Summary / Explanation
Current research indicates minimal long-term harm from extended magnetic resonance imaging (MRI) exposure, yet minor reversible effects from its magnetic, gradient, and radiofrequency (RF) fields have been described.[1] These findings underscore the necessity of a detailed investigation to determine this technology's long-term biological and health repercussions. Assessments should encompass not only basic MRI functions like the primary magnetic and RF fields and magnetic field gradients but also screening protocols, monitoring, and assessment methods. Adopting this comprehensive strategy will deepen the healthcare professional's understanding of MRI's biological implications to promote its safe and efficient use in clinical settings.
Basic MRI Principles
MRI technology operates based on the magnetic properties of hydrogen protons in body tissues. Protons act like miniature magnets when exposed to a strong magnetic field. Thus, during an MRI procedure, tissue protons align themselves with the equipment’s magnetic field. However, factors like temperature prevent the hydrogen nuclei's magnetic moments from aligning perfectly, giving rise to the net magnetization vector. This vector represents the sum of the protons’ magnetic moments within the sample.[2]
Net magnetization is crucial as it enables MRI technology to produce clear, detailed images of the body's internal structures. The principles of proton alignment and net magnetization comprise the foundational elements of MRI, providing unparalleled insights into the human body’s inner workings.
Relaxation times, specifically T1 and T2, influence image contrast and are indispensable in MRI technology. T1, or longitudinal relaxation time, refers to how long protons take to realign with the equipment's main magnetic field.[3] T2, or transverse relaxation time, is the time it takes protons to lose phase coherence perpendicular to this field.
Different tissues have unique T1 and T2 times, leading to variable signal intensities in MRI images and enabling detailed internal visualization. MRI images can be 'weighted' to emphasize these differences. T1-weighted images make tissues with shorter T1 times appear brighter. Meanwhile, T2-weighted images highlight those with longer T2 times. Proton-density weighting focuses on hydrogen proton density in tissues.[4] Switching from one mode to another enables tailoring scans to specific medical needs.
MRI can create 2-dimensional (2D) and 3-dimensional (3D) images. In 2D MRI, slice selection, frequency encoding, and phase encoding gradients define the imaged body slice. In contrast, 3D MRI replaces the slice-selection gradient with a second phase-encoding gradient, allowing volumetric data acquisition for creating detailed images.[5]
The filling k-space is a central MRI element that affects image contrast and quality. Strategies like spiral imaging, starting from the k-space center and spiraling outward, influence tissue contrast in the final image and impact resolution.[6] Advanced MRI sequences like gradient (GRE) and spin (SE) echo manipulate protons differently, creating images with unique characteristics. GRE is fast and sensitive to magnetic field variations, while SE provides high tissue contrast.[7]
Contrast agents like gadolinium-based compounds enhance MRI imaging by increasing contrast, especially in tissues or blood vessels. Contrast agents change the hydrogen protons' relaxation properties, visually enhancing specific structures.[8] Equipment setting variations and contrast agent administration allow for precise and comprehensive internal body examinations using the MRI.
Biological and Health Effects of MRI
The energy frequencies involved in clinical MRI are lower than in x-rays, visible light, and microwaves. Tissue heat accumulation occurs primarily through cellular RF absorption. The amount of absorbed energy increases with frequency, making RF heating a concern. Humans regulate temperature through convection, conduction, radiation, and evaporation, relying on surface area for effective heat dissipation. Higher surface-area-to-volume ratios enhance this ability. Thus, rapid fast spin echo (FSE) sequences in MR imaging, which involve increased RF pulses, raise concerns about amplified RF effects.[9]
The magnetic field strength in MRI is measured in gauss and kilogauss, with higher field strengths measured in Tesla. Field strengths below 2.0 Tesla have no significant biological effects on humans. However, reversible effects like ECG changes have been reported, possibly linked to the magnetic hydrodynamic effect in blood moving across a magnetic field. This effect can interfere with cardiac gating techniques in high-field scanners.[10]
Field strengths greater than 2.0 Tesla are known to cause reversible biological effects, such as fatigue, headache, hypotension, and irritability, and risk disturbing intracellular molecules like DNA and hemoglobin.[11] Hemoglobin disruptions in patients with sickle cell disease can precipitate vaso-occlusion.
Natural magnetic fields influence lower life forms. When using MRI, small electrical potentials in large blood vessels have been observed to develop perpendicular to the static field.[12] Studies show no significant effects on cell growth and morphology at field strengths below 2.0 Tesla.[13] Meanwhile, research into high-level microwave radiation and electromagnetic fields' effects on humans has yielded controversial and inconclusive results regarding carcinogenesis.[14]
The 5-gauss line deserves a special mention. This line represents a point where the magnetic field strength diminishes to a level considered safe for human exposure. Magnetic fields above this level may pose health risks such as implanted device malfunction, particularly if exposure is prolonged. This area around the scanner is often marked with a line.
MRI Screening
Implementing a meticulous screening process protects everyone accessing the area surrounding the MRI equipment. This process involves in-depth questioning and educating patients and staff, ensuring a safe environment. The most reliable method of preventing health risks during imaging procedures is a thorough screening of patients and MR staff.[15] Those with potentially ferromagnetic foreign objects inside or on their bodies must undergo an extensive examination to prevent health hazards or accidents.
Everyone, including patients, volunteers, visitors, MR healthcare workers, and cleaning staff, should be meticulously screened by qualified professionals before entering the MRI zone. Routine service checks and ongoing education are likewise crucial. Careful MR facility management and regular maintenance are vital to creating a safe space for all. Most MR-related injuries are due to inadequate screening.[16] However, not all MRI users adhere to stringent screening processes, and no consensus has been established regarding the best screening protocol.[17]
MRI Zones
The four "zones" around an MRI magnet represent restriction levels on patients as they approach the equipment (see Image. MRI Safety Zones):
Zone I constitutes areas where the MRI machine’s magnetic field poses no hazard to a patient. No restriction on patient movement needs to be imposed in this area. Zone I includes the hallway leading to the MRI department and hospital areas freely accessible to the public.
Zone II generally constitutes the MRI department’s waiting room and, often, dressing rooms. Patient movement restrictions are not required in this zone, either.
Zone III is a restricted space immediately beyond the MRI room. For example, the control room where MRI technicians operate the scanner is considered to be within Zone III.
Zone IV is the MRI scanner room proper, which poses the greatest hazard to patients and facility workers. Stringent safety precautions are required while operating in this zone.
The MRI Screening Form
A detailed questionnaire for patients and staff helps identify potential adverse reactions to strong magnetic field exposure. The form must ask about past surgeries, metallic foreign body injuries, and pregnancy status. The presence of implants, materials, devices, or objects that magnetic fields can damage, including electrically, magnetically, or mechanically activated gadgets, must be elicited. The questionnaire should include a body diagram to indicate the hazardous implants’ locations. This form can also help gather crucial information for assessing other MRI-related risks, such as past adverse reactions to contrast media.
The MRI Screening Interview
Verbal interviews are necessary when written questionnaires have limited utility. For example, language barriers and visual and literacy issues can make patients answer written questionnaires incompletely or incorrectly. An interview can bypass these problems if conducted verbally in the individual’s native language. An MR technologist or trained staff can further ensure safety by clarifying and confirming responses. This "oral phase" is crucial in verifying the reliability of the provided information.
Prescreening for Metallic Implants
Each MRI facility must have a standard MRI screening policy for individuals suspected of having foreign metallic implants or devices. The policy should apply regardless of the MR system's field strength, magnet type, or magnetic shielding presence. Protocols regarding the radiographic procedures and evaluation of the metal object’s potential risks when exposed to strong magnetic fields must be outlined.[18]
Pregnant Patients
MRI for pregnant patients is usually recommended to avoid ionizing radiation exposure, eg, from computed tomography or fluoroscopy, or when alternative nonionizing diagnostic methods are insensitive. MRI’s benefits and risks must be determined in patients who are pregnant or may be expecting. MRI can image the fetus in various planes. Additionally, large fetal and placental fields of view may be examined with excellent resolution through this modality. MRI affords detailed imaging with less ionizing radiation risk, which may account for this diagnostic tool’s rising popularity in obstetrics.[19]
However, concerns about MRI’s safety in this cohort exist, particularly when gadolinium-based contrast agents (GBCAs) are administered.[20] The safety of MRI and gadolinium administration in pregnant patients is a subject of ongoing research, given the potential risks to the fetus.[21] GBCAs can cross the placental barrier. The concern is greatest during the first trimester due to the contrast agent’s possible teratogenic effects.[22]
MRI Safety for Ancillary Equipment and Implants
Ancillary equipment must meet one of 3 criteria: manufacturer's safety declaration, FDA approval, or previous testing. “Manufacturer declaration” means the maker tested the equipment for safety.[23] Metallic implants can acquire significant torque in magnetic fields, posing risks if not anchored properly.[18] The metal type determines the magnetic field’s effects on the implant. Strong magnetic fields can cause significant deflection in ferrous metals. Meanwhile, nonferrous metals tend to accumulate heat in the MRI environment due to RF absorption.[18]
The patient's surgical history must be obtained before an MRI, particularly if implants were used. Even without significant health effects, implants can produce MRI artifacts that can lead to misinterpretation. The metal type and implant size influence the artifact size.[24]
Understanding the interactions between various medical implants and the MRI environment can help healthcare professionals enhance patient safety while maintaining imaging quality. Below are some key considerations for different types of implants and devices.
Aneurysm clips
The aneurysm clip material largely determines its safety within the MRI environment. Ferromagnetic clips, like those made from certain stainless steel types, pose serious risks and are contraindicated. Nonferromagnetic or weakly ferromagnetic clips, eg, specific alloys or titanium, are generally safe. MRI with aneurysm clips should only proceed after confirming the clip type as nonferrous.[25]
Hemostatic vascular clips
Hemostatic clips, typically made from nonferromagnetic materials, have shown no deflection in magnetic fields and are considered safe in MRI environments.[26]
Intravascular coils, filters, and stents
Some intravascular devices are ferromagnetic but can become embedded in vessel walls over time, reducing dislodgement risks. MRI is usually safe for patients with these devices after a sufficient postimplantation period.[27]
Carotid artery vascular clips
Most carotid artery clamps are safe in MRI environments. The exception is the Poppen-Blaylock clamp, which shows significant magnetic attraction.[28]
Vascular access ports
Common bioimplants facilitating medication administration are made from various materials. Most vascular access ports show minimal magnetic deflection, indicating they may be safe to expose to MR energy.[29]
Heart valves
Most heart valve prostheses show minor attraction to MR system magnetic fields. Attractive forces are minimal compared to cardiac forces. Thus, MRI is generally safe for patients with these prostheses, including previously considered hazardous types like the Starr-Edwards Model Pre-6000.[30]
Dental devices and materials
Dental materials show measurable deflection, but only materials with magnetic components may pose problems during MRI procedures. Dental devices held in place nonmagnetically generally do not cause issues, though they might affect image quality.[31]
Penile implants
Among tested penile implants, only one model showed significant magnetic deflection.[32] MRI is inadvisable if the penile implant might cause discomfort during the procedure.[32]
Otologic implants
Certain otologic implants are approved for 3-Tesla MR systems. Such models include the Baha and Ponto Pro osseointegrated implants and Sophono Alpha 1 and 2 implanted magnets. Cochlear implants with removable magnets are approved for 1.5-Tesla MRI as long as the magnetic component is removed before the procedure.[33]
Ocular implants
Some eyelid springs and retinal tacks are attracted to magnetic fields and may cause ocular discomfort or injury during an MRI procedure. Protective measures are advised in some cases.[34]
Intraocular ferrous foreign bodies
Metal fragments in the eye pose risks due to magnetic field-induced movement. Small fragments are detectable via radiography. X-rays usually suffice for detecting metal fragments that might cause damage even though computed tomography is more accurate than radiography.[35]
Metallic foreign objects
Radiographic confirmation is essential for suspected metallic foreign bodies. The location, mass, orientation, and proximity to sensitive structures are crucial in assessing MRI safety in the presence of a foreign object.[36]
Bullets, pellets, and shrapnel
Caution is advised for patients with embedded bullets or shrapnels, especially considering the varying metallic compositions. Some of these objects are ferromagnetic.[37]
Orthopedic implants, materials, and devices
Orthopedic implants typically show no magnetic field deflection. Additionally, heating due to magnetic and RF field exposure is unlikely, making most MRI procedures safe for patients with these implants.[38]
Pacemakers
The use of MRI in patients with cardiac pacemakers has historically been a complex issue due to safety concerns. While MRI has traditionally been contraindicated for this cohort, recent advancements have led to the development of MRI-conditional pacemakers.[39] These novel devices can undergo MRI scans, though the appropriate energy settings may be limited.[40] Protocols should be established to ensure patient safety, including pre- and post-MRI device checks and close monitoring during the procedure.[41]
MRI is often critical to diagnostic accuracy, but whether this modality’s benefits in patients with pacemakers outweigh the risks must always be considered. Managing such patients requires coordination between radiology and cardiology specialists and using appropriate safety mechanisms.
Assessment and Monitoring
Patients undergoing MRI must be monitored visually and verbally. The technologist should interact with the patient before, during, and after the procedure. Patients unable to communicate, eg, due to sedation, coma, age, hearing impairment, and language barriers, also require physiologic monitoring.
The choice of monitoring methods depends on the patient's condition, clinical indications, MRI procedure type, and available equipment. Options include visual inspection, pulse rate, electrocardiography, respiratory rate, pulse oximetry, capnometry, and temperature. Patients under sedation or anesthesia require appropriate monitoring to manage potential adverse drug reactions.[42][43]
MR-safe monitoring devices—instruments that do not interfere with MRI processes—are available. MR-safe electrodes should be used during cardiovascular MRI, ie, ECG-based gating is required, to ensure safety and data accuracy. Sedated patients must be monitored with pulse oximetry, as magnet-hemodynamic effects can cause ECG artifacts, and many sedation drugs depress respiration.[44]
Claustrophobia is a concern in MRI due to RF heating, gradient noise, and space confinement. Some patients develop persistent claustrophobia after MRI, requiring psychiatric treatment.[45] Controllable air movement in the magnet bore, along with good patient communication and education, can help reduce these reactions.
Gadolinium-Based Contrast Agents
Gadolinium is the most frequently used MRI contrast agent. Gadolinium side effects are generally minimal but may include temporary bilirubin and blood iron elevation, headaches, nausea, and anaphylaxis.[46] Gadolinium chelates are administered intravenously, followed by a saline flush. Generally, gadolinium chelates are safe with few absolute contraindications. However, caution is advised in individuals with sickle cell disease, renal failure, and allergies, and pregnant or lactating women.[20]
Emergency Procedures for MRI Environments
The most vital step in medical or technical MRI emergencies is promptly removing the patient from the scan room. The patient should be moved to a safer location to avoid the risks posed by the strong magnetic field inside the area. Administering emergency medical care inside the MR scan room can be dangerous due to the magnetic nature of most crash cart items.[47]
Emergencies like a fire or quench—the sudden loss of superconductivity in the MRI magnet, leading to a rapid helium release—necessitate immediate patient evacuation.[48] Two instances require manually quenching a magnet: a sizeable fire endangering human life and, rarely, a ferromagnetic object pinning an individual to the scanner.
Evacuation Protocol
Running a code, ie, performing emergency resuscitation, within the MRI environment is highly discouraged. Using ferromagnetic objects like monitors and defibrillators near highly magnetic equipment can cause injury to both the patient and healthcare personnel. An area outside the MRI scan room must be designated for medical emergencies to allow healthcare teams to focus on patient care without the risk of magnetic interference. The designated area should be prepared and equipped to handle full emergency procedures.
A code is not a reason to consider quenching an MRI machine. Emergency resuscitation is typically performed in Zone II.
Conclusion
Understanding and addressing the possible causes of adverse events during an MRI procedure can help enhance this diagnostic modality’s safety and effectiveness. Overall, an interprofessional approach to safety, guided by ongoing research, is essential in applying this technology.
Close collaboration among radiologists, physicians, and safety experts to craft MRI screening and monitoring protocols helps protect patients, particularly individuals with implants. Technologists and physicists ensure that MRI equipment is properly maintained and calibrated for safety and optimal imaging quality.
The judicious use of GBCAs in sensitive patients also demands a collaborative approach between radiologists, nephrologists, nurses, and pharmacists. In emergencies, coordinated efforts between different medical teams are crucial for prompt and appropriate responses within the unique MRI environment.
Interprofessional teams can collaborate to make informed decisions regarding patient care, balancing the need for diagnostic imaging with safety considerations. Clear and seamless interprofessional communication can ensure better outcomes for patients undergoing MRI procedures.
