The "missile effect" refers to the capability of the fringe field component of the static magnetic field of an MR system to attract a ferromagnetic object, drawing it rapidly into the scanner by considerable force. Obviously, the missile effect can pose a significant risk to the patient inside the MR system and/or anyone who is in the path of the projectile. Furthermore, considerable damage to the MR system may result due to the impact of ferromagnetic object.
Therefore, a strict policy should be established by the MR facility to detect metallic objects prior to allowing individuals or patients to enter the MR environment in order to avoid accidents and potential injuries related to the missile effect. In addition, to guard against accidents from metallic projectiles, the immediate area around the MR system should be clearly demarcated, labeled with appropriate warning or danger signs, and secured by trained staff aware of proper MR safety procedures.
For patients preparing to undergo MR procedures, all metallic personal belongings (i.e., hearing aids, analogue watches, jewelry, etc.) and devices must be removed as well as clothing items that have metallic fasteners or other metallic components (e.g., clothing with metallic threads). The most effective means of preventing a ferromagnetic object from inadvertently becoming a missile is to require the patient to wear a gown without pockets.
Nonambulatory patients must only be allowed to enter the area of the MR system using a nonferromagnetic wheelchair or nonferromagnetic gurney. Wheelchairs and gurneys should also be inspected for the presence of a ferromagnetic oxygen tank or other similar components or accessories before allowing the patient into the MR setting. Fortunately, there are several commercially available, MR-safe or MR-compatible devices that may be used to transport and support patients to and from the MR system room.
Any individual accompanying the patient must be required to remove all metallic objects before entering the MR area and should undergo a careful and thorough screening procedure. All hospital and outside personnel that may need to enter the MR environment periodically or in response to an emergency (e.g., custodial staff, maintenance workers, housekeeping staff, bioengineers, nurses, security officers, fire fighters etc.) should be educated about the potential hazards associated with the magnetic fringe field of the MR system. These individuals should, likewise, be instructed to remove metal objects before entering the MR environment, especially the MR system room in order to prevent missile-related accidents.
Many serious incidents have occurred when individuals, who were unaware of the powers of the fringe field, entered the MR environment with items such as oxygen tanks, wheel chairs, monitors, and other similar ferromagnetic objects. In July 2001, a fatal accident, widely reported in the news media, illustrated the extreme importance of careful attention to ferromagnetic objects that may pose hazards in the MR environment. According to several newspaper reports, a young patient suffered a blow to the head from a ferromagnetic oxygen tank that became a projectile in the presence of a 1.5-Tesla MR system.
While MR safety guidelines and procedures are well known, accidents related to the missile effect continue to occur. Guidelines and recommendations for preventing these hazards are presented in Table 1.
Table 1. Guidelines and recommendations for preventing accidents and hazards related to the missile effect in the MR environment.*
(1) Appoint an MRI safety officer or other person responsible for ensuring that proper procedures are in effect, enforced, and updated to ensure safety in the MR environment.
(2) Establish and routinely review MRI safety policies and procedures, and assess the level of compliance by all staff members.
(3) Provide all MRI staff, along with other personnel who would have an opportunity or need to enter the MR environment (e.g., transport personnel, security officers, housekeeping staff, maintenance workers, fire department personnel, etc.), with formal training on MRI safety. This should be done especially for new employees and repeated on a regular basis (i.e., yearly).
(4) Understand and emphasize to all personnel that the static magnetic field of the MR system is always ON and treat the MR environment, accordingly.
(5) Don't allow equipment and devices containing ferromagnetic components into the MR environment, unless they have been tested labeled MR safe.
(6) Adhere to any restrictions provided by suppliers regarding the use of MR-safe and/or MR-compatible equipment and devices in your MR environment. A label of MR safe means that -the device, when used in the MR environment, has been demonstrated to present no additional risk to the patient or other individuals, but may affect the quality of diagnostic information- (CDRH Magnetic Resonance Working Group 1997). MR-compatible equipment, on the other hand, is not only MR safe, but also can be used in the MR environment with no significant effect on its operation or on the quality of diagnostic information.
(7) Maintain a list of MR-safe and MR-compatible equipment, including restrictions for use. This list should be kept in every MR center by the MR safety officer.
(8) Bring nonambulatory patients into the MR environment using a nonmagnetic wheelchair or gurney. Ensure that no oxygen tanks, sandbags with metal shot, or other ferromagnetic objects are concealed under blankets or sheets or stowed away on the transport equipment.
(9) Ensure that IV poles accompanying the patient into the MR environment is nonferromagnetic.
(10) Carefully screen all individuals and patients entering the MR environment for magnetic objects in their bodies (e.g., implants, bullets, shrapnel), on their bodies (e.g., hair pins, brassieres, buttons, zippers, jewelry), or attached to their bodies (e.g., body piercing jewelry). Magnetic objects on or attached to the bodies of patients, family members, or staff members should be removed, if feasible, before the individuals enter the MR environment.
(11) Have patients wear hospital gowns or scrubs without pockets and metallic fasteners for MR procedures. Patients' regular clothing can contain magnetic objects or threads that may pose a hazard in the MR environment.
*Adapted and published permission from ECRI Institute Report, 2001 and Shellock FG, 2001. Reviewed and updated 2016.
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