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MRI Safety-Related Emergency

MRI is a medical imaging unit that uses a strong magnetic field and RF pulse to produce images of internal anatomical structures.

MRI scans usually take a long time and are louder, they need the subject to enter a narrow, confining tube. In addition, a person with some medical implants or other non-removable metallic implants inside their body is not allowed to undergo an MRI examination safely.

MRI scans may not consider as life-threatening because there are no ionizing radiation sources but may cause a serious and life-threatening accident if safety guidelines are not followed.

Biological effects (Potential risks)

Exposure to static magnetic fields of up to 4T is not thought to be injurious. Biological effects that are relevant to clinical imaging include:

Distorted ECG: Distorted ECG can cause magneto-hydrodynamic imaging. These can be caused by the effect of a static magnetic field on moving blood systole as a conducting fluid.

There is also an effect of gradient and RF fields on the configuration of ECG. Therefore, Morphological ECG changes are difficult to detect and diagnose, but rhythm can be recognized.

If there is any concern related to rhythm, remove the patient from the scanner and perform a 12 lead ECG.

Pregnancy:

  • For Patients:

Pregnant patients should avoid MRI during their 1st trimester (First 3 months), wherever it is possible.

Avoid MRI during your first trimester. After that in 2nd and 3rd trimester, the decision is made on a risk versus benefit determination. For example: If it avoids the patient being subjected to X-rays.  

  • For Health care workers

All pregnant the health care workers may enter MR room regardless of trimester but they should not remain in the room when the scanner is operational, they need to avoid exposure to gradient and radiofrequency fields.  

Mechanical effects (very rare risk)

These effects include translational or attractive forces on metallic objects when brought into the magnetic field. These effects include:

  • Projectile or missile effect:

The attractive forces that are exerted by the static magnetic field give the greatest potential for patient injury. Metallic objects will be pulled out of hands, pockets, etc. and also fly into the magnet which may cause injury and death.

  • Effect on ferromagnetic implants

Pacemakers: Pacemaker is an electro-mechanical device, patients with pacemakers are restricted to the MR room.

Other ferromagnetic biomedical implants include are valves, stents, etc.

Contraindications to MRI

Implants and metal     

Electro-mechanical Implants

Cerebral aneurysm clips

Metallic foreign body    

Shrapnel, bullets (in critical area)

Ocular implants

Swan-Ganz

Pacemakers

Pacing wires

Cochlear implants

Neuro-stimulators

Hydrocephalus shunts

 

Effects due to the static magnetic field

  • Gradient field: Gradient field can cause auditory damage so we need to provide hearing protection to the patients i.e. headphone mandatory above 90dB.
  • Thermo-genic effects: These can cause a physiological tissue healing response
  • Most of the transmitted RF power is transferred into heat within the patient’s body tissue. All MR systems consist of a safety threshold to avoid dangerous levels. Patients who have compromised thermoregulatory systems are at higher risk.

Quench

Quenching refers to an incident that can occur when the liquid cryogens (cryogens maintain the magnetic field- helium) that cool the magnet coils, boil off rapidly. This can result in escaping helium very rapidly from the cryogen bath. In those cases, the coil ceases to be superconducting and becomes resistive. In general, a quench will be accompanied by a loud bang or thundering or hissing or rushing sound with the cold gas expulsion.

Causes of quench

  • Physical
  • Human error (accidental)
  • Intervention (elective)

Effects of quench

  • Asphyxia
  • If there is no ventilation is present, the pressure will be a build-up
  • Elective quench

The magnet should only be quenched in the following two situations:

  1. If someone is trapped in the scanner by a ferromagnetic object and is injured and distressed. For example, oxygen cylinder, piece of equipment.
  2. If there is a fire in the immediate vicinity in order to lower the risk to Fire Brigade.

What actions may be taken at the time of quench:

  • Evacuate the room as soon as possible.
  • Make sure the door should be open during evacuation.
  • Close the door after evacuation.
  • If someone trapped in the room needs to stay close to the floor level.
  • Seek advice from senior physicians immediately.
  • Call the scanner engineer.

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