Magnetic Resonance Imaging (MRI) is a diagnostic technique that provides detailed images of organs and soft tissues inside the body without using ionizing radiation like X-rays. The technology relies on an extremely powerful magnet, radio waves, and a computer to generate these images. Because the scanner contains a strong magnetic field, the presence of metal presents a significant safety risk that must be addressed before any scan.
Why Metal Reacts to MRI Scanners
The danger of metal in the MRI suite is rooted in the machine’s physics, which involves two primary forces. First, the static magnetic field of the machine is thousands of times stronger than Earth’s magnetic field, and this field exerts a powerful attractive force on ferromagnetic metals, such as iron, nickel, and cobalt. This attraction creates a torque on the metal object, which can cause it to move violently, turning it into a dangerous projectile, an effect sometimes called the “missile effect.”
Second, the MRI scanner uses radiofrequency (RF) pulses to create the images, and these pulses can induce electrical currents in conductive materials like metal. If a metal object forms a loop or is close to the skin, the induced current can cause the metal to heat up rapidly. This thermal effect can result in severe burns to the surrounding tissue, even if the metal object is not strongly attracted to the static magnetic field. The risk of heating is especially high in long, thin conductors that can act as antennae.
Common External Items That Must Be Removed
Patients are required to remove all external metal objects before entering the magnetic field to mitigate both the projectile and heating risks. All jewelry, such as rings, necklaces, watches, and body piercings, must be removed as they can heat up or be pulled off the body. Items of clothing containing metal, like zippers, snaps, belt buckles, and underwire bras, must also be removed, often requiring the patient to change into a hospital gown.
Even small items can pose a risk or compromise the scan quality. Hairpins, keys, coins, and eyeglasses are all routinely removed due to their metal content. Furthermore, some cosmetics, like mascara and certain permanent makeup or tattoos, contain metallic pigments, which can cause skin irritation or minor burns during the scan. Patients are advised to remove these products before the procedure, especially if the scan area is near the face.
Navigating Internal Medical Devices and Implants
Internal metal, in the form of medical devices or implants, requires a careful screening process. Medical devices are categorized using standardized labels established by organizations like the American Society for Testing and Materials (ASTM). The three main categories are “MR Safe,” “MR Conditional,” and “MR Unsafe,” providing a quick guide to their compatibility.
An “MR Safe” designation means the device poses no known hazard in the magnetic resonance environment. Conversely, “MR Unsafe” devices, typically older or highly ferromagnetic implants like certain aneurysm clips, must not enter the MRI room. The most common and complex category is “MR Conditional,” which means the device is safe only if specific conditions are met, such as limits on the magnetic field strength, the specific absorption rate (SAR), and the time spent scanning.
Examples of common MR Conditional devices include most modern pacemakers, implantable cardioverter-defibrillators (ICDs), and many joint replacements and spinal hardware. The safety of these conditional devices depends on the specific model number, manufacturer, and date of implantation, requiring comprehensive documentation from the patient. Failure to confirm the exact specifications of a conditional device means the device must be treated as unsafe.
Potential Hazards If Metal Is Present
If metal is inadvertently present during an MRI scan, the consequences include patient injury and image degradation. The physical movement of ferromagnetic objects presents the most immediate danger, as the powerful magnetic field can cause implanted screws, clips, or shrapnel fragments to shift, potentially causing internal bleeding or severe tissue damage. Electronic implants, such as pacemakers or neurostimulators, can malfunction due to the magnetic field interfering with their circuitry.
The risk from induced currents can lead to severe burns, particularly around long, thin wires or small metallic foreign bodies near the skin. Beyond direct physical injury, the presence of metal can compromise the diagnostic quality of the scan. Metal objects create artifacts—dark areas or distortions on the image—by locally disrupting the uniformity of the magnetic field. This image distortion can obscure the area of interest, making accurate diagnosis impossible.