All individuals with a history of potential foreign body penetration or implantation must undergo further investigation before being permitted entrance into Zone III of the MRI environment
Risk of patient injury
- Non-MR Personnel may include patients, visitors, or facility staff who have not undergone formal safety training (within the last 12 months) as designated by the MRI safety director or appropriate representative.1
- Non-MR Personnel with a history of previous injury by a metallic foreign body such as a bullet, shrapnel, or other metallic object could be at risk for injury in the MRI environment2:
- A foreign metallic body or implant may cause damage to tissue as it moves due to translational forces (missile effect) and/or rotational forces (torque) from the magnetic field.3
- The risk is particularly prevalent in areas where movement of the item has the potential to cause hemorrhage or damage to delicate anatomical structures such as vessels or organs.1
- Injuries can and have occurred related to dislodged ferromagnetic implants such as aneurysm clips, pins in joints, and drug infusion devices.4
- Electromechanically activated devices such as cardiac pacemakers, Implantable Cardioverter Defibrillators (ICDs), and drug infusion pumps may experience temporary or permanent modification of the function of the device.2
- If the object is made from conductive material, and located within the volume of the radiofrequency energy, the RF and time-varying magnetic fields may cause heating of the object and the surrounding tissue, which could lead to burns.3,5,6
- The risk of injury is dependent on2:
- properties of the object or implant (ferromagnetic properties of the foreign body, geometry, dimensions);
- strength of the static magnetic field, and the strength of the spatial gradient magnetic field of the MRI system;
- location of the object (in or adjacent to a sensitive site of the body such as vital neural, vascular, or soft tissue structures); and
- amount of force with which the object is “fixed” or retained within the tissue (i.e., counterforce or retention force from scarring or encapsulation may prevent migration of the metal).
- Undergoing a previous MRI examination without incidents does not guarantee a safe subsequent MR examination.2
- Various factors such as the MRI field strength, the orientation of the patient, and the orientation of a metallic implant or object can substantially change the scenario.
- Comprehensive screening must be completed each time a patient prepares to undergo an MRI examination.
Screening follow-up questioning
- A comprehensive screening process must be used to identify any person with a foreign body injury or implant.
- Acceptable screening methods include obtaining a documented verbal history, radiographic imaging (plain film or digital x-ray, prior CT or recent MR studies of the area in question), or written documentation detailing the type of implant or foreign body present within the patient.3
- Any individual with a history of potential ferromagnetic foreign object penetration must undergo further investigation before being permitted entrance into Zone III of the MRI environment.3
- If the implant or foreign body has been positively identified, best efforts should be made to determine the conditions for MR safety of the implant or object.3
- If no reliable patient metal exposure history can be obtained and the MRI examination cannot be delayed, it is recommended that such patients undergo plain-film or digital radiographic imaging (if recently obtained plain films, computed tomography CT studies, or MR studies of the following areas are not already available).3
- Radiographic imaging should include the head/neck, chest, abdomen/pelvis, and upper arms and thighs.3
- If there are obvious post-traumatic changes to the distal extremities, those regions should also be radiographically examined prior to MRI examination.3
- Each such patient should be reviewed and cleared by an attending MRI radiologist, and a risk versus benefit ratio assessment should be performed prior to permitting them access to the MRI scanner.3
- Any findings from the screening process should be documented and recorded in accordance with facility guidelines and reporting requirements.
Special considerations for foreign orbital bodies
- Special attention must be given to screening individuals for intra-orbital foreign bodies since high-velocity ferromagnetic fragments have been known to penetrate the globe of the eye and, in at least two cases, have interacted with the static magnetic field resulting in unilateral blindness.1
- The procedure to screen for foreign orbital bodies is subject to a degree of controversy since radiographs deliver a lens dose, and performing radiographic screening for orbital foreign bodies in patients or Non-MR Personnel simply because of a history of occupational exposure is not cost-effective.1,2
- Studies of screening for metallic foreign body in the orbit show that simple screening questions to determine the need for radiographic investigation increase the efficiency and cost-effectiveness of the screening process.2,7,8
- Individuals with a suspected metallic foreign body in the orbit are to be asked about potential injuries to the eye, including2:
- history of eye injury, and
- whether they had a medical examination at the time of injury.
- If the individual did not have an injury, then they can proceed to MR imaging.2
- If the individual did sustain an injury and was told that their ophthalmologic examination was normal, and/or if the foreign body was removed entirely at the time of the injury, it is still recommended that the individual have their orbits cleared by either plain x-ray orbit films or by a radiologist’s review and assessment of prior CT or MR images (obtained since the suspected traumatic event), if available.3
- All patients who report having suffered a penetrating eye injury should be considered for plain radiography if no recent imaging is available for review.1
Unanticipated foreign bodies
- It is possible that during an MRI examination an unanticipated ferromagnetic implant or foreign body is discovered.
- This is typically suspected or detected by means of a sizable field-distorting artifact.3
- In such cases, the MRI examination should be put on hold immediately and the MRT should notify the radiologist of the suspected findings.3
- Metallic implants made of ferromagnetic and even diamagnetic materials experience force and torque in magnetic fields. All metallic implants are heated by the radiofrequency field and to a negligibly small degree by the time-varying magnetic field used in MR systems.9
- MRTs must take care while removing the patient from the scanner. To minimize the risk of injury from magnetic translational and rotational forces that might move or dislodge strongly ferromagnetic objects3:
- the patient is removed slowly;
- the patient body position parallel to the scanner is maintained during removal;
- the patient is moved as far down the table as possible before transfer to a stretcher; and
- once safely out of the MRI scanner, the patient is taken out of MRI Zone IV.
- Assessment by an MRI radiologist of the risks and benefits for each patient and their clinical situation are paramount for appropriate medical decision-making in these scenarios.3
References
- Westbrook C, Talbot J. MRI in practice. 5th ed. Wiley-Blackwell; 2018. Accessed August 26, 2020. https://www.wiley.com/en-us/MRI+in+Practice%2C+5th+Edition-p-9781119392002
- Shellock FG. Reference manual for magnetic resonance safety, implants, and devices. Biomedical Research Publishing Group; 2020. Accessed August 7, 2020. http://www.mrisafetybook.com/
- ACR Committee on MR Safety. ACR manual on MR safety. Published online 2020. Accessed August 7, 2020. https://www.acr.org/-/media/ACR/Files/Radiology-Safety/MR-Safety/Manual-on-MR-Safety.pdf
- The Joint Commission. Sentinel event alert 38: Preventing accidents and injuries in the MRI suite. Published online February 2008. Accessed August 27, 2020. https://www.jointcommission.org/-/media/deprecated-unorganized/imported-assets/tjc/system-folders/topics-library/sea_38pdf.pdf?db=web&hash=11FD2BD7B5D766004867096BF00FB3B9
- Health Canada. Safety code – 26: Guidelines on exposure to electromagnetic fields from magnetic resonance clinical systems. Minister of National Health and Welfare; 1987. Accessed August 7, 2020. https://www.canada.ca/en/health-canada/services/publications/health-risks-safety/safety-code-26-guidelines-electromagnetic-fields-magnetic-resonance-clinical-systems-exposure.html#a45
- US Food & Drug Administration. Benefits and risks. FDA. Published December 2017. Accessed August 27, 2020. https://www.fda.gov/radiation-emitting-products/mri-magnetic-resonance-imaging/benefits-and-risks
- Seidenwurm D, McDonnell C, Raghavan N, Breslau J. Cost utility analysis of radiographic screening for an orbital foreign body before MR imaging. Am J Neuroradiol. 2000;21(2):426-433.
- Elmquist C, Shellock FG, Stoller D. Screening adolescents for metallic foreign bodies before MR procedures. J Magn Reson Imaging. 1995;5(6):784-785. doi:10.1002/jmri.1880050629
- Panych LP, Madore B. The physics of MRI safety. J Magn Reson Imaging. 2018;47(1):28-43. doi:10.1002/jmri.25761
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