High Yield Overview
MRI Safety: Contraindications & Implants
—MR Safe
—No hazard in any MRI
—MR Conditional
—Safe under specific conditions
—MR Unsafe
—Contraindicated
—Screening
100%of patients
MRI Device Classification
MR Safe: Non-conductive, non-metallic (safe everywhere)
MR Conditional: Safe with specific conditions (field, SAR, position)
MR Unsafe: Ferromagnetic or active electronic device without MR approval
Key: Most modern implants are MR Conditional - check manufacturer documentation
Critical Must-Knows
- Absolute contraindications: Non-MR safe pacemakers, cochlear implants (most), ferromagnetic foreign bodies
- MR Conditional devices safe under specific field strength and SAR limits
- All patients must complete MRI safety screening questionnaire
- Ferromagnetic projectile hazard from loose objects in scanner room
- Thermal injury risk from conductive loops and monitoring cables
Examiner's Pearls
- "Most modern orthopaedic implants are MR Conditional at 1.5T
- "Pacemakers: MR-conditional devices increasingly common
- "6-week wait after implant for healing before MRI
- "Pregnancy: MRI preferred over CT if imaging needed
- "Tattoos: Small risk of heating, monitor patient
Clinical Imaging
Imaging Gallery
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Exam Warning
MRI safety is commonly examined. Know the three MRI device categories (Safe, Conditional, Unsafe), absolute contraindications, and how to screen patients. Most orthopaedic implants are MR Conditional at 1.5T - the key is verifying device details.
The magnetic field is ALWAYS ON. Ferromagnetic objects become projectiles near the scanner. All persons entering the scanner room must be screened. Deaths have occurred from projectile injuries.
MRI Safety Zones
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ACR MRI Safety Zones
| Zone | Access | Description |
|---|---|---|
| Zone I | Public | General public areas outside MRI suite |
| Zone II | Supervised | Reception, screening area, staff offices |
| Zone III | Restricted | Control room, areas near scanner |
| Zone IV | MRI room | Magnet room itself - highest restriction |
Zone IV Access
Only screened patients and trained personnel may enter Zone IV. All ferromagnetic objects removed. Emergency equipment must be MR Safe or kept outside. Patient trolleys must be MR Conditional.
Contraindications
Absolute Contraindications to MRI
| Device/Condition | Hazard | Notes |
|---|---|---|
| Non-MR safe pacemaker/ICD | Device malfunction, heating, movement | MR-conditional devices available now |
| Ferromagnetic intracranial aneurysm clip | Movement, rupture | Most modern clips non-ferromagnetic |
| Metallic intraocular foreign body | Movement, vision loss | Orbital X-ray if history of metalwork |
| Cochlear implant (most) | Magnet displacement, demagnetisation | Some newer models MR Conditional |
| Implanted drug pumps (most) | Malfunction, dose alteration | Check specific model |
Orthopaedic Implants
General Guidance
Most modern orthopaedic implants (titanium plates, screws, joint replacements) are MR Conditional and safe for MRI at 1.5T and often 3T. Wait 6 weeks post-implant for tissue healing before MRI. The main issue is image artefact, not safety. Verify specific implant if documentation available.
Common Orthopaedic Implants and MRI
| Implant | MRI Safety | Notes |
|---|---|---|
| Titanium plates/screws | MR Conditional (most) | Safe at 1.5T/3T, causes minimal artefact |
| Stainless steel implants | MR Conditional (most) | Safe but causes more artefact |
| Total hip replacement | MR Conditional | Significant artefact, use MARS protocols |
| Total knee replacement | MR Conditional | Artefact, MARS helpful |
| Spinal fusion hardware | MR Conditional | Artefact around implants, STIR better than fat-sat |
| External fixators | Generally MR Unsafe | Remove if possible or confirm specific model |
Patient Screening
Mnemonic
IMPLANTSMRI Screening Checklist
I
I = Implanted devices (pacemaker, defibrillator, pump)
M
M = Metallic foreign bodies (eyes, shrapnel)
P
P = Pregnancy status
L
L = Loop forming potential (hearing aids, jewelry)
A
A = Aneurysm clips
N
N = Neurostimulators (cochlear, spinal)
T
T = Tattoos (heating risk)
S
S = Surgery history (when, what implanted)
Memory Hook:All patients must complete a written screening questionnaire. Verbal confirmation is also required.
Specific Hazards
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MRI Hazards and Mitigation
| Hazard | Mechanism | Prevention |
|---|---|---|
| Projectile | Ferromagnetic objects attracted to magnet | Screen all objects, no ferromagnetic items in Zone IV |
| Thermal injury | RF heating of conductive materials | Avoid loops, insulate cables, monitor skin temperature |
| Device malfunction | Electromagnetic interference | Screen for active implants, use MR Conditional devices |
| Acoustic damage | Loud gradient noise (100+ dB) | Ear protection mandatory for all |
| Peripheral nerve stimulation | Rapidly switching gradients | Adjust parameters if symptomatic |
| Claustrophobia | Enclosed scanner bore | Open MRI, sedation if needed, communication |
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
EXAMINER
"A patient with a total hip replacement from 2 years ago is referred for MRI of the affected hip to assess for adverse local tissue reaction around a metal-on-metal bearing."
EXCEPTIONAL ANSWER
Yes, MRI is safe for this patient. Modern hip replacements are MR Conditional and safe at 1.5T and usually 3T. The implant has been in place for 2 years, well beyond the 6-week healing period. For ALTR assessment, I would specifically request MRI with MARS (Metal Artefact Reduction Sequences) at 1.5T (less artefact than 3T). Key considerations: (1) Confirm the implant details if documentation available. (2) Use STIR not chemical fat saturation (fails near metal). (3) MARS protocols (MAVRIC, SEMAC, WARP) significantly reduce artefact. (4) The safety concern is minimal; the practical issue is image quality due to metal artefact.
KEY POINTS TO SCORE
Most hip replacements are MR Conditional
Safe at 1.5T (preferred) and often 3T
6-week wait post-implant (already satisfied)
MARS sequences reduce artefact
1.5T has less artefact than 3T
COMMON TRAPS
✗Thinking joint replacements are contraindicated
✗Not knowing about MARS protocols
✗Using 3T when 1.5T would be better
VIVA SCENARIOStandard
EXAMINER
"A patient with a cardiac pacemaker requires an MRI scan for a suspected disc prolapse. How would you approach this?"
EXCEPTIONAL ANSWER
This depends on whether the pacemaker is MR Conditional or not. Many modern pacemakers (since approximately 2010) are MR Conditional and can safely undergo MRI under specific conditions. Approach: (1) Identify the specific pacemaker model from the patient's device card or cardiology records. (2) Check manufacturer documentation for MRI conditions. (3) If MR Conditional: Arrange with cardiology for device interrogation before and after, program to MRI mode if required, specific field strength and SAR limits apply, continuous monitoring during scan, resuscitation equipment available. (4) If non-MR safe: MRI is contraindicated. Consider alternative imaging (CT myelography for disc assessment). (5) The scan should be at a centre experienced in MRI with cardiac devices.
KEY POINTS TO SCORE
Many modern pacemakers are MR Conditional
Identify specific model from device card
Cardiology involvement for programming/monitoring
Specific field strength and SAR limits apply
Non-MR safe devices: MRI contraindicated
COMMON TRAPS
✗Automatically refusing MRI for pacemaker patients
✗Not checking specific device compatibility
✗Not involving cardiology for MR Conditional devices
VIVA SCENARIOStandard
EXAMINER
"A metalworker with a history of 'something in his eye years ago' from grinding metal is scheduled for brain MRI."
EXCEPTIONAL ANSWER
My concern is a ferromagnetic intraocular foreign body. If a metallic fragment is in the orbit and is ferromagnetic, the magnetic field could cause it to move, potentially causing severe eye injury or blindness. Management: (1) Detailed history: When did it occur? Was it treated? Was imaging done at the time? Does he have symptoms? (2) If uncertain: Order plain X-rays of the orbits (lateral and Waters view) to screen for metallic foreign bodies. This is low radiation and highly sensitive for metal. (3) If X-ray negative and history not concerning: Proceed with MRI. (4) If X-ray shows metal: MRI contraindicated until ophthalmology review and possible removal. (5) CT orbit could characterise the fragment if needed, though CT also has radiation.
KEY POINTS TO SCORE
Intraocular metal is an absolute contraindication
Ferromagnetic fragments can move causing blindness
Orbital X-ray screening if history of metal exposure
X-ray negative + low concern = proceed
Metal visible = MRI contraindicated
COMMON TRAPS
✗Proceeding without screening high-risk patients
✗Not knowing orbital X-ray is the screening tool
✗Over-investigating low-risk patients
MRI Safety Quick Reference
High-Yield Exam Summary
Device Classification
- •MR Safe: No hazard in any MRI
- •MR Conditional: Safe with specific conditions
- •MR Unsafe: Contraindicated
Absolute Contraindications
- •Non-MR safe pacemaker/ICD
- •Ferromagnetic intracranial aneurysm clip
- •Intraocular metallic foreign body
- •Most cochlear implants
Orthopaedic Implants
- •Most are MR Conditional (safe)
- •Wait 6 weeks post-implant
- •1.5T preferred for less artefact
- •MARS sequences reduce artefact
Screening Requirements
- •Written questionnaire for all patients
- •Orbital X-ray if metalwork history
- •Verify implant documentation
- •All objects removed before Zone IV