MRI Safety Risks and Effects

MRI safety risks

• Static Magnetic Field Effects:
  • Projectile or missile effect: Ferromagnetic metals are drawn to magnets, posing a serious danger.
  • Torque forces: Implants, like coils, can be moved or aligned with the magnetic field, potentially causing harm.
  • Medical implant malfunction: Implants such as pacemakers and hearing aids might be damaged by magnetic fields.

Biological Effects

• RF fields:
  • Interactions with biological processes.
  • Effects are dependent on magnetic energy intensity.
  • A threshold exists where no biological response occurs.
  • Effects are predictable.

Tissue Heating (Thermal Effects)

• RF field influence: Energy absorption by the body can exceed heat loss, impacting tissues.
  • Factors like frequency, exposure time, and object mass influence heating.
  • Specific Absorption Rate (SAR) measures energy absorption per mass unit.
  • Outer body parts are more prone to RF exposure effects, including abnormal blood cells and cataracts.

Tissue Heating Increase

• Increased by:
  • Malfunctioning or improperly set-up RF coils.
  • Conductive materials like ECG leads.
  • Fast imaging sequences.

SAR (Specific Absorption Rate)

• Measures the strength of RF energy.
• Measures how quickly energy is absorbed by each unit of mass.
• SAR limits are set to 0.4 W/kg for workers and 0.08 W/kg for the public to minimize biological effects.

SAR Tolerance

• Influenced by oxygen supply and blood flow in tissues.
• Tissues like the eye lens are more vulnerable to RF energy due to lower oxygen supply.

Gradient Field Effects

• Create currents in conductive materials within the body.

Gradient Field Current Effects

• Involuntary nerve stimulation:
  • Muscle twitch and pain.
  • Potential for other neurological effects.
  • Safety procedures mitigate this risk.

MRI Safety: Magnetic Field Effects

• Three effects of a static magnetic field:
  • Projectile or missile effect: Ferromagnetic metals are drawn to the magnet, posing a significant risk.
  • Torque forces: Medical implants (coils) can move or align with the field, causing potential life-threatening consequences.
  • Medical implant malfunction: Implants like pacemakers and hearing aids can be damaged by magnetic fields.

MRI Safety: Biological Effects

• Two primary causes of biological effects in MRI:
  • Radiofrequency (RF) fields
  • Gradient fields

RF Field Effects: Tissue Heating (Thermal Effects)

• RF field effects result from interactions between the MRI field and biological processes within the patient.
• The intensity of the magnetic energy determines the severity of the effect.
• There's a threshold below which no response is observed.
• Effects are deterministic (predictable based on exposure level).
• Specific Absorption Rate (SAR): Measures the amount of RF energy absorbed by a unit of mass.
  • Higher SAR indicates more energy absorbed, leading to increased tissue heating.
• Factors influencing tissue heating:
  • Frequency of the RF field
  • Exposure time
  • Mass of the object
• Effects of RF exposure:
  • More noticeable on the outer parts of the body
  • Can lead to abnormal blood cells and cataracts.

Factors Increasing Tissue Heating

• Malfunctioning RF coils or improper setup: Increases the intensity of RF energy.
• Conductive materials (e.g., ECG leads): Can concentrate RF energy, leading to higher SAR.
• Fast imaging sequences: Shorter scan time means higher RF power for the same image quality, resulting in increased SAR.

SAR Limits and Tolerance

• SAR limits: Set to minimize biological effects.
  • Workers: 0.4 W/kg
  • Public: 0.08 W/kg
• SAR tolerance: Varies based on tissue oxygen supply and blood flow.
  • Tissues with lower oxygen supply and blood flow (like the eye lens) are more susceptible to RF energy effects.

Gradient Field Effects

• Gradient fields create currents in conductive materials within the body.
• Effects of gradient field currents:
  • Involuntary nerve stimulation: Can cause muscle twitching and pain.