Prevention of Thermal Injuries and Burns
Conductive materials (e.g., metals) when exposed to the time varying magnetic field of the MR scanner will increase in temperature. Tissue, as a conductive material, will also undergo heating. Heating occurs due to induction of voltage and current in the conductive material, caused by the time-varying magnetic fields of the MRI scanner, principally due to the radiofrequency (RF) magnetic field. Resistance to conduction manifests as heat. The composition of the conductive material and the amount of RF power, determined by RF amplitude, frequency of switching and duration, will determine the amount of heat produced. Heating risk is only present when the conductive material is actually exposed to RF power. Thus, entering the static magnetic field of the MR scanner room, but remaining outside the bore of the scanner, does not confer any risk of heating. Moreover, body parts containing conductive material, which are located completely outside the bore of the MR scan throughout the time the scanner is actively imaging, will not experience heating.
Any increase in tissue temperature caused by heating of the conductive material will be further modified by the nature of the tissue composition. Some scenarios, such as intravascular devices exposed to constantly flowing blood, for example, rapidly dissipate heat through the convection. Most metallic implants and foreign bodies that are contained entirely within the body (not protruding through the skin surface) will not, under conditions used for standard clinical scanning, generate sufficient heat to cause symptoms or injury. Conductive material in contact with the skin surface, on the other hand, readily rise to temperatures that cause frank burns at the interface of the conductive material and skin.
Scanner Safeguards to Prevent Thermal Injury (Burn) During MRI:
The amount of radiofrequency magnetic field power to which the patient is exposed will determine the degree of temperature increase. RF power deposition can be estimated as the specific absorption rate (SAR), which is dependent upon the patient’s size and mass. Alternatively, the RF power can be quantified as B1+RMS, which is not patient independent. SAR remains the most prevalent approach, particularly on older scanners. However, at higher field strengths (e.g., 3 Tesla), SAR is not as accurate and B1+RMS is preferred, if available. Whichever approach is used for assessing patient RF exposure, the scanner software contains built in safeguards – defined by the FDA – that prevent the user from applying RF power at a level that could overheat tissue. Two limitations of the RF safeguards must be borne in mind:
- RF limits are designed to prevent direct overheating of tissue. These limits do NOT account for the presence of highly conductive material, such as metal. Thus, even if scanning is performed within FDA guidelines, overheating of a metal implant or foreign body could occur and cause injury.
- If the MR scanner is used in “research mode”, safeguards may not remain in place.
Key Risk Factors for Thermal Injury (Burn) During MRI:
- Any conductive material that touches the skin surface (e.g., jewelry, piercings, clothing)
- Conductive material located in a sensitive location (e.g., brain, myocardium)
- Very long scan times utilizing large RF exposure (e.g., turbo spin echo)
- Patients who are unable to perceive or report sensations of heating (e.g., altered mental status, diabetic neuropathy)
- Looping or coiling of conductive material (e.g., cables)
- Proximity of conductive material to the walls of the scanner bore
Approaches to Mitigate Risk of Thermal Injury (“Thermal Injury Precautions”):
- Communication between the patient and technologist before, during, and after the scan is the cornerstone of burn prevention. Instruct the patient before entering the scanner bore that they should report any unusual sensation. The technologist should also verbally check in with the patient periodically during the scan. If the patient reports discomfort during the scan, STOP and reassess.
- Removal: Inspect the patient’s skin surface prior to MRI. Any conductive material outside the patient should be removed (e.g., piercings, medication patches). Unnecessary leads and cables should be removed.
- Positioning: Do not allow the patient’s skin to touch the inner walls of the scanner bore. Uncross arms and legs. Position any necessary cable as close to the center of the bore as possible. Eliminate any crossing or loops of cable.
- Insulation: Any necessary cables or devices with conductive components must be distanced and insulated from the patient’s skin. Separate the patient’s skin from the inside of the scanner bore. Use sponges, pads, blankets, etc.
Heat Sink: Dry cold packs can be used to decrease risk for thermal injury in the presence of superficial devices (e.g., skin staples) that cannot be removed and tattoos.