Thermal Injuries and Patient Monitoring during MRI Studies
Hazard [Health Devices Sep 1991;20(9):362-3]
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.
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
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.
- 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.
- Do not loop leads or cables.
- Place the sensor well away from the RF coil, and run
the cables away from the coil whenever possible.
- 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.
- Keep the cables off the patient and run them over
blankets whenever possible.
- Remove all unused sensors, cables, and surface coils
from the MRI system.
- 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
- Cables/Leads, ECG
- ECG Monitors [12-599]
- Electrodes, Electrocardiographic [11-439]
- Magnetic Resonance Imaging (MRI) Units [16-260]
- Magnetic Resonance Imaging (MRI) Units, Extremity
- Magnetic Resonance Imaging (MRI) Units, Full-Body
- Magnetic Resonance Imaging (MRI) Units, Mammographic
- 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