Magnetic Resonance Imaging (MRI) provides detailed images of the body’s soft tissues using an extremely strong magnetic field and radio waves. Because of the magnet’s strength, the presence of metal inside the body presents a significant safety concern. Any metallic object must undergo careful assessment before a patient can safely enter the magnetic environment.
Understanding Magnetic Resonance and Metal Interaction
The primary mechanism of an MRI involves a large, superconducting magnet that creates a static magnetic field, measured in Tesla (T). This powerful force directly interacts with ferromagnetic metals, such as iron, nickel, and cobalt. This interaction creates three distinct risks for the patient: movement, heating, and image distortion.
The most dangerous risk is the mechanical force, or movement, exerted on ferromagnetic objects. This force can cause the metal to rotate, shift, or be forcefully pulled toward the center of the magnetic bore, potentially dislodging an implant. The second major concern is the thermal risk, where radiofrequency (RF) coils can induce an electrical current in metallic implants, especially long wires or leads. This current causes the metal and surrounding tissue to heat up rapidly, leading to severe thermal burns.
Even non-ferromagnetic metals can interfere with the imaging process. Any metallic material disrupts the homogeneity of the static magnetic field, resulting in significant signal loss and image distortion, known as artifact. This artifact appears as a dark void around the metal, which can obscure the surrounding anatomy and render the scan non-diagnostic. Thus, the concern for metal is twofold: physical danger to the patient and compromise of the scan’s diagnostic quality.
High-Risk Internal Medical Devices
Devices containing ferromagnetic materials or sensitive electronics are considered high-risk, or “MR-unsafe,” often requiring exclusion from the MRI environment. Older models of intracranial aneurysm clips, made from ferromagnetic alloys, pose a direct threat as the magnetic field can cause them to twist or migrate, leading to catastrophic bleeding. While nearly all modern clips are non-ferromagnetic, the specific material of older implants must always be verified.
Cardiac devices represent a significant concern due to their electronic components and long wires. Traditional pacemakers and implantable cardioverter-defibrillators (ICDs) can malfunction, as the magnetic field can damage the battery or internal circuitry. The leads are particularly susceptible to thermal risk, acting as antennae that focus RF energy, causing tissue heating at the electrode tip. Deep brain stimulators and certain implantable drug infusion pumps are also high-risk because the powerful magnetic fields can cause them to malfunction.
Common Metals That Are Conditionally Safe
Many modern implants are designed to be “MR-conditional,” meaning they can be scanned safely under specific, controlled conditions. This safety is due to the use of non-ferromagnetic metals that are not attracted by the static magnetic field. Titanium and its alloys, widely used for orthopedic fixation devices like plates, rods, and screws, are non-ferromagnetic and safe for MRI. These materials may still cause minor image artifacts, but they do not pose a risk of movement or heating.
Most joint replacements, including hips and knees, are considered MR-conditional, commonly made of titanium or non-ferromagnetic stainless steel alloys. While they produce some image distortion, the risk of movement or heating is negligible, allowing the scan to proceed. Dental hardware, such as braces, crowns, and fillings, rarely poses a safety risk, as the metal volume is small and often made from non-ferromagnetic materials like gold or porcelain. However, certain non-precious dental alloys containing nickel or cobalt may still create significant image artifacts, especially during a head or neck scan.
The Pre-Scan Safety Screening Process
A mandatory screening process is the most important step in ensuring patient safety before an MRI. Every patient must complete a detailed questionnaire inquiring about their surgical history and any potential metallic foreign bodies. This questionnaire is designed to uncover items that might not be obvious, such as shrapnel, metallic fragments from welding accidents, or older, unverified implants.
Patients are advised to bring documentation for implanted medical devices, such as an implant card provided by the manufacturer. This card specifies the exact model, material, and the MRI safety status (MR-safe or MR-conditional) and the field strength at which it can be safely used. The MRI technologist and supervising radiologist use this documentation and the patient’s history to make the final determination. If an implant’s safety status cannot be verified, it is treated as MR-unsafe, and an alternative imaging method will be considered to protect the patient.