Magnetic Resonance Imaging (MRI) is a powerful diagnostic tool that provides highly detailed images of the body’s internal structures without using ionizing radiation. Instead, the process relies on an extremely strong magnetic field and radio waves to create these images. The strength of this magnet, which can be tens of thousands of times greater than the Earth’s magnetic field, is the reason metal objects pose a significant safety risk during a scan.
The strong static magnetic field of the scanner creates three primary hazards when metal is present. First, ferromagnetic materials—those containing iron, nickel, or cobalt—can be violently pulled toward the magnet in what is known as the “projectile effect” or “missile effect.” Small objects like hairpins or paper clips can reach speeds of up to 40 mph, turning them into dangerous projectiles that can cause serious injury to the patient or staff.
A second risk involves heating, which can occur even with non-ferromagnetic, conductive materials. The radiofrequency (RF) pulses used during the scan can induce electrical currents in metal objects, especially those that are elongated like wires or certain pieces of jewelry. This phenomenon, often called the “antenna effect,” can cause localized heating and lead to severe thermal burns where the metal is in contact with the skin.
Finally, the presence of metal can severely degrade the diagnostic quality of the images. Any metallic material distorts the magnetic field in its vicinity, creating dark or distorted areas known as artifacts on the resulting scan. These artifacts can obscure the very tissue the radiologist needs to examine, potentially leading to a missed or delayed diagnosis and necessitating a repeat scan.
External Metal Items Requiring Removal
Given the extreme forces at play, the immediate safety protocol requires the removal of all external, portable metal items before entering the MRI suite. This practice prevents the most common type of accidental injury and image interference. Patients are routinely asked to remove all jewelry, including rings, necklaces, watches, and especially body piercings, which can cause severe localized burns.
Even items that seem non-metallic can contain hidden ferromagnetic components. These include personal accessories:
- Keys
- Coins
- Glasses
- Removable dental work like retainers and dentures
These items must be stored securely outside the scan room. Electronic devices like cell phones, pagers, and analog watches must also be left behind, as the magnetic field will destroy them.
Clothing itself can be a source of prohibited metal. Patients must change out of street clothes that contain metal zippers, snaps, belt buckles, or metallic threads. Women’s underwire bras are a particular concern due to the conductive metal wiring that can heat up and cause burns on the skin. Patients are required to change into facility-provided, metal-free gowns or scrubs before the procedure.
Internal Implants and Medical Device Screening
The complexity increases significantly when considering metal that is permanently or semi-permanently inside the body. Internal implants require meticulous screening because their safety depends on the material composition, size, and specific conditions of the MRI scanner. Informing the technologist about all internal devices is a non-negotiable first step in the safety process.
Medical devices are classified using standardized labels: “MR Safe,” “MR Conditional,” and “MR Unsafe.” MR Safe devices pose no known hazard in any MRI environment, while MR Unsafe items must never enter the room. The most common category is MR Conditional, meaning the device is safe only under specific operating conditions, such as a maximum magnetic field strength or a limit on the radiofrequency power used.
Certain devices, particularly those that are electronic or have long wires, present the highest risk. Pacemakers, implantable cardioverter-defibrillators (ICDs), and neurostimulators are often MR Conditional and require a cardiologist or specialist to reprogram the device before and after the scan. The wire leads for these devices are susceptible to the antenna effect, which can result in dangerous heating or unintended stimulation of the heart or nerves.
Orthopedic hardware, such as surgical plates, screws, pins, and joint replacements, is generally made from non-ferromagnetic metals like titanium, making it safe for MRI. However, this metal can still cause significant image artifacts, potentially obscuring the area of interest. The specific manufacturer information and model of the hardware must be verified to ensure it meets the MR Conditional standards for the scanner being used.
Aneurysm clips are a different concern, as older models were often made of ferromagnetic materials and could dangerously move or dislodge due to the powerful magnetic force. Modern neurosurgical clips are generally non-ferromagnetic. However, due to the location near sensitive brain tissue, definitive documentation of the clip’s safety classification is always required before a scan proceeds to prevent a potential life-threatening event.
Identifying Hidden Metal and Pre-Scan Protocols
Beyond the obvious items, several less apparent sources of metal can pose a risk in the MRI environment. Tattoos and permanent makeup are a concern because some inks contain iron oxide or other metallic pigments, especially black and brown shades. The metallic components can react to the magnetic field, causing a burning sensation or localized swelling in the tattooed area due to induced currents and heating.
Permanent eyeliner is a specific example of permanent makeup that has been linked to eye irritation or low-grade burns during a scan. While a reaction is rare, the metal in the ink can interfere with the magnetic field, causing image artifacts that distort the scan. Patients with large or dark tattoos are advised to inform the technologist and report any heating sensation immediately during the procedure.
Patients with certain occupations, such as welders or metal workers, may have tiny metallic fragments or shrapnel embedded in their eyes or skin. These fragments can become dangerous projectiles or heat up during the scan. Safety verification is completed through a detailed screening questionnaire that covers all internal and external metal sources. The MRI technologist uses this information, often with the patient changed into a metal-free gown, before the patient is allowed into the high-field magnet room.