Patient evaluation for metallic or electrical implants, devices, or foreign bodies before magnetic resonance imaging

INTRODUCTION — Magnetic resonance (MR) images are created by measuring the response of atomic nuclei of tissues placed in a strong magnetic field to high frequency radio waves. However, for patients with metallic or electrical objects in their body, the strong magnetic field associated with the scanner poses potential hazards [1-4].

Safety issues from metallic or electrical implants, devices, or foreign bodies, if unanticipated until the patient presents for MR imaging (MRI), can lead to exam delays and, rarely, cancellation. Many common issues can be addressed before the exam visit if the referring health care provider elicits the relevant history at the time of referral and communicates it to the imaging service. The safety screening performed by radiology requires time and documentation by the patient and the imaging service personnel; for outpatients, a significant proportion of their MRI visit will likely be spent on the safety screening and preparation procedures.

This topic describes how to evaluate a patient at the time of MRI referral for metallic or electrical objects that could represent hazards during imaging. Both the common safety issues and those that are most likely to lead to exam delay or cancellation are discussed. Safety procedures for the common objects are described so that the referring health care provider can counsel the patient and work with the imaging service to anticipate them. If there is uncertainty regarding the appropriate procedures, health care providers should consult with the radiologist, as MRI services have established safety protocols in place. The safety profile of many medical devices are also described in MRIsafety.com.

Related UpToDate content discusses:

●Safety issues related to gadolinium contrast agents given for MRI. (See "Patient evaluation before gadolinium contrast administration for magnetic resonance imaging".)

●Prophylaxis for patients with claustrophobia who undergo MRI.

●Patient monitoring and sedation during MRI. (See "Anesthesia for magnetic resonance imaging and computed tomography procedures".)

●MRI scanner technology and physics. (See "Principles of magnetic resonance imaging".)

PATIENT SAFETY CONCERNS — When metallic or electrical implants, devices, or foreign bodies are near an MRI scanner, potential safety risks that arise are projectile or torque injuries, thermal injury, and device malfunction. In addition, artifacts resulting from imaging patients with indwelling foreign bodies can significantly impair image quality and result in missed or delayed diagnoses. These safety risks originate from three key components of the scanner: the main magnetic field, gradient coils, and radiofrequency (RF) coils (figure 1) [1-3]. All three components are required to produce a magnetic resonance (MR) image and each represent a potential hazard to anyone near the scanner (table 1).

●Projectile or torque injuries – An MRI scanner exerts a powerful magnetic field on its surroundings. Magnetic strength of most clinical scanners are 1.5 or 3.0 Tesla (T), which is approximately 100,000 times the strength of the Earth's magnetic field and 10 to 100 times more powerful than the surface of the strongest available toy or kitchen magnets.

MRI exerts strong translational or rotational forces on ferromagnetic objects [5,6]. Thus, objects outside the body can become projectiles and hardware inside the body could be displaced and injure adjacent tissue [7-9]. As the magnet is always on, the hazard is constant, even when the scanner is not acquiring images. The hazard is also somewhat unpredictable as the strength of the magnetic field increases rapidly as one approaches the scanner. These changes in magnetic forces with proximity can sometimes be quite sudden.

Higher magnet strength scanners (ie, 7T) are also available for clinical use to image the head and extremities, and there are significantly higher translational and rotational forces associated with 7T environments [10-12].

●Thermal injury – The two possible sources of thermal injury in an MRI scanner are the gradient and the RF coils. Three orthogonal sets of gradient coils generate gradient magnetic fields in three dimensions. RF coils transmit pulses to stimulate MR signal from the hydrogen protons within tissue and transmit these signals back to the scanner console. RF coils are often separate devices placed on or around the patient in the anatomic region to be imaged.

Both gradient and RF coils create large fluctuating electromagnetic fields and can cause electrical currents in nearby conductive materials, tissue, and perspiration. This phenomenon, known as Faraday's Law of Induction, leads to heating and, when excessive, can cause burns [13,14]. Examples include direct skin contact with coil cables, wire leads from cardiac devices, body piercing jewelry, tattoos, and dermal drug patches. Clothing containing metal fibers (eg, athletic wear such as yoga pants) are another common hazard [15].

●Device malfunction – The main magnetic field, the RF coil, and the gradient coils can all potentially interfere with electrical or mechanical components of medical devices. Cardiovascular implantable electronic devices (CIED), cochlear or otologic implants, and drug infusion pumps are the most common examples. MRI has been shown to stimulate permanent pacemakers or implantable cardioverter-defibrillators and cause inadvertent or incorrect pacing [16,17]. Mechanical components of medical devices (eg, drug infusion pump motor) can also be damaged by the main magnetic field [18].

●Imaging artifacts – Ferromagnetic substances interact with the magnetic fields of the scanner and gradient coils and compromise imaging of nearby structures. If an implant is within the anatomical region of the body part of interest (eg, hip prothesis for pelvic MRI, dental amalgam for head MRI, and spinal fusion hardware for spine MRI), the resulting images may be either nondiagnostic or of limited quality. Certain pulse sequences, in particular gradient-dependent sequences (eg, gradient-echo, susceptibility-weighted, and diffusion), are more affected. These artifacts will occur irrespective of the safety profile of the device.

ASSESSING IMPLANTS, DEVICES, OR FOREIGN BODIES FOR MRI — The MRI safety profile of most medical implants and devices can be found in searchable, updated databases [19]. Many manufacturers also publish MRI safety information for their products online. The identifying information on the hardware is contained in the medical record describing the device placement procedure. Patients may have been given a card with the identifying information and the MRI safety profile at the time of the procedure. A description of an implant by brand name and manufacturer (eg, endovascular stent graft, FLAIR, Bard) is sometimes sufficient to determine its safety profile. However, in some cases, a lot and/or model number will be necessary.

Safety labeling — Medical implants and devices have been manufactured and tested for MRI safety beginning around 2000. In 2009, the United States Food and Drug Administration (FDA) adopted a standard labeling scheme to describe the MRI safety of an object [1,20,21]. Medical devices produced since the adoption of this system are labeled under one of three categories as (figure 2):

●MRI safe – The object poses no hazard in all MRI environments. These are nonconducting and nonmetallic items (eg, plastic syringe).

●MRI conditional – The object has been shown to be safe in a specific MRI environment under certain conditions of use. Main magnetic field strength, radiofrequency (RF) or gradient field strength, or specific configuration of the object itself (eg, routing of leads for a neurostimulator) are among the features of the MRI environment that could be defined as a condition of use. Thus, MRI-conditional objects are considered compatible with MRI that adheres to specific safety guidelines. For a device or implant in this category, the guidelines for safe MRI are available, usually online, from the manufacturer.

●MRI unsafe – The object is not safe in any MRI environment. These are most ferromagnetic items (eg, hospital gurneys, scissors) or electrical devices (eg, mobile phones).

The safety profile of a particular object can vary from one manufacturer to another, from one model to another, and can even change between manufacturing lots. Thus, obtaining records detailing the specifics of the implant or device under question is often necessary. With this information, the MRI center can ascertain the safety profile of the object [22].

Legacy implants and devices — Most objects manufactured before 2010 do not carry a safety label, and those implanted before 1996 were made before MRI compatibility became a manufacturing concern. While this may not necessarily preclude the MRI, these legacy implants and devices are classified and handled as MRI unsafe. Nevertheless, MRI compatibility information on some of these legacy objects may be available on searchable online databases [19]. Objects without safety labeling are sometimes tested, and the findings are published from institutions separate from the manufacturer (eg, at academic centers).

Procedures for common implants, devices, and foreign bodies — If the object is considered MRI unsafe or if the safety profile of the object is unknown, the referring health care provider should discuss the exam indications and alternative diagnostic strategies with the radiologist at the time of referral. (See 'Patient safety evaluation procedure' below.)

The following is a list of some common implants or devices and how they are usually handled for MRI:

Jewelry, tattoos, permanent makeup, and surgical skin staples — All metallic jewelry should be removed before the MRI. Body piercing removal may require that the patient seek help from a professional. A nonmetallic stent or catheter can replace the piercing tract temporarily to lower the risk of closure [23]. If a patient cannot or does not wish to remove a metallic piercing, some facilities will decline to perform the exam because of the risk of a skin burn (table 2).

In most cases, dermal adornments such as tattoo or permanent makeup (eg, eyeliner) do not hinder an MRI. A prospective multicenter study of 300 patients with 932 tattoos measuring <2 cm imaged in a 3 Tesla scanner with body or head coil resulted in one adverse reaction (ie, warm, tight feeling at the site of the tattoo) [24]. Estimated probability of an adverse reaction was <0.3 percent (95% CI 0.01-1.68). However, rare cases of severe burns have been reported [25-28]. Dermal adornments that are in close proximity may increase the risk of thermal injury if they are in the volume associated with RF energy deposition. Conductive loops may be created by tattoos, particularly with dark colors of ink and curved patterns [29]. If the affected area is relatively small, many facilities will image the patient after obtaining consent and apply cold compresses to minimize the risk of burning. In addition, ferromagnetic material in the ink can cause significant imaging artifacts underneath the tattoo or permanent makeup (eg, orbital MRI in patient with permanent eyeliner). In such cases, the exam may be nondiagnostic.

Surgical skin staples become fixed with granulation tissue after six to eight weeks and can be safely imaged after that time. While imaging artifact is observed, the risk of heating is negligible for most skin closure hardware [30].

Bullets, shrapnel, and other metal fragments — Because of the concern for torque and thermal injuries, MRI is contraindicated if bullet fragments and shrapnel lie too close to vital structures (eg, eye, artery) [31]. Sheet metal work confers a small risk of metallic fragments in the orbit which, if present, is a contraindication for MRI. Radiography of the area of concern is necessary if the patient's history or exam (eg, scar) suggest embedded metal fragments. Prior radiographs or computed tomography (CT) images, if available, should be obtained and reviewed (table 2).

Intracranial aneurysm clip — As death has been attributed to a misidentified aneurysm clip, accurate identification of all intracranial clips, coils, and hardware before MRI is essential [9]. Thus, the manufacturer and model is required to determine the safety profile of each device unambiguously on MRIsafety.com. A radiograph of the head should be obtained if the patient's history suggests that an uncharacterized aneurysm clip might be present (table 2) [1].

Cerebrospinal fluid shunt — Cerebrospinal fluid (CSF) shunts are rated as MRI safe or MRI conditional (figure 2). The shunt should be identified by model name and manufacturer and the safety profile verified on MRIsafety.com. Patients with magnetic externally-adjustable CSF shunt valves have a very small risk of experiencing an unintended change in their valve setting when exposed to strong magnetic fields [32]. In such patients, the valve settings should be checked both before and immediately after the MRI procedure per the manufacturer's recommendations (table 2).

Cochlear implants — Cochlear implants are electronically active and vary widely in their design and components. The safety concern is device malfunction with MRI or the presence of ferromagnetic material that could cause torque or heating injury. The manufacturer, model, and year are required to determine the safety profile of each device unambiguously on MRIsafety.com [33]. MRI is contraindicated if the implant is classified as MRI unsafe (table 2).

Neurostimulators — Most neurostimulators are rated as MRI unsafe or MRI conditional (figure 2). Safety concerns are device malfunction causing pain or injury, and lead heating. The model name, number, and manufacturer should be used to verify the safety profile of the neurostimulator on MRIsafety.com. MRI-unsafe devices must be removed. MRI-conditional neurostimulators can remain in place but imaging should be performed in accordance with the manufacturer's recommendations. For this, discussion with the radiologist on the safety issues at the time of referral is advised (table 2).

Dental implants — Any detachable implants (eg, dentures) should be removed prior to imaging. Most permanent dental implants contain ferromagnetic material but are generally safe because they are securely anchored to bone or teeth. However, they do cause significant imaging artifact, which may compromise diagnostic performance if the area of interest includes the face, upper neck, or skull base.

Cardiovascular implantable electronic device — When considering MRI for a patient with cardiovascular implantable electronic device (CIED), a discussion with a radiologist is strongly advised first since the necessary resources or expertise for safe imaging is not available at many centers. Moreover, patients with a pacemaker or intracardiac defibrillator in place who undergo MRI should be under the care or supervision of a cardiology practitioner specializing in electrophysiology before, during, and after their examination (table 2).

The majority of the CIEDs in the United States population are legacy devices and are classified as MRI unsafe [34,35]. In most practice settings, this is a contraindication to MRI. However, MRI can be safely performed in these patients at some centers, if the necessary electrophysiology expertise and imaging resources are available [36,37]. Guidance regarding the performance of MRI in patients with non-conditional CIEDs is derived from the Heart Rhythm Society recommendations [38]. Before the patient is referred for MRI, a cardiologist with electrophysiology expertise should review the patient's history to determine if there are any issues (eg, temporary transvenous leads, a device without any history of MRI safety testing) that would preclude the exam. Imaging must be performed in a 1.5 Tesla (T) magnet and the device programming is interrogated before and after the exam. The pacemaker settings are sometimes changed temporarily by the cardiologist for the duration of imaging. While in the scanner, the patient's blood pressure, pulse oximetry, and cardiac rhythm are monitored and a health care professional with expertise in pacemaker/implantable cardiac device programming and cardiac rhythm analysis is present.

Many modern CIEDs have been designed to be MRI conditional (figure 2). These can be identified by model name, number, and manufacturer at MRIsafety.com. When referring a patient with MRI-conditional devices, a discussion with a radiologist and a cardiologist about the necessary safety preparations is advised, as the site may not have the necessary resources or expertise to meet the requirements. Recommendations on device settings and required level of monitoring are published by each manufacturer and can vary considerably. Some MRI-conditional CIED systems have an MRI-safe mode to be used during imaging, while others do not require a specific setting change.

In certain patients with a CIED, MRI may be contraindicated, even if the device is labeled MRI conditional. CIEDs result in significant artifact compromising the diagnostic quality of most cardiac or chest, breast, and upper abdominal MRIs [39]. For these scenarios, MRI should be avoided. Abandoned (disconnected) pacer leads are at risk of heating and remain contraindicated at most imaging centers, although there are some facilities that have the resources to perform these scans under close supervision, as there is increasing evidence that such patients can be scanned safely [40,41].

Cardiac valves — Heart valves are rated as MRI safe or MRI conditional (figure 2) [35]. The model name, anatomic location (eg, aortic, mitral), and manufacturer are needed to review the safety profile of a heart valve [19]. With MRI-conditional devices where imaging must be performed in accordance with the manufacturer's recommendations, a discussion with the radiology service on safety issues at the time of referral is advised (table 2).

Arterial stents, coils, and clips — Endovascular stents, grafts, coils, and biliary stents are usually embedded within the tissues six to eight weeks following implantation and are typically cleared for MRI after that period [35]. If the implantation procedure has occurred within the past eight weeks, scheduling of the MRI exam should be discussed with the radiologist or the MRI safety officer. Required wait time following implantation varies with the specific type and model of the implant and its anatomic location relative to the region to be imaged. Coronary stents, including bare metal stents, are often considered safe for MRI even immediately after deployment, and steel embolization coils have been shown to be safe for imaging right after implantation [42-44]. Information specific to each type of model of implant can be found online (table 2) [19].

Vascular ports and intravenous catheters — Vascular access devices are rated MRI safe or MRI conditional (figure 2) [35]. Safety profile based on the model and manufacturer of each device can be found online [19]. With MRI-conditional devices, where imaging must be performed in accordance with the manufacturer's recommendations, a discussion with radiology on the safety issues at the time of referral is advised.

Inferior vena cava filters — Inferior vena cava filters are rated MRI safe or MRI conditional (figure 2). Safety profile based on the model and manufacturer of each device can be found online [19]. For MRI-conditional devices, the labeled safety recommendations, including the maximum magnet strength, should be adhered to for imaging. For this, discussion with radiology on the safety issues at the time of referral is advised. In addition, if possible, imaging should be performed >8 weeks after placement to ensure that the filter is embedded in the vessel (table 2) [35].

Breast prostheses and tissue expanders — The most widely used breast implants are rated MRI safe. However, there are many breast implants and tissue expanders with metallic components and/or magnetic injection site guides. These are rated MRI unsafe and would be contraindications to MRI [45-47]. These are identified by model name, style number, and manufacturer [19].

Orthopedic implants — Most metallic orthopedic hardware is labeled MRI conditional and is comprised of titanium, which is not ferromagnetic (figure 2) [48]. To prevent overheating, the labeled safety recommendations including maximum magnet strength and RF and gradient power must be adhered to for imaging. These can be found at MRIsafety.com by identifying the implant by model name, anatomic location, and manufacturer [33].

Screws and anchors of the hardware that have more ferromagnetic components are embedded securely into bone and do not pose a threat of projectile injury. Imaging artifact is significant and, if the hardware is within or adjacent to the area of interest, the exam may be nondiagnostic [49,50]. In such cases, a discussion of alternative imaging strategies with a radiologist is likely to be helpful.

Drug infusion pumps and patches — Patients must remove all infusion pumps, glucose monitors, drug patches and all other removable drug delivery and monitoring devices because of the concerns for device malfunction and thermal injury. Since the patient may not have access to these devices for as long as two hours, this should be anticipated in advance (table 2).

Radiotherapy fiducial markers — Radiotherapy fiducials are rated MRI conditional (figure 2) [51]. They are comprised of minimal, if any, ferromagnetic components and embedded in tissue. Thus, they are subject to insignificant torque or translational forces or heating.

Contraceptive devices — Most intrauterine devices (IUDs) are rated MRI safe or MRI conditional (figure 2) [52]. Safety profile of most commonly encountered models can be found online [19,33]. No adverse events have been experienced with either the copper-based IUDs or the permanent steel ring IUDs common in China [52-55]. However, the steel ring IUDs result in substantial imaging artifacts and precludes a diagnostic-quality pelvic MRI.

The Essure fallopian tube microinsert for permanent contraception is rated as MRI conditional at field strengths at and below 3T [52,56]. It is weakly ferromagnetic and does cause some imaging artifact but usually does not preclude diagnostic imaging [57].

The commonly used subdermal contraceptive implants, Implanon and Nexplanon, are MRI safe [52,58].

Urinary catheters and devices — Urinary catheters are MRI safe and usually comprised entirely of plastic or silicone. Artificial urinary sphincters are rated MRI safe or MRI conditional (figure 2) [51]. Safety profile of most commonly encountered models can be found online [19,33].

Penile prostheses — Most penile prostheses are rated MRI safe or MRI conditional (figure 2) [59]. Two notable exceptions are the Duraphase and OmniPhase models, which are rated MRI unsafe because of the concern for displacement or torque injury. Patients with these prostheses should not undergo MRI. Safety profile of most commonly encountered models can be found online [19,33].

PATIENT SAFETY EVALUATION PROCEDURE — The process for identifying potential hazards for MRI begins with the health care provider at the time of exam ordering. A more detailed safety screening is performed by the imaging service, usually on the day of the exam (form 1). This may uncover unexpected issues that could necessitate additional procedures for the patient (eg, medical record retrieval or radiography) before the MRI.

At time of referral for MRI — Potential hazards, if unanticipated when the patient presents to an imaging facility, can result in exam delay or cancellation. Thus, the healthcare provider should counsel the patient on the safety screening process and obtain the relevant history at the time that MRI is ordered. If safety concerns arise, the referring healthcare provider should consult the radiologist or the MRI safety officer at the imaging site to appropriately prepare the patient (table 2).

Common issues to be addressed at the time that MRI is ordered include:

●Assistance for the patient to complete the safety questionnaire – Some patients may need the help of a health care proxy, guardian, or a translator to navigate the safety screening process. These needs can be anticipated if the referring health care provider counsels the patient and informs the imaging service about communication concerns at the time of MRI referral.

●Cardiovascular implantable electronic device (CIED) – Consultation with radiology is advised before referral to MRI, as many sites will not have the resources to meet the safety requirements for the exam. If MRI is to be performed, a cardiologist should evaluate the patient before and after the exam and electrophysiology monitoring is usually performed during the examination [36,37]. (See 'Cardiovascular implantable electronic device' above.)

●Safety profile of an implant or a medical device – Certain devices or implants must be identified by the manufacturer and lot and/or model number and their safety labeling individually verified on databases such as MRIsafety.com. This applies to CIEDs, cochlear or otologic implants, and metallic implants near vital structures (eg, intracranial aneurysm clip or vascular coil). The identifying information on the hardware is contained in the medical record describing the device placement procedure. Some patients may have this information as the device specifications are often given to the patient at the time of placement. MRI is contraindicated in patients with objects classified as MRI unsafe or where the safety profile is unknown. The exception is CIEDs where imaging may be possible at certain centers with the necessary expertise (see 'Cardiovascular implantable electronic device' above). For objects classified as MRI conditional, the referring health care provider should discuss with the radiologist the risks versus benefits of proceeding with the MRI and the necessary conditions for minimizing the risks at the time of exam ordering.

●Recent surgery or interventional procedure – Most implants are thought to imbed in tissue between six to eight weeks after placement. Thus, many objects, even if they are ferromagnetic, can be cleared for MRI after an eight-week delay. If possible, the MRI should be scheduled at least eight weeks following surgery or an interventional procedure that placed a foreign body.

●Bullet, shrapnel, or other metal fragments – Radiographs may be needed to demonstrate the location of embedded metallic objects for safety purposes or to determine the feasibility of obtaining a diagnostic-quality MRI. If prior radiographs or computed tomography (CT) exams already exist, the health care provider should inform the radiologist so that these images can be obtained and reviewed beforehand. If prior imaging is not available, the health care provider should advise the patient and notify the imaging service so that radiography prior to MRI can be arranged.

●In all patients who have had orbital trauma by a metallic foreign body, use of orbital radiography is recommended. One or two radiographic views are recommended in cases involving an injury to the eye. In addition, the successful completion of MRI after orbital trauma involving a metallic foreign body is not considered sufficient proof of safety for a subsequent MRI examination [11,12].

●Temporary removal of an implant or device – Patients should be forewarned that they will be asked to remove dentures, hearing aids, insulin infusion pumps, continuous glucose monitors, and medication patches for the duration of the MRI visit. Since most visits last between two to three hours, the patient should plan accordingly. Patients should know in advance that body piercing jewelry must be removed and they may need to seek professional help to do so.

Upon patient arrival at the imaging facility — Healthcare providers should inform their patient about the safety screening process that precedes the MRI examination.

Patients must complete a detailed questionnaire about their medical history, especially prior surgeries, interventional procedures, and trauma (form 1). This form also includes a long checklist of the common implants and devices, both medical (eg, contraceptive device, orthopedic hardware, ports, stents) and nonmedical (eg, tattoo, jewelry). In addition to the written screening, the MRI technologist or nurse will take a focused verbal history after review of the screening form.

Unanticipated safety issues discovered by the detailed screening may involve additional procedures for the patient. Radiographic imaging may be necessary to locate a suspected foreign body (eg, orbital radiography for metal workers). The patient may need to work with the imaging service to retrieve identifying information of some implanted objects (ie, vendor and model and/or lot number). If MRI is to be undertaken in a patient with an indwelling object that could be ferromagnetic, the radiologist will usually counsel the patient and, in some cases, obtain formal written consent.

Patients are usually asked to arrive at the imaging facility an hour to 90 minutes in advance of their scheduled MRI to allow sufficient time for the safety screening. Some facilities will send the questionnaire to the patient several days before the exam date to facilitate the process.

Most patients will be asked to change out of their street clothes and into a hospital gown. Metallic microfibers (eg, antimicrobial silver and copper) in fabrics used in some athletic clothing such as yoga pants can cause burns [60]. For areas in or around the volume of transmitted radiofrequency (RF) power deposition, patients should wear MR-safe gowns or scrubs that can be supplied by the MRI facility. All devices (eg, medication pumps or patches, dentures, pessaries) and jewelry will must be removed. Once prepared for the exam, some facilities will also scan a patient with a hand-held or wall-mounted metal detector before allowing them to enter restricted areas around the scanner. (See 'Scanner environment' below.)

SPECIAL POPULATIONS

Patients with altered mental status — Patients who cannot provide reliable histories because they are unconscious or confused require special safety screening procedures (table 2).

If the patient's condition is likely to improve, the exam should be delayed, if possible, until a reliable history can be obtained. If waiting is not an option, available medical records and imaging should be reviewed for evidence of a foreign body. Caregivers and family members should be interviewed for relevant history of surgery, trauma, or interventional medical procedures. Before imaging, the patient should be carefully examined for any signs (eg, scar, deformity) that might indicate the presence of an implant or device. Any anatomic areas of concern should undergo radiography.

Patient monitoring during scanning requires MRI-compatible equipment and personnel with the necessary expertise and training in MRI safety procedures. (See "Anesthesia for magnetic resonance imaging and computed tomography procedures", section on 'Anesthetic challenges for magnetic resonance imaging'.)

Children — For most pediatric patients, the relevant history of underlying implants or devices is obtained from the parent, caregiver, or guardian. Older patients (ie, teenagers) should also be questioned separately to ensure that all potential hazards are disclosed (table 2).

Most imaging services strongly discourage or do not allow comfort items (eg, toys, pillows) in the scanner environment. If the patient wishes to be accompanied by an adult during imaging, that individual must also undergo the complete MRI safety screening process. (See 'Patient safety evaluation procedure' above.)

Sedation is necessary for many children so that they are motionless for the duration of MRI. Patient monitoring during scanning requires MRI-compatible equipment and personnel with the necessary expertise and training in MRI safety procedures. (See "Anesthesia for magnetic resonance imaging and computed tomography procedures", section on 'Anesthetic challenges for magnetic resonance imaging'.)

Prisoners and security personnel

●Restraints and tracking devices - Many restraints or tracking devices are metallic and can pose safety risks to patients who are prisoners. The presence of ferromagnetic components increases the risk of projectile injury with the static main magnetic field, and metallic shackles are also often comprised of interconnected circular links and linked across limbs, greatly increasing risk of thermal injury from radiofrequency (RF) and gradient fields. Tracking bracelets can also be damaged by the gradient and RF fields.

Thus, when prisoners are scheduled for MRI:

•Ferromagnetic restraints and devices should be removed before entering restricted areas (safety zones III and IV). (See 'Safety zones' below.)

•If restraints are required during the MRI session, arrangements should be made in advance for use of nonmetallic restraints and for the supervising officer to be available within the restricted areas to maintain security.

•Both prisoners and accompanying officers need to be screened (similar to any patient) prior to entering any restricted areas [11].

●Firearms - Security personnel accompanying the prisoner are often armed.

Firearms should not be brought into restricted areas unless necessary for security and cannot be brought into the magnet room [11]. Firearms are composed of sufficient ferromagnetic materials to become projectiles from the static magnetic field of an MRI scanner. These weapons can be carried with a round in the chamber so that they are ready for use in emergency situations, which adds to the risk of an accidental discharge [61]. While a mechanical safety is often engaged, this lowers but does not eliminate the risk of spontaneous discharge should a firearm be pulled into the magnet [62].

SCANNER ENVIRONMENT

Restricted access for patients and health care providers — Because of safety concerns, access to the area around the MRI scanner is restricted. Individuals working in these areas undergo MRI safety training regularly. Patients or healthcare providers without MRI safety training must undergo screening for metallic or electrical objects before entering and must also be accompanied by qualified MRI personnel while in these areas (see 'Patient safety evaluation procedure' above). They must also be accompanied by qualified MRI personnel while in these areas.

Restricted access to the scanner environment becomes particularly relevant during medical emergencies since code teams cannot safely access these restricted areas. Thus, most imaging facilities have a designated location for resuscitation away from scanner that can be accessed by all health care providers.

Safety zones — An MRI facility is divided geographically into four safety zones, labeled I to IV [1].

●Zone I – This area is freely accessible to the public without supervision. An example would be the reception area to an MRI center.

●Zone II – This is a public area that serves as an intermediate space between Zone I and the restricted areas. This is where the final safety screening is performed and patients are required to change out of their street clothes.

●Zone III – This is a restricted area. All individuals undergo safety screening prior to entering and are always accompanied by trained personnel. The magnetic field within this zone can cause harm and can exceed 5 G (0.0005 Tesla), the upper limit for safety designated by the US Food and Drug Administration (FDA) [63].

●Zone IV – This is a restricted area and only accessed from Zone III. This is the room where the MRI scanner is located and, hence, the magnetic field is the strongest. The entrance to this room is usually marked with a red light, physical barrier, and a warning sign stating that "the magnet is always on."

SUMMARY AND RECOMMENDATIONS

●Unanticipated safety issues from metallic or electrical implants, devices, or foreign bodies, can lead to exam delays and, rarely, cancellation at the time the patient presents for magnetic resonance imaging (MRI). Many common issues can be addressed before the exam visit if the referring health care provider elicits the relevant history at the time of referral and communicates it to the imaging service.

●When metallic or electrical implants, devices, or foreign bodies are near an MRI scanner, potential safety risks that arise are projectile or torque injuries, thermal injury, and device malfunction. In addition, artifacts resulting from imaging patients with indwelling foreign bodies can significantly impair image quality and result in missed or delayed diagnoses (table 1). (See 'Patient safety concerns' above.)

●The MRI safety profile of most medical implants and devices can be found in searchable, updated databases such as MRIsafety.com. Many manufacturers also publish online MRI safety information for their products. The identifying information on the hardware is contained in the medical record describing the device placement. Patients may have been given a card with the identifying information and the MRI safety profile at the time of procedure. (See 'Assessing implants, devices, or foreign bodies for MRI' above.)

●The United States Food and Drug Administration (FDA) has adopted a standard labeling scheme to indicate the degree of MRI compatibility of an object (figure 2). All medical implants and devices are labeled under one of three categories as MRI safe, MRI conditional, or MRI unsafe. (See 'Safety labeling' above.)

●If the object is considered MRI unsafe or if the safety profile of the object is unknown, the referring health care provider should discuss the exam indications and alternative diagnostic strategies with the radiologist at the time of referral. (See 'Procedures for common implants, devices, and foreign bodies' above.)

●The process for identifying potential hazards for MRI begins with the health care provider at the time of exam ordering (table 2) (see 'Patient safety evaluation procedure' above). A more detailed safety screening is performed by the imaging service, usually on the day of the exam (form 1). This may uncover unexpected issues that involve additional procedures for the patient (eg, medical record retrieval, radiography) before the MRI.

●Because of safety concerns, access to the area around the MRI scanner is restricted. Patients or health care providers without MRI safety training must undergo screening for metallic or electrical objects before entering. They must also be accompanied by qualified MRI personnel while in these areas. (See 'Scanner environment' above.)