MR Safety: Essential Guidelines for Radiology Practice

Magnetic Resonance Imaging (MRI) is a powerful diagnostic tool; however, its operation is associated with safety risks that both patients and healthcare staff must understand. This article outlines critical safety considerations that every radiology professional should be aware of.

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Overview of MRI Hardware Components

MRI systems rely on four essential components:

1. Main Magnet: Generates a strong static magnetic field, usually maintained at low temperatures by superconducting technology. Standard clinical MRI scanners range from 0.2 to 3.0 Tesla (T), with some higher-field systems reaching 7T or beyond.
2. Gradient Coils: Used to spatially encode the MR signal. Rapid switching of these coils creates peripheral nerve stimulation.
3. Radiofrequency (RF) Coils: Stimulate tissues to emit MR signals. Surface coils are used for specific body parts, while volume coils cover larger areas.
4. Cooling Systems: Use liquid helium to maintain superconducting magnet coils. Any rise in temperature may disrupt magnet function, leading to system failure.

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Magnetic Fields and Related Safety Risks

MRI systems generate static and time-varying magnetic fields, each associated with specific risks.

1. Static Magnetic Fields

   • Fields above 1.5T may induce sensory effects, such as nausea, vertigo, and flashing lights.
   • Projectile Risks: Ferromagnetic objects can become dangerous projectiles, causing severe injury or equipment damage. All metallic items must be strictly screened to prevent such accidents.

2. Time-Varying Gradient Fields

   • Can cause peripheral nerve stimulation and cardiac effects, including ventricular fibrillation.
   • Visual flashes (magnetophosphenes) may occur due to induced retinal currents.

3. RF Energy and Specific Absorption Rate (SAR)

   • RF energy heats tissues, with risks of burns from conductive materials (e.g., tattoos or metal leads).
   • SAR limits ensure safe RF exposure:
     • 4 W/kg for the whole body over 15 minutes
     • 3 W/kg for the head over 10 minutes
     • 12 W/kg for extremities over 5 minutes

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Zone Safety System

The American College of Radiology (ACR) defines four safety zones to restrict access to MRI environments.

• Zone 1: General public access (waiting areas, hallways).
• Zone 2: Transition area where patients are screened.
• Zone 3: Restricted access; only trained personnel can enter.
• Zone 4: The MRI scanner room itself, with high magnetic fields and strict access control.

These zones help ensure that only appropriately screened individuals enter high-risk areas, reducing accidents.

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Emergency Procedures and Quenching

A quench refers to the rapid release of helium from the MRI system, usually during emergencies, to stop the magnet. It can cause oxygen depletion in the room, leading to asphyxiation risks. Quenches are only performed if:

1. A fire is present that cannot be controlled without emergency services.
2. A person is trapped by a large ferromagnetic object.

In both cases, evacuation procedures must be followed immediately.

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Patient and Staff Screening

Comprehensive screening is essential to prevent injuries. Key considerations include:

1. Metallic Objects: All jewelry, hairpins, piercings, and devices must be removed before entering the MRI room.
2. Implantable Devices: Some devices (e.g., pacemakers, neurostimulators) are contraindicated unless MR-conditional.
3. Pregnancy: MRI during pregnancy is safe if necessary, though gadolinium-based contrast agents should be avoided unless absolutely required.

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Handling Cryogens Safely

MRI machines use liquid helium to maintain superconducting magnets. Handling cryogens requires specific safety measures:

• Protective Clothing: Use gloves, goggles, and overalls to avoid frostbite.
• Ventilation: Ensure the quench pipe is functioning to prevent helium from venting into the room.
• Training: Only authorized personnel should handle cryogen systems.

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Dealing with Implants and Foreign Objects

Implants and devices must be classified based on MR safety labeling:

• MR Safe: Items that pose no risks (non-metallic and non-conductive).
• MR Conditional: Devices that can be safely used under specific conditions.
• MR Unsafe: Items that are hazardous in the MR environment (e.g., pacemakers without MR compatibility).

Additional considerations include:

• Aneurysm Clips: MRI can only be performed if documented as MR-compatible.
• Pacemakers: Generally contraindicated unless specifically designed for MRI.
• Shrapnel and Bullets: Embedded metal fragments pose risks of heating and movement, especially near vital structures.

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RF Burns and Prevention Strategies

Burns occur when conductive materials come in contact with the patient's skin. To prevent this:

• Use foam insulation between cables and the patient.
• Ensure skin-to-skin contact is avoided by placing padding between body parts.
• Avoid looping cables across the patient.

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Key Takeaways

1. MRI Hardware: Critical components include the magnet, gradient coils, RF coils, and cooling systems.
2. Magnetic Field Risks: Strong magnetic fields can cause projectile injuries and peripheral nerve stimulation.
3. RF Energy and SAR: Monitoring SAR ensures tissue heating stays within safe limits.
4. Zone Safety System: Adhering to safety zones prevents unauthorized access to high-risk areas.
5. Emergency Procedures: Know when and how to perform a quench safely.
6. Patient Screening: Thorough screening identifies contraindicated devices and metallic objects.
7. Pregnancy: MRI is safe when medically necessary, but contrast agents should be used cautiously.
8. Cryogen Safety: Proper handling prevents frostbite, hypothermia, and asphyxiation.

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MRI safety is a shared responsibility, requiring vigilance from healthcare professionals to prevent accidents and ensure the well-being of both patients and staff. By following these guidelines, radiologists can ensure safe and effective imaging practices.