Why are metallic implants contraindicated in magnetic resonance imaging (MRI)?

Why Metals Are Contraindicated in MRI

Metallic objects are contraindicated in MRI primarily because ferromagnetic materials can become dangerous projectiles in the powerful magnetic field, cause severe thermal burns from radiofrequency heating, and may be displaced or rotated near vital structures, potentially causing catastrophic injury or death. 1, 2

Three Primary Mechanisms of Harm

1. Projectile Effect (Magnetic Attraction)

• Ferromagnetic metals experience strong deflection forces in the static magnetic field, transforming everyday objects into high-velocity projectiles that can strike patients or staff 2, 3
• The risk is highest at the portal (entrance) of the MRI scanner where magnetic field gradients are steepest 4
• Objects like external fixator clamps exhibit significant ferromagnetism and must be completely avoided 5

2. Thermal Injury from Radiofrequency Heating

• Conductive materials in contact with tissue can cause severe burns when exposed to radiofrequency magnetic fields during scanning 2
• This occurs even without visible displacement of the metal 2
• Transdermal medication patches containing metal backing must be removed before MRI to prevent thermal burns 2
• Heating effects can damage surrounding tissue, particularly problematic for implants near sensitive structures 5

3. Movement and Displacement of Implanted Devices

• Ferromagnetic implants near vital structures pose catastrophic risk if displaced 1
• Ferromagnetic cerebral aneurysm clips can move near vital brain structures and are absolutely contraindicated 1
• Steel fragments near the brain or retina can cause catastrophic injury if displaced 1
• Certain older aneurysm clips (Drake, Mayfield, McFadden, Sundt-Kees) have sufficient ferromagnetism to warrant absolute exclusion from 1.5T MRI systems 4

MRI Safety Classification System

The three-tier system determines whether scanning is permissible 6, 1:

MRI Unsafe (Absolute Contraindication)

• Most cardiac pacemakers and implantable cardioverter-defibrillators due to heating of pacemaker leads 1
• Ferromagnetic cerebral aneurysm clips 1
• Steel fragments or bullets near brain or retina 1, 7
• Any device containing ferromagnetic wire reinforcement 2

MRI Conditional (Relative Contraindication)

• Device can be scanned only when manufacturer-specified conditions are met (specific field strength limits, patient positioning, scanning protocols) 6, 1
• Most contemporary neurostimulators fall into this category 1
• Requires consultation with device manufacturer and strict protocol adherence 6

MRI Safe (No Contraindication)

• Coronary artery stents are MRI-safe at field strengths up to 3 Tesla with no waiting period required 1
• Carotid artery stents are MRI-safe, though they cause local artifacts 1
• Most modern orthopedic hardware made of titanium or non-ferromagnetic materials 1, 5
• 75% of tested metallic implants (95 of 127) were deemed safe for MRI in systematic evaluations 3

Mandatory Pre-MRI Screening Protocol

Every patient must undergo standardized safety screening using a written questionnaire before any MRI examination 6, 1, 2:

• Screen for implants, ports, catheters, metallic implants, vascular stents, coils, active devices, cardiac pacemakers, bullets, and claustrophobia 6, 1
• Obtain and verify the patient's implant pass to determine device manufacturer, specific model, material composition, and MRI safety classification 6, 1
• Check online resources (https://mrisafety.com/) to confirm implant safety level if documentation is unavailable 6, 1
• For metal-working occupations, obtain orbital radiographs to exclude intraocular foreign bodies before scanning 1

Removal of All Metallic Items

All metallic objects must be removed from patients before entering the MRI suite 2, 8:

• Remove jewelry, watches, credit cards, dental prostheses 2, 8
• Remove clothing with metallic components including zippers, buttons, and underwire bras 2, 8
• Provide patients with cotton clothing free of metal components 6, 2
• Empty bladder before scanning for comfort and reduced radiation exposure 6, 2

Image Quality Considerations (Not a Safety Issue)

• Implants can create artifacts, signal voids, and geometric distortions that complicate image interpretation 6, 8
• Coronary stents cause local artifacts but this is an image quality concern, not a safety hazard 1
• Scanning on 1.5T systems is preferred for patients with non-removable metallic prostheses to limit susceptibility artifacts 1

Critical Pitfalls to Avoid

• Do not confuse vascular stents with cardiac pacemakers—they have completely different safety profiles 1
• Do not assume all neurostimulators are absolutely contraindicated; many modern devices are MRI-conditional and can be scanned safely when protocol requirements are met 1
• Recognize that artifact does not equal danger; coronary stents cause local artifacts but pose no safety risk 1
• Do not assume heating effects only occur with visible displacement; conductive loops can cause burns without movement 2
• For ferromagnetic projectiles in or near vital anatomic structures, MRI remains absolutely contraindicated due to movement risk 7