Magnetic Resonance Imaging (MRI) uses a strong magnetic field and radio waves to create detailed images of the body’s internal structures. While the presence of metal implants historically raised concerns, modern technology and screening protocols have made MRI procedures safe for many individuals. Safety depends entirely on the specific implant material, its exact location, and the conditions of the scan.
Understanding the Physical Interaction of Metal and MRI
The MRI environment introduces three physical interactions that metal implants can experience, dictating the required safety precautions. The first concern is movement or torque, which occurs when a ferromagnetic (iron-containing) object is pulled or twisted by the scanner’s static magnetic field. This effect can dislodge older implants or metallic foreign bodies, such as shrapnel, especially if located near sensitive organs like the eye.
The second concern is heating, caused by the radiofrequency (RF) energy used to generate the image signal. This RF energy can induce electrical currents within conductive materials, particularly elongated structures like wires or leads, causing them to heat up. Temperature rises may cause tissue damage or burns, especially at the tips of device leads.
The third interaction is the creation of an image artifact. Metal distorts the magnetic field uniformity in its vicinity, leading to signal loss, geometric distortion, and blurring on the scan. While not a direct patient safety hazard, these artifacts can obscure the area of interest, potentially making the scan diagnostically useless.
Implant Safety Categorization
To standardize safety assessment, the American Society for Testing and Materials (ASTM) International established three categories for medical devices in the MRI environment. MR Safe items pose no known hazards and are typically made of non-metallic materials like plastic. These items can be brought into the scanning room without restriction.
MR Unsafe devices are those known to pose a direct threat in the MRI environment, primarily because they contain older ferromagnetic materials. Examples include certain older aneurysm clips or non-MRI-approved tools, which could become dangerous projectiles or experience severe torque. These devices must never enter the magnetic field.
MR Conditional means the device is safe only under specific, defined conditions set by the manufacturer. These conditions may include limits on the static magnetic field strength (e.g., only safe at 1.5 Tesla), limits on the radiofrequency energy level (Specific Absorption Rate or SAR), or restrictions on the body part being scanned. Most orthopedic hardware, such as titanium hip replacements, joint prostheses, rods, and modern surgical clips, fall into this category and require adherence to these parameters for a safe scan.
Essential Patient Screening and Verification
Patient safety relies on a thorough pre-MRI screening process conducted by trained staff. The first step is completing a questionnaire about all metal and electronic devices in the body. Patients must disclose every piece of metal, including seemingly minor items like permanent makeup, dental implants, or any history of metallic foreign bodies.
The most essential step for a patient with an implant is providing documentation to verify the device’s identity. This verification requires the exact make, model, and manufacturer of the implant, which can often be found on an implant ID card provided after surgery or in surgical reports. Without this identification, the implant’s safety status cannot be reliably determined, and the MRI scan may have to be postponed or canceled.
The patient’s role is to communicate this information accurately, allowing the MRI technologist and radiologist to assess the risk. They will cross-reference the implant details with established safety lists to ensure the scan parameters are within the manufacturer’s specified conditional limits. A prior successful MRI does not guarantee future safety, as scan protocols or the magnetic field strength may change.
High Risk Electronic and Active Implants
A distinct category of implants posing the highest risk are active electronic devices, which contain batteries, circuitry, and delicate wiring. These include cardiac pacemakers, implantable cardioverter-defibrillators (ICDs), neurostimulators for conditions like Parkinson’s disease, and certain programmable drug infusion pumps. The magnetic fields can cause these devices to malfunction, potentially leading to unintended pacing, inhibition of pacing, or inappropriate shock delivery.
Even if these devices are labeled MR Conditional, the procedure often requires advanced planning and specialized personnel. A cardiologist or electrophysiologist must typically interrogate the device before the scan to temporarily reprogram it into a safe mode, such as asynchronous pacing, and turn off any sensing or shock therapies. Continuous patient and device monitoring is necessary throughout the scan, and the device must be reprogrammed back to its normal settings immediately afterward to ensure proper function.