Cause of Device-Related Incident
Device factors; External factors; User errors

Clinical Specialty or Hospital Department
Clinical/Biomedical Engineering; Radiology / Ultrasound / Nuclear Med.

Device Factors
Design / labeling error

Document Type
Hazard Reports

External Factors
Electromagnetic or radio-frequency interference (EMI and RFI)

Mechanism of Injury or Death
Burn (electrical, thermal, chemical); Failure to deliver therapy

Support System Failures
*Not stated

Tampering and/or Sabotage
*Not stated

User Errors
Incorrect clinical use

UMDNS
Cables/Leads, ECG [15-754]; ECG Monitors [12-599]; Electrodes, Electrocardiographic [11-439]; Magnetic Resonance Imaging (MRI) Units [16-260]; Magnetic Resonance Imaging (MRI) Units, Extremity [18-109]; Magnetic Resonance Imaging (MRI) Units, Full-Body [18-108]; Magnetic Resonance Imaging (MRI) Units, Mammographic [18-110]; Oximeters, Pulse [17-148]; Physiologic Monitoring Systems, Acute Care [12-647]

Thermal Injuries and Patient Monitoring during MRI Studies



Hazard [Health Devices Sep 1991;20(9):362-3]

Problem

We received several reports of incidents in which patients undergoing MRI studies have sustained second- and third-degree burns. Such thermal injuries usually occur where the skin is in contact with a monitoring sensor or cable (e.g., ECG electrode or cable, pulse oximeter sensor) or an MRI accessory (e.g., surface coil). Although some of the reported thermal injuries have been serious enough to require skin grafts, no life-threatening incidents have been reported.

Little work has been done to determine the actual mechanisms that cause the burns; however, certain precautions are being recommended based on theoretical considerations and simple empirical observations. Below, we discuss the most likely cause of such thermal injuries and recommend ways of minimizing their occurrence.

Discussion

Thermal injuries caused by sensors, cables, or other accessories placed on the patient may arise from the currents induced in the conductive medium by the MRI environment. Currents are induced by two magnetic fields: the pulsed magnetic-gradient field and the pulsed radio-frequency (RF) field. These fields vary with time, and if the changing magnetic flux lines intercept an electrically conductive loop, an electromotive force (EMF) will be induced in the loop. Heating will result from the current flowing through the loop; the magnitude of the current is determined by the loop's resistance.

With most MRI systems, the currents induced by the pulsed magnetic-gradient field are about 1,000 times smaller than those induced by the pulsed RF field; therefore, they are not significant sources of thermal injury. The pulsed magnetic-gradient field is produced by a large gradient coil that encloses the RF coils, including the smaller surface coils that are located in the vicinity of the body to be imaged. The risk of burns is increased by proximity to the RF coils. In addition, the power and frequency of the pulsed RF field increases with the field strength of the magnet, so that thermal injuries seem to be more likely with high-field MRI systems.

In using physiologic monitoring equipment during MRI studies, a number of precautions can be taken to minimize the likelihood of burns. These precautions include not looping sensor cables, using high-resistance graphite electrodes and cables, and placing the sensor and cable away from the RF coil. Also, an electrically conducting path can be eliminated altogether by using certain monitoring equipment. For example, end-tidal CO2 monitors often require only a plastic nasal cannula on the patient and a plastic airway tube to the monitor; noninvasive blood pressure measurements require only a plastic tube connected between the monitor and the pressure cuff on the patient.

By following the recommended precautions and commonsense practices listed below, personnel can help reduce the risk of thermal injury to patients.

Recommendations

  1. Be aware of the potential for thermal burns during MRI when selecting and using monitors and sensors for MRI use. Systems using electrically nonconducting paths (e.g., fiberoptic cable and plastic tubing) or high-resistance paths (e.g., carbon ECG leads) are preferred, provided that they meet monitoring needs. Be sure to use these devices according to the instructions provided by the manufacturers.
  2. Do not loop leads or cables.
  3. Place the sensor well away from the RF coil, and run the cables away from the coil whenever possible.
  4. Check all sensors and cables to ensure that the electrical insulation around them is intact. Be sure that no other bare metal surface is in contact with the patient.
  5. Keep the cables off the patient and run them over blankets whenever possible.
  6. Remove all unused sensors, cables, and surface coils from the MRI system.
  7. Instruct conscious patients to call out if they experience uncomfortable levels of warming anywhere, especially at sites of sensor application. Heed such calls. Install intercom systems and check them regularly. Operators should periodically check the sites of sensor locations on unconscious patients.

UMDNS Terms

  • Cables/Leads, ECG [15-754]
  • ECG Monitors [12-599]
  • Electrodes, Electrocardiographic [11-439]
  • Magnetic Resonance Imaging (MRI) Units [16-260]
  • Magnetic Resonance Imaging (MRI) Units, Extremity [18-109]
  • Magnetic Resonance Imaging (MRI) Units, Full-Body [18-108]
  • Magnetic Resonance Imaging (MRI) Units, Mammographic [18-110]
  • Oximeters, Pulse [17-148]
  • Physiologic Monitoring Systems, Acute Care [12-647]

Cause of Device-Related Incident

Device factor: Design/labeling error

User error: Incorrect clinical use

External factor: Electromagnetic or radio-frequency interference (EMI and RFI)

Mechanism of Injury or Death

Burn; Failure to deliver therapy


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