Magnetic Resonance Imaging (MRI) is a diagnostic tool that uses a powerful magnetic field and radio waves to generate detailed images of organs and tissues within the body. Metal objects pose a serious hazard in this environment. For this reason, all metal, particularly jewelry, must be removed before a person enters the MRI scanning suite. The magnetic field can interact with metal, risking patient safety, damaging expensive equipment, and distorting the resulting images.
The Magnetic Mechanism: Why Metal Reacts
The danger of metal near an MRI machine is rooted in how the strong magnetic field interacts with different materials. Ferromagnetic materials, including iron, nickel, and cobalt, pose the greatest risk because they are strongly attracted to the magnet. This attraction results in a translational force, known as the “missile effect,” where objects are violently pulled toward the machine.
Even if an object is firmly attached to the body, such as an internal device, the magnetic field can exert a rotational force, or torque. This force causes the object to twist or move within the tissue. Such movement could cause severe internal injury, particularly near sensitive organs or blood vessels.
A different hazard is Radiofrequency (RF) heating, which affects metals regardless of their magnetic properties. Metal objects can act as antennas that absorb the radio waves used by the MRI machine. This absorption induces eddy currents and Joule heating, which can lead to severe thermal burns to the surrounding tissue.
The presence of any metal also interferes with the homogeneity of the magnetic field necessary for clear imaging. This interference creates dark spots or distortions on the scan, known as artifacts. Artifacts can obscure the anatomy being examined and make the images diagnostically useless.
Removable Jewelry Risks and Materials
Easily removable jewelry items like rings, necklaces, watches, and earrings must be taken off before an MRI scan. The risk depends heavily on the material composition. Ferromagnetic metals, such as steel, iron, and nickel, are highly magnetic and must be removed to prevent the danger of becoming a projectile or twisting.
Other metals, while not strongly attracted to the magnet, still present a safety concern. Gold, silver, and platinum do not experience the missile effect, but these electrically conductive materials can absorb radiofrequency energy, leading to thermal burns. Even non-ferromagnetic items like underwire bras or clothing with metal zippers can cause image distortion or heating.
Patients often do not know the exact alloy composition of their jewelry, especially costume pieces. Since many jewelry alloys contain trace amounts of ferromagnetic nickel or iron for strength, the standard protocol is to remove all external metal objects. This precaution eliminates the risk of movement, heating, and image artifacts.
Non-Removable and Embedded Metallic Items
Metallic items that are not easily removable or are embedded within the body require specific review before a scan. Body piercings, especially microdermals, should ideally be replaced with non-metallic retainers made of glass or plastic. If a piercing cannot be removed, staff must confirm its material; implant-grade titanium may be considered safe, though it still risks heating. Tattoos and permanent makeup also present a risk because some older inks contain ferromagnetic iron oxides.
During the scan, these metallic pigments can absorb RF energy, potentially causing a burning sensation or swelling. Patients should inform the technologist about tattoos so the area can be monitored. Internal dental work, such as fillings, crowns, or permanent retainers, rarely poses a safety risk for movement because they are securely fixed. However, these non-ferrous dental metals can cause significant image artifacts, requiring the technologist to adjust the scanning sequence.
Internal medical devices like pacemakers, cochlear implants, surgical clips, and stents pose the most complex challenge. These items require pre-screening to confirm their safety status. They must be confirmed as “MR Conditional,” meaning they can be scanned safely only under specific conditions, or “MR Safe,” meaning they pose no known hazard.
Patient Screening and Preparation Protocol
Patient safety begins with a mandatory screening process that starts before the patient enters the MRI room. Every patient must complete a detailed safety questionnaire about internal and external metal objects, prior surgeries, and potential metal fragments. Full disclosure is paramount, as withholding information about metal can be life-threatening.
Patients are typically required to change into a pocketless, MR-safe hospital gown to eliminate concealed metal on their clothing, such as zippers or buttons. This step ensures all personal items, including keys, phones, and credit cards, are secured outside the scanning suite.
Many facilities also utilize Ferromagnetic Detector Systems (FMDS) before the patient enters the scanner room. These detectors specifically detect the presence of ferromagnetic materials. This final check serves as an augmented safety measure to catch any small, forgotten items missed during the screening process.