Magnetic Resonance Imaging (MRI) uses a strong magnetic field and radiofrequency waves to generate detailed cross-sectional images of the body’s internal structures. The system uses an immensely powerful superconducting magnet that is always active, generating a field thousands of times stronger than the Earth’s natural magnetic field. This extreme magnetic environment poses a significant risk if individuals carry or have certain materials implanted within their body. Therefore, screening is conducted for every individual before they are permitted to enter the MRI suite.
Active Electronic and Stimulating Implants
Devices that contain batteries, electronic circuitry, or leads represent the primary risk within the strong magnetic field. The magnetic and radiofrequency fields can induce electrical currents in the wires or components, causing the device to malfunction, suffer permanent damage, or dangerously heat the surrounding tissue. Screening must confirm the presence of any such device, including cardiac pacemakers, implantable cardioverter-defibrillators (ICDs), and deep brain neurostimulators.
Other electronic devices that require careful screening include cochlear implants, vagus nerve stimulators, and medication pumps. The powerful magnetic force can cause the device itself to move, potentially tearing leads or dislodging the implant from its intended position. Thorough screening involves verifying the exact model and manufacturer of the device to determine its safety classification.
An implanted device is classified as either “MR Safe,” “MR Conditional,” or “MR Unsafe,” with most modern devices falling into the conditional category. “MR Conditional” means the device can be safely scanned only under specific conditions, such as a maximum static magnetic field strength or a radiofrequency power level. Patients are often required to provide detailed implant documentation or an identification card so the technologist can confirm the parameters and proceed safely. If a device is classified as “MR Unsafe,” the patient cannot undergo the procedure.
Passive Metallic Implants and Foreign Objects
Beyond electronic devices, non-electronic metallic objects can also pose significant risks within the magnetic environment due to movement and thermal effects. Ferromagnetic implants can experience a strong pulling force, which can cause them to move violently inside the body. This is a particular concern for older intracranial aneurysm clips, which historically were made from ferromagnetic materials and could be displaced, leading to severe neurological damage or hemorrhage.
Foreign bodies, such as shrapnel or metal fragments from a prior injury, must be carefully assessed, especially if they are located near sensitive structures like the eyes or major blood vessels. Even a tiny metallic sliver in the eye can be forcibly displaced by the magnet, potentially leading to vision loss. Heating is also a concern, as the radiofrequency energy transmitted during the scan can be absorbed by metallic implants, leading to localized thermal injury or burns to the surrounding tissue.
While many modern orthopedic implants, such as joint replacements or spinal rods, are constructed from non-ferromagnetic alloys like titanium, screening is still mandatory. These materials are considered “MR Conditional,” but the presence of large amounts of metal can still cause signal voids or artifacts that obscure the image quality. The screening process must confirm the material composition and location of the implant to mitigate both the risk of heating and image distortion before proceeding with the scan.
External Items and Personal Accessories
The screening process also focuses on personal items worn or carried by the patient, all of which must be removed before entering the magnetic field. Any item containing metal, no matter how small, can become a high-velocity projectile or cause localized heating and burns. This includes common accessories like watches, jewelry, hairpins, and all external hearing aids, which must be secured outside the imaging room.
Clothing must also be screened, and patients are typically changed into a hospital gown, as street clothes may contain metallic components such as zippers, snaps, or specialized threads. Even removable dental work must be taken out before the scan. Certain cosmetics and tattoos require specific questioning because some mascaras, permanent makeup, or tattoo inks contain metallic iron oxide pigments that can heat up or cause localized skin irritation.
Transdermal patches also require specific screening, as some varieties contain a thin metallic foil backing. This foil can absorb the radiofrequency energy and cause skin burns to the patient. All external items that could pose a risk of projectile motion, heating, or image artifact creation must be safely removed from the patient and the vicinity of the magnetic field.
Screening for Specific Medical Conditions
Beyond the risks associated with metal, screening also addresses physiological conditions and medical history essential for patient safety and procedure planning. Pregnancy is a significant consideration, especially during the first trimester, when the fetus is undergoing rapid development. While no definitive evidence of harm exists, the procedure is often avoided or postponed due to the cautious principle of limiting exposure to radiofrequency energy.
Screening also includes questions related to psychological factors, such as claustrophobia, which is common given the confined nature of the imaging bore. Identifying this condition allows the clinical team to plan supportive measures, such as providing mild sedation or utilizing an open-bore scanner. This ensures the patient can tolerate the procedure and remain completely still. Patient movement can severely compromise image quality, necessitating a repeat of the scan.
Assessing kidney function is the primary non-metal screening, particularly if the procedure requires a Gadolinium-Based Contrast Agent (GBCA). These agents are injected to enhance the visibility of certain tissues or lesions. In patients with severely impaired kidney function, the body cannot efficiently clear the contrast agent, increasing the risk of developing Nephrogenic Systemic Fibrosis (NSF). NSF involves the thickening and hardening of skin and connective tissues. A recent blood test, such as a creatinine level or estimated Glomerular Filtration Rate (eGFR), is mandatory before administering the contrast to confirm adequate renal function.