Why do you need to remove metal objects during an MRI scan? Learn about the dangers of strong magnetic fields for metals and how to reduce them.
Everyone has seen the sign that says, “Please remove all metal objects that may interfere with the MRI scan, such as earrings, necklaces and watches, before entering the MRI room. But why do we need to remove these metal objects? To understand why, let’s first take a look at what MRI is and how it works, and then look at the dangers of the presence of metallic objects during MRI scans in relation to magnetic fields.
MRI (Magnetic Resonance Imaging) is a high-tech medical device or machine that uses a powerful magnetic field to create tomographic images of living organisms. In this article, we will explain the former. The principle of MRI is as follows. The atomic nuclei in the body tissues normally rotate, but in the strong magnetic field of the MRI machine, they undergo precession. When a high frequency is applied, the atomic nuclei become in a high energy state, and after the high frequency is turned off, they return to their original state, releasing energy that is collected by an antenna and visualized by a computer.
So why should you not wear metal jewelry during an MRI scan? There are three main reasons.
The first reason is “rearrangement of metallic materials due to strong magnetic fields”. Under the influence of the strong magnetic field of an MRI, metal materials are reoriented in the same direction as the magnetic field. Not only metal jewelry, but also metal materials implanted in the body can tear the surrounding body tissues. In fact, there was an accident in which a patient had a metal aneurysm clip installed in his brain and his brain tissue was torn during an MRI scan.
The second reason is “strong magnetic field attraction”. The strong magnetic fields of an MRI affect not only the inside of the machine, but also the surrounding area. If you approach a metal object near an MRI, the magnetic field will suck the metal object in like a bullet. There have been cases where a handgun, scissors, and a patient’s bed have been sucked into the machine and caused accidents, and there was also an accident where an oxygen tank brought in by a doctor was sucked into the boy in the MRI and hit the boy’s head. In addition, the magnetic field of the MRI does not disappear even when the power is turned off. This is because the magnetic field is generated by liquid helium in the MRI. It takes a long time to remove the liquid helium to remove substances that have adhered to the MRI machine.
The third reason is “eddy current heat generation”. Eddy currents are swirling currents that occur in a conductor when it is placed in a changing magnetic field. The magnetic field generated by MRI causes eddy currents to flow in the metal materials in the device, and heat is generated by the resistance of the metal materials. Not only metal jewelry, but also iron oxide ink tattoos and some nicotine patches contain metal, so there is a risk of heat burns during an MRI.
Thus, the presence of metallic materials during an MRI scan poses many risks. These risks are even more problematic in situations that require urgent care. For example, if the internal condition of an area injured in a serious car accident needs to be quickly determined by MRI, the treatment time will be delayed because people with metal materials such as pacemakers or iron cores in their bodies must first have them removed.
Is there a way to reduce the risks of MRI? One existing solution is to use a substance called liquid crystal polymer (LCP). Replacing the metal material in the body with LCP can reduce the amount of eddy currents and thus reduce heat generation. In addition, unlike metals, LCP does not rearrange itself and is not moved by magnetic fields, greatly reducing the risk. Research is also underway with polyaniline, a plastic that conducts electricity, and if commercialized, all bioelectronic systems such as pacemakers could be replaced with this material, dramatically reducing the risk of MRI.
