The electromagnetic fields associated with the magnetic resonance (MR) environment can pose serious risks to individuals with certain types of implants, devices, or materials. Most injuries that occur in this setting are the result of magnetic field-induced movement (i.e., including "missile effect" accidents) or dislodgment of ferromagnetic objects. Other hazards or problems may result from the induction of electrical currents, excessive heating, and/or the misinterpretation of an imaging artifact as an abnormality.
Maintaining a safe MR environment is a daily challenge for MR technologists and healthcare workers. The types of medical implants and devices that are encountered in patients continue to grow. Up-to-date safety documentation must be obtained for these implants and devices to ensure patient safety, particularly with regard to the proper implementation of an effective pre-MR procedure screening policy.
This article presents basic information for some of the more commonly encountered implants and devices that have been evaluated for MR safety. In addition, other hazards associated with the presence of metallic objects in the MR environment are described.
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http://www.mrisafety.com -- A Web site devoted to MR safety, bioeffects, and patient management. Key features of this Web site include: |
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The List |
A searchable database that contains information for implants and other objects tested for MR safety. The List is updated on an on-going basis. |
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Safety Information |
Useful information that pertains to patient care, policies, and procedures for the MR environment. |
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Research Summary |
A presentation and summary of more than 100 articles on MR bioeffects and safety. |
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Pre-MR Screening Program |
Important screening information and a comprehensive screening form that’s available to download for use at your imaging facility. |
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Answers to Safety Questions |
Concise answers provided in a timely manner via e-mail from Dr. Frank G. Shellock. |
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This web site was created and is maintained by Frank G. Shellock, Ph.D. |
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Potential MR environment-related hazards
The missile effect
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 into the scanner by considerable force. 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 moving object.
Many types of common objects have been involved in missile effect incidents, including oxygen cylinders, floor buffers, IV poles, mop buckets, laundry carts, chairs, ladders, monitors, tools, scissors, traction weights, and sand bags (i.e., those filled with metal shot). In several cases, injuries have occurred to patients and healthcare professionals after being struck by these projectiles. A recent fatal accident, widely reported in the news media, illustrates the extreme importance of careful attention to 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-T MR system. While MR safety guidelines and procedures are well-known, missile accidents continue to occur.
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Recommendations for preventing hazards related to "missile effects" in the MR environment (adapted from ECRI report and Shellock FG, 2001). |
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Appoint a safety officer or other person responsible for ensuring that procedures are in effect, enforced, and updated to ensure safety in the MR environment. |
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Establish and routinely review MR safety policies and procedures, and assess the level of compliance by all staff members. |
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Provide all MR staff, along with other personnel who would have an opportunity or need to enter the MR environment (such as transport, security, housekeeping, maintenance, and fire department personnel), with formal training on safety considerations in the MR environment. |
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Understand that the MR system’s static magnetic field is always "on" and treat the system and environment, accordingly. |
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Don’t allow equipment and devices containing ferromagnetic (especially ferromagnetic) components into the MR environment, unless they have been tested and labeled MR safe. |
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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, "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. |
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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 (see http://www.mrisafety.com for comprehensive information). |
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Bring nonambulatory patients into the MR environment using a nonmagnetic wheelchair or wheeled stretcher. Ensure that no oxygen bottles, sandbags, or any other ferromagnetic objects are concealed under blankets/sheets or stowed away on the transport equipment. |
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Ensure that IV poles accompanying the patient for the MR procedure are nonmagnetic |
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Carefully screen all people entering the MR environment for magnetic objects in their bodies (such as implants, bullets, and shrapnel), on their bodies (such as hair pins, brassieres, buttons, zippers, and jewelry), or attached to their bodies (such as body piercings). Magnetic objects on, or attached to, patients’, family members’, or staff members’ bodies should be removed if feasible before such individuals enter the MR system room. |
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Have patients wear hospital gowns -- those without metallic fasteners -- for MR procedures. Patients’ regular clothing can contain magnetic objects or threads that may pose a hazard in the MR environment. |
Magnetic field interactions
Numerous studies have assessed the magnetic field interactions for implants, devices, and materials by measuring deflection forces, translational attractions, or torque. In general, these investigations have demonstrated that MR procedures may be performed safely in patients if the metallic objects are nonferromagnetic. Additionally, MR examinations may be performed safely if the object is ferromagnetic and only minimally attracted in relation to its in vivo application or intended use (i.e., the associated attractive force is insufficient to move or dislodge the object in situ or affect its intended function).
Various factors influence the risk of performing an MR procedure in a patient with a metallic object including the strength of the static and gradient magnetic fields, the degree of ferromagnetism of the object, and the mass of the object. Additional issues relate to the geometry of the object, the location and orientation of the object in situ, the presence of retentive mechanisms (i.e., fibrotic tissue, bone, sutures, etc.), and the length of time the object has been in place.
These factors should be considered carefully before subjecting a patient or individual with a ferromagnetic object to an MR procedure, or allowing entrance into the MR environment. This is particularly important if the object is located in a potentially dangerous area of the body, such as near a vital neural, vascular, or soft tissue structure where movement or dislodgment could injure the patient. Furthermore, in certain cases, there is the possibility of changing the operational or functional aspects of the implant, material, or device as a result of exposure to the electromagnetic fields used by the MR systems.
Induced electrical currents
The potential for MR procedures to injure patients by inducing electrical currents in conductive devices or materials is well-documented. Various reports have indicated that substantial electrical currents may be generated during MR procedures in electrocardiographic leads, indwelling catheters with metallic components (e.g., thermodilution catheters), guide wires, disconnected or broken surface coils, certain cervical fixation devices, or improperly used physiologic monitors.
Heating
Temperature elevations produced during MR procedures have been studied using ex vivo testing techniques to evaluate various implants, devices, and materials of a variety of sizes, shapes, and metallic compositions. In general, reports have described only minor temperature changes associated with the use of conventional MR procedures (i.e., those that use standard pulse sequences and techniques) involving metallic objects. Therefore, heat generated during an MR procedure involving a patient with a passive (i.e., there is no electrical means of operation) metallic implant, particularly if it is small, does not appear to be a substantial hazard.
By comparison, first-, second-, and third-degree burns have occurred in association with conductive devices (e.g., monitoring systems, surface coils, etc.) that were not used according to manufacturer’s recommendations, or were damaged. Therefore, MR healthcare workers should remain vigilant in their patient screening procedures, especially for patients with electronically-activated implants or whenever using electrically-conductive devices.
Artifacts
Artifacts caused by the presence of implants, devices, and materials have been described and are well-recognized on MR images. Signal loss and distortion of the image by the presence of a metallic object is predominantly caused by the disruption of the local magnetic field that perturbs the relationship between position and frequency, that are crucial for proper image reconstruction. The relative amount of artifact seen on an MR image is dependent on the magnetic susceptibility, quantity, shape, orientation, and position of the object in the body as well as the technique used for imaging.
Aneurysm clips
The presence of an intracranial aneurysm clip in an individual or patient that needs to enter the MR environment represents a situation that requires the utmost consideration because of the serious problems related to possible movement or dislodgment of an aneurysm clip. Certain types of intracranial aneurysm clips (e.g., those made from martensitic stainless steels such as 17-7PH or 405 stainless steel) are an absolute contraindication to the use of MR procedures because excessive, magnetic field interactions can displace these clips and cause serious injuries or fatalities. By comparison, aneurysm clips classified as "nonferromagnetic" or "weakly ferromagnetic" (e.g., those made from Phynox, Elgiloy, austentitic stainless steels, titanium alloy, or commercially pure titanium) are considered safe for patients undergoing MR procedures.
Only a single fatality has occurred due to the presence of a ferromagnetic aneurysm clip in a patient in the MR environment. This unfortunate incident was the result of erroneous information pertaining to the specific type of aneurysm clip that was used in the patient.
Coils, stents, and filters
Various types of coils, filters, and stents have been evaluated for safety with MR systems. Several of these studies reported positive findings for magnetic field interactions associated with exposures to MR systems. Fortunately, these particular devices typically become incorporated securely into the vessel wall primarily due to tissue in-growth within approximately six to eight weeks after their introduction. Therefore, it is unlikely that these implants would become moved or dislodged by static magnetic fields of MR systems up to 1.5-Tesla. However, it should be noted that because new coils, stents, and filters are currently being developed, there may be additional devices that prove to be hazardous for individuals and patients in the MR environment.
Heart valve prostheses
Many heart valve prostheses have been evaluated for the presence of attraction to static magnetic fields of MR systems. Of these, many heart valve prostheses displayed measurable yet relatively minor magnetic field interactions to the static magnetic field of the MR systems that were used for testing. Because the actual attractive forces exerted on these heart valves were minimal compared to the force exerted by the beating heart, an MR procedure is not considered hazardous for a patient who has any of the heart valve prostheses that have been tested to date. With respect to clinical MR procedures, there has never been a report of a patient incident or injury related to the presence of a heart valve prosthesis.
External hearing aids
External hearing aids are included in the category of electrically-activated implants or devices that may be found in patients referred for MR procedures. The magnetic fields used for MR examinations can easily damage these devices. Therefore, a patient or other individual with an external hearing aid must not enter the MR environment due to the possible risk of damage to the device. Fortunately, an external hearing aid can be readily identified and removed from the patient or individual prior to entering the MR environment to prevent damage to the device.
Hemostatic clips
To date, none of the various hemostatic vascular clips that have been evaluated were attracted by static magnetic fields up to 1.5-Tesla. Hemostatic clips tend to be made from nonferromagnetic materials such as tantalum, commercially pure titanium, and nonferromagnetic forms of stainless steel. Additionally, some forms of ligating or hemostatic clips are made from biodegradable materials. Therefore, patients who have hemostatic vascular clips made from nonferromagnetic materials are not at risk for injury during MR procedures and may undergo MR procedures immediately after these clips are placed surgically.
Intrauterine contraceptive devices
Intrauterine (IUD) contraceptive devices may be made from nonmetallic materials (e.g., plastic) or a combination of nonmetallic and metallic materials. Copper is typically the metal used in an IUD. The Copper T and Copper 7 both have a fine copper coil wound around a portion of the IUD. Testing conducted to determine the MRI safety aspects of IUDs with metal indicated that these objects are safe for patients in the MR environment using MR systems operating at 1.5-Tesla or less. This includes the Multiload Cu375, the Nova T (containing copper and silver), and the Gyne T IUDs. An artifact may be seen for the metallic component of the IUD.
Orthopedic implants, materials, and devices
Most of the orthopedic implants, materials, and devices that have been evaluated for ferromagnetism are made from nonferromagnetic materials and are, therefore, safe for patients undergoing MR procedures. Only the Perfix interference screw used for reconstruction of the anterior cruciate ligament has been found to be highly ferromagnetic. Because this interference screw is firmly imbedded in bone for its specific application, it is held in place with sufficient force to counterbalance it and to prevent movement or dislodgment.
MR procedures and post-op patients
There is a lot of confusion regarding the issue of performing an MR procedure during the post-op period in a patient with a metallic implant, material, or device. In general, if the metallic object is a "passive implant" (i.e., there is no power associated with the operation of the object) and made from a nonferromagnetic material (e.g., elgiloy, phynox, MP35N, titanium, titanium alloy, nitinol, tantalum, etc.), the patient may undergo an MR procedure using an MR system operating at 1.5-Tesla or less immediately after surgery. In fact, at some centers, metallic implants such as intravascular stents, are actually placed using MR guidance.
For implants that are "weakly" magnetic, it is typically necessary to wait six to eight weeks before performing an MR procedure. In this case, "retentive" or counter forces provided by tissue in growth, scarring, or granulation serve to prevent the object from presenting a risk or hazard to the patient in the MR environment. For example, certain types of coils, stents, filters and cardiac occluders that are "weakly" ferromagnetic typically become firmly incorporated into the tissue six to eight weeks following placement. Therefore, it is unlikely that these objects will be moved or dislodged by magnetic field interactions associated with MR systems operating at 1.5-Tesla or less. If there is any concern regarding the integrity of the tissue with respect to its ability to retain the object in place during an MR procedure or during exposure to the MR environment, the patient should not be exposed to the MR environment.
Conclusions
To date, more than 900 implants, devices, and materials have been evaluated for safety associated with MR procedures. While this article provides comprehensive and accurate information for several of the more commonly encountered metallic and electronically activated objects, many additional implants and devices still require evaluation for MR safety.
Furthermore, new objects are continually being developed that will require testing to determine possible risks in the MR environment. Thus, to ensure patient safety, MR technologists and healthcare workers should perform MR procedures only in patients with metallic objects that have been previously tested and demonstrated to be safe.
By Frank G. Shellock, Ph.D.AuntMinnie.com contributing writer
August 21, 2001
Frank G. Shellock, Ph.D., is an adjunct clinical professor of radiology at the University of Southern California in Los Angeles. He is the creator of the MRIsafety.com Web site.
References
G. Chaljub, L.A. Kramer, et. al., "Projectile cylinder accidents resulting from the presence of ferromagnetic nitrous oxide or oxygen tanks in the MR suite," American Journal of Roentgenology, July 2001, Vol. 177:7, pp. 27-30.
ECRI, "Patient Death Illustrates the Importance of Adhering to Safety Precautions in Magnetic Resonance Environments," ECRI meeting, Plymouth, PA, August 6, 2001.
F.G. Shellock, Guide to MR Procedures and Metallic: Update 2001 (7th edition), Lippincott Williams & Wilkins, Philadelphia, 2001.
F.G. Shellock FG, Magnetic Resonance Procedures: Health Effects and Safety, CRC Press, Boca Raton, FL, 2001.
Copyright © 2001 AuntMinnie.com
