of an MRI does not disappear even when the power is turned off, as it is generated by liquid helium inside the MRI.
To remove substances adhering to the MRI equipment, the liquid helium must be removed, which takes a significant amount of time. The third reason is “heat generated by eddy currents.” Eddy currents are swirling currents that occur inside a conductor when it is placed in a changing magnetic field.
The magnetic field generated by MRI causes eddy currents to flow through metallic substances inside the equipment, and heat is generated due to the resistance of the metallic substances. Not only metallic jewelry but also tattoos containing iron oxide ink and some nicotine patches contain metal, posing a risk of burns due to heat generation during MRI imaging.
As such, the presence of metallic substances during MRI scans poses significant risks. These risks become even more critical in emergency situations requiring immediate treatment. For example, in cases where rapid assessment of internal conditions is necessary following a severe traffic accident, individuals with metallic substances such as pacemakers or metal implants must first have these removed, potentially delaying treatment.
Is there a way to reduce the risks associated with MRI scans? One existing solution involves using a material called LCP (Liquid Crystal Polymer). By replacing metallic objects inside the body with LCP, eddy currents are reduced, thereby minimizing heat generation. Additionally, unlike metal, LCP does not rearrange itself or move under a magnetic field, significantly reducing risks. Research is also underway using “polyaniline,” a conductive plastic, and if commercialized, this material could replace all biomedical electronic systems like pacemakers, innovatively reducing risks associated with MRI scans.
