Guidelines to Prevent Excessive Heating and Burns Associated with Magnetic Resonance Procedures*
Magnetic resonance (MR) imaging is a relatively safe diagnostic modality. However, damaged radiofrequency coils, physiologic monitors, electronically-activated devices, and external accessories or objects made from conductive materials have caused excessive heating, resulting in burn injuries to patients undergoing MR procedures. Heating of implants and similar devices may also occur, but this tends to be problematic primarily for objects made from conductive materials that have elongated shapes or that form loops of a certain diameter. For example, excessive MRI-related heating has been reported for leads, guidewires, catheters (e.g., catheters with thermistors or other conducting components), and external fixation systems, and cervical fixation systems.
In the United States, many incidents of excessive heating have been reported in patients undergoing MR procedures that were unrelated to equipment problems or the presence of conductive external or internal implants or materials [review of data files from U.S. Food and Drug Administration, Center for Devices and Radiological Health, Manufacturer and User Facility Device Experience Database, MAUDE]. In a review of the MAUDE database over a 10 year period, Hardy and Weil (2010) indicated that 419 thermal injuries were associated with MRI.
These incidents included first, second, and third degree burns that were experienced by patients. In many of these cases, the reports indicated that the limbs or other body parts of the patients were in direct contact with transmit body radiofrequency (RF) coils or other transmit RF coils of the MR systems. In other cases, skin-to-skin contact points were suspected to be responsible for these injuries, however, the exact mechanism responsible for these incidents is unknown.
MR systems require the use of RF pulses to create the MR signal. This RF energy is transmitted through free space from the transmit RF coil to the patient. When conducting materials are placed within the RF field, a concentration of electrical currents sufficient to cause excessive heating and tissue damage may occur. Therefore, only devices with carefully designed current paths can be made safe for use during MR procedures. Simply insulating conductive material (e.g., wire or lead) or separating it from the patient may not be sufficient to prevent excessive heating or burns from occurring for some devices.
Furthermore, certain elongated shapes (i.e. depending on the length and the transmit RF frequency) exhibit the phenomenon of “resonance” that increases their propensity to concentrate RF currents. At the operating frequencies of present day MR systems, conducting loops of tens of centimeters in size can create problems and must be avoided, unless high impedance techniques are used to limit the RF current. Importantly, even loops that include small gaps separated by insulation may still conduct currents.
Pietryga et al. (2013) reported a case of a thermal burn that occurred during MRI that was likely caused by invisible silver embedded microfibers in the fabric of an undershirt. As the prevalence of fabrics containing nondetectable metallic microfibers increases for use in athletic clothing or other garments, the importance of having patients change into gowns or other appropriate attire that do not contain metallic materials is advised as another means of preventing MRI-related burns.
To prevent excessive heating and possible burns in association with MR procedures, the following guidelines are recommended:
- The patient should change into a gown or other appropriate attire that does not contain metallic material.
- Prepare the patient for the MR procedure by ensuring that there are no unnecessary metallic objects contacting the patient’s skin (e.g., drug delivery patches with metallic components, jewelry, necklaces, bracelets, key chains, etc.).
- Prepare the patient for the MR procedure by using insulation material (i.e. appropriate padding) to prevent skin-to-skin contact points and the formation of “closed-loops” from touching body parts.
- Insulating material (minimum recommended thickness, 1-cm) should be placed between the patient’s skin and transmit RF coil that is used for the MR procedure (alternatively, the transmit RF coil itself should be padded). There should be no direct contact between the patient’s skin and the transmit RF body coil of the MR system. This may be accomplished by having the patient place his/her arms over his/her head or by using elbow pads or foam padding between the patient’s tissue and the transmit RF body coil of the MR system. This is especially important for MR examinations that use the transmit RF body coil or other large RF coils for transmission of RF energy.
- Use only electrically conductive devices, equipment, accessories (e.g., ECG leads, electrodes, etc.), and materials that have been thoroughly tested and determined to be safe or otherwise acceptable for MR procedures.
- Carefully follow the MR Safe or MR Conditional criteria and recommendations for implants and devices made from electrically-conductive materials (e.g., bone fusion stimulators, neurostimulation systems, cardiac devices, cochlear implants, etc.).
- Before using electrical equipment, check the integrity of the insulation and/or housing of all components including surface RF coils, monitoring leads, cables, and wires. Preventive maintenance should be practiced routinely for such equipment.
- Remove all non-essential electrically conductive materials from the MR system prior to the MR procedure (i.e. unused surface RF coils, ECG leads, EEG leads, cables, wires, etc.).
- Keep electrically conductive materials that must remain in the MR system from directly contacting the patient by placing thermal and/or electrical insulation between the conductive material and the patient.
- Keep electrically conductive materials that must remain within the transmit body RF coil or other transmit RF coil from forming conductive loops. Note: The patient’s tissue is conductive and, therefore, may be involved in the formation of a conductive loop, which can be circular, U-shaped, or S-shaped.
- Position electrically conductive materials to prevent “cross points”. A cross point is the point where a cable crosses another cable, where a cable loops across itself, or where a cable touches either the patient or sides of the transmit RF coil more than once. Even the close proximity of conductive materials with each other should be avoided because cables and RF coils can capacitively-couple (without any contact or crossover) when placed close together.
- Position electrically conductive materials (e.g., cables, wires, etc.) to exit down the center of the MR system, not along the side of the MR system or close to the transmit RF body coil or other transmit RF coil.
- Do not position electrically conductive materials across an external metallic prosthesis (e.g., external fixation device, cervical fixation device, etc.) or similar device that is in direct contact with the patient.
- Allow only properly trained individuals to operate devices (e.g., monitoring equipment) in the MR environment.
- Follow all manufacturer instructions for the proper operation and maintenance of physiologic monitoring or other similar electronic equipment intended for use during MR procedures.
- Electrical devices that do not appear to be operating properly during the MR procedure should be removed from the patient immediately.
- RF surface coil decoupling failures can cause localized RF power deposition levels to reach excessive levels. The MR system operator will recognize such a failure as a set of concentric semicircles in the tissue on the associated MR image or as an unusual amount of image non-uniformity related to the position of the transmit RF coil.
- Do not permit patients to wear clothing items (e.g., sportswear, underwear, yoga pants, etc.) that have metal-based fibers.
- Closely monitor the patient during the MR procedure. If the patient reports sensations of heating or other unusual sensation, discontinue the MR procedure immediately and perform a thorough assessment of the situation.
The adoption and regular practice of these guidelines will ensure that patient safety is maintained, especially as more conductive materials and electronically-activated devices are used in association with MR procedures.
[*The document, Guidelines to Prevent Excessive Heating and Burns Associated with Magnetic Resonance Procedures, was developed by the Institute for Magnetic Resonance Safety, Education, and Research (IMRSER) and published with permission. Reviewed and updated 2017.]
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