MRI is primarily a noninvasive diagnostic technique that uses the magnetic properties of atomic nuclei to produce high-resolution, multiplanar cross-sectional images of the body. In certain fields of medicine though, MRI is being utilized as an interventional aid in diagnosis or in disease therapy, in lieu of conventional approaches. Atoms having an odd number of protons and/or neutrons in their nuclei, for example, hydrogen, have an associated electrical charge, and the net rotation of protons or neutrons produces a local magnetic field similar to the electromagnetic field produced by the flow of electrons in a wire loop. Normally, the magnetic fields surrounding these nuclei are randomly oriented. When placed in the setting of a powerful static magnetic field of the MRI scanner, the nuclei align themselves longitudinally so they lie parallel or antiparallel to the magnetic field.

Within the MRI, a specific radiofrequency (RF) pulse of energy (a second magnetic field) is directed toward the patient at right angles to the static magnetic field, thereby displacing the orientation of the aligned nuclei from the longitudinal magnetic field. As the RF pulse is removed, the nuclei return to their original magnetic alignment positions. This is termed relaxation. The energy released from the tissue as the nuclei relax is termed an RF signal. It can be detected by the receiver coil of the scanner and used to produce the MR image. There are two ways to measure relaxation time. T1 relaxation represents the time to revert to the resting magnetic state, and T2 relaxation represents the time to revert to the resting axial spin. These values vary for specific body tissues, especially water and fat. This allows for differentiation of body structures as well as between normal and pathologic tissue. Different atoms respond to different RFs, with the response being proportional to the strength of the static magnetic field. Hydrogen is present in high concentrations in biologic tissues and is the atom most often used for imaging. Resolution of the image requires a strong magnetic field. The magnetic field strength used to quantify the scanners is in tesla units. One tesla is equivalent to 10,000 gauss. Most MRI scanners used now have a magnetic field of 0.5 to 3.0 T (the magnetic field of the earth is 5 × 10^-5 T). Weaker field scanners have the advantage of having an open construction, whereas the stronger closed ones generate better image quality. Some research scanners vary in strength from 4 T to 9.4 T. The magnetic field of an MRI takes several days to establish. Therefore, it is constantly applied even in the absence of a patient, and it is deactivated only in an absolute emergency.

The advantages of MRI include the lack of ionizing radiation, the capability of using noniodinated intravenous contrast agent, and the production of multiplanar, temperature sensitive, contrast amplified images. MRI also provides excellent resolution of normal tissues (e.g., gray and white matter and soft tissues) without image artifact from bone, and it can differentiate
between normal and pathologic tissue. MRI may be used to evaluate blood and cerebrospinal fluid flow, to assess contraction and relaxation of organs such as the heart, and to provide images of tissues in proximity to bone because calcium does not interfere with MRI.

MRI-guided intervention is another major potential advantage in cases where conventional computed tomography (CT) imaging is lacking. These images of the soft tissues and tumors are exceptionally delineated, thus facilitating interventions for biopsies of these areas. In addition, tumor ablation via cryoablation modalities has been performed safely in some centers in the MRI suite. MRI offers better capture of the temperature-related changes that occur during cryoablation and has been effectively used in safely treating liver, kidney, breast, and prostate tumors as well as in uterine fibroids.

MRI has also proven advantageous during interventional electrophysiology studies and catheter ablation procedures. The MRI offers structural real-time three-dimensional (3D) images of the anatomy, presence of scar tissues in the heart and the nearby structures, and exact localization of the ablation catheter. It can also delineate any potential gaps in ablation for a successful therapy. Delayed enhancement (DE) MRI is currently the gold standard for outlining the scar/fibrosis in the heart that potentially may serve as an arrhythmogenic source for patients with ventricular tachycardia (VT). Based on the staging of atrial fibrosis, DE MRI can also provide vital preprocedural information in extrapolating those patients who might be at risk for relapse of atrial fibrillation. In experienced hands and centers, this real-time MRI provides vital information that traditional fluoroscopy could not supply in a radiation-free environment.

There are many hazards associated with an MRI suite. The constant static magnetic field, the pulsed (time-varied) gradient magnetic fields, and the high-frequency RF waves all interact and when combined with a ferromagnetic object can pose significant risk to the patient or operator. Not only can the attraction of these external objects become fatal but also implantable objects can dislodge and malfunction or burn the patient. Depending on the distance from the core of the magnet, the magnetic field strength diminishes. Distances less than five gauss line can make pacemakers malfunction. Distances less than 50 gauss line can make ferromagnetic gas cylinders become lethal projectile missiles.

Owing to the potential for catastrophic events, recommendations for MRI safety have been proposed by the American College of Radiology. Briefly, the MRI suite should be separated into four zones: zone I is the public zone immediately outside the MRI suite, zone II is the reception area monitored by MRI personnel, zone III encompasses the control room with access to this area being restricted from the public, and zone IV is the physical MRI scanner room. To ensure everyone’s safety, entry into zones III and IV should be screened by MRI personnel. Ferromagnetic objects should not be brought into zone III, but if necessary, they should be brought in with extreme caution. Items such as credit cards, pens, keys, laryngoscopes, scissors, stethoscopes, paper clips, and ferromagnetic gas cylinders should be excluded from zone IV. At Weill Cornell Medical Center, all oxygen E-gas cylinders have been replaced by aluminum cylinders to further ensure patient and personnel safety. Adherence to these zones must be maintained especially during emergent situations.

Safety terminology for devices and implants were devised by the U.S. Food and Drug Administration (FDA), Center for Devices and Radiologic Health in 1997 to distinguish the safety of the device (MRI safe) and the compatibility of the device (i.e., the lack of artifacts on the exam) in a particular MR environment (MRI compatible). Over the years though, these labels were potentially being misused, and due to safety concerns, a set of new terminologies were developed by the American Society for Testing and Materials (ASTM) International.
MR safe refers to items with no known hazards in any MR environment. MR conditional refers to items that pose no known hazards in a specified MR environment (i.e., strength of the magnetic field). MR unsafe refers to items that pose definite hazards in all MR environments (i.e., ferromagnetic scissors). Concerns about a specific device and its potential hazards should be addressed by the staff members, on-site radiologist, and/or manufacturer to determine whether the exam can be conducted in a safe manner.

Obviously, with the growing use of implantable medical devices (e.g., pacemakers, vascular clips, implantable cardioverter-defibrillators [ICDs], mechanical heart valves, and implanted infusion pumps), a potential exists for injury to the patients. Most anesthesiologists agree that MR should not be conducted on patients with an implanted device, but that is not an absolute. In 2011, MR conditional pacemakers and leads became available and are available in the United States. They are approved in certain settings and up to 1.5 T MRI examinations. Nevertheless, the majority of cardiac implantable devices are not MR safe, so caution must be used. Currently, there are no ICDs that are MR conditional. Clips used in vascular surgery and neurosurgery must be verified and the model, and type of clip needs to be documented. All intracranial aneurysm clips manufactured after 1995 that the company identifies it as MR conditional need no further testing. Clips manufactured before 1995 require some pretesting and may need the approval of an MR attending radiologist. Despite a prior MR history of a patient, variations can occur in different magnetic fields under the influence of different pulsed frequencies; so, verification may still be needed.

In addition to these devices, many patients have stents, filters, and grafts. Coronary artery stents, especially after 2007, are proven safe for patients having an MRI procedure at ≤3 T with an average whole-body specific absorption rate (SAR) of 2.0 to 4.0 W per kg for 15 minutes of scanning time. Most aortic stent grafts are MR safe, but the Zenith AAA and the TAA endovascular graft (Cook Medical, Bloomington, IN) are MR conditional, and their timing for scanning needs to be individualized. Coil embolization materials, such as platinum, iridium, and nitinol, have been shown to be safe for MR examinations. Most heart valves and annuloplasty rings can be safe under certain conditions, that is, ≤3 T, but if possible, delay the scan for up to 6 weeks after implantation.

Recently, the ASTM has deemed certain cochlear implants as MR conditional and may be safe if the patient is older than 8 years of age and ≤1.5 T is utilized. Care must be taken with patients containing shrapnel and bullets; there may be potential risks if it is ferromagnetic due to movement and heating near vital organs. Patients with implanted stimulators need to be investigated, and some may be MR unsafe or conditional. MRI may be performed on a limited basis for patients with certain deep brain stimulators and vagal nerve stimulators. In each case, care must be individualized and decisions made with the on-site level 2 MR attending radiologist.

Other disadvantages of the MRI include the potential for malfunction of electronic equipment (e.g., monitors and infusion pumps) as well as limited access to the patient. MR safe/conditional monitors and machines should be used in the scanner. Thermal injuries have been reported from electrocardiogram (ECG) electrodes and pulse oximeter probes. All cables and monitoring equipment should be placed in a straight manner without coiling to ensure no such injuries. MR safe/conditional infusion pumps can be used in zone IV, but otherwise, pumps can be left in zone III and the tubing passed through a wave guide. The relatively long scan time necessary to obtain images (as much as 15 minutes per image, with a total scanning period of 1 to 3 hours) and image degradation from any motion including cardiac contractions, respirations, cerebrospinal fluid flow, and bowel peristalsis are additional disadvantages.

Another potential hazard to both the patients and the operators is the high decibels levels of sound emanating from the scanner due to the RF pulses (>90 dB). Hearing protection for both is mandatory even if the patient is under anesthesia. Due to the small bore of most MR scanners (i.e., 50 to 60 cm in diameter), scanning obese patients may be impossible and a sense of claustrophobia and anxiety can be experienced.

Patients with certain implanted biologic devices, such as pacemakers, ICDs, and certain pumps, should be excluded from MRI studies because of the possibility of device malfunction, inactivation, or damage in the magnetic field. In an MR environment, the device may be moved, rotated, dislodged, or accelerated toward the magnetic core. In the case of certain metallic devices (i.e., leads), they can act as an “antenna” and concentrate the RF energy, which can lead to thermal injuries. Fractured leads are absolutely contraindicated due to the high risk of thermal injuries. The safety of various cardiovascular devices for MR examination was reviewed based on ex vivo, animal, and in vivo studies and reported by the American Heart Association (AHA). Although many coronary stents, peripheral stents, sternal wires, inferior vena cava filters, prosthetic heart valves, and annuloplasty rings have been deemed MR safe or conditional, it is important to review each type carefully and decide whether the proposed MR exam is in line with the specified milieu for the particular device. Currently, the AHA and the American Society of Anesthesiologists (ASA) practice advisory task force on the anesthetic care for MRI regard the presence of a pacemaker or an ICD as a strong relative contraindication to routine MR examination. Although some MR conditional pacemakers and leads are available, the majority are still MR contraindicated. MR is currently contraindicated in patients with an artificial heart or ventricular assist device.

If the benefits far outweigh the risks of the exam, then the scanning should be performed in an experienced center with expertise in MR imaging and electrophysiology. The care of the patient should be performed in collaboration with the cardiologist, radiologist, device manufacturer, and the technician. Written consent must detail all risks to the patient including death. Full resuscitation equipment must be readily accessible in the MRI suite in case of an arrhythmia or equipment malfunction. For a more thorough evaluation of these devices, please refer to the recommendations in the reference article or online at www.MRIsafety.com. Orthodontic braces and dentures and tattoos or cosmetics that contain metallic dyes, although safe, can degrade the image quality significantly.

Although an area of controversy and ongoing investigation, most human data to date suggest that there are no significant deleterious effects to patients or health care professionals from exposure to the static magnetic field of an MRI. However, MRI capabilities were developed relatively recently (the 1960s), and epidemiologic evidence of harmful exposure may not yet be apparent. It seems prudent, therefore, to limit repetitive and readily avoidable exposure to the magnetic field.

Also controversial is the use of MRI during pregnancy because the effects of magnetic fields on the human fetus cannot be easily determined. However, current evidence does not
support the suggestion that routine clinical exposure of the pregnant patient to MRI can cause developmental abnormalities in the fetus. Pregnant patients have undergone MRI safely during all stages of pregnancy. Nevertheless, caution is advised. Studies have shown that gadolinium-based contrast agents enter the fetal circulation and are filtered in the fetal kidneys into the amniotic fluid. It is unclear the duration of the substance in the fluid, but it can potentially form free gadolinium ions that can theoretically pose risks to the developing fetus. The appropriateness of MRI examination and the acuteness of the diagnostic need must be considered. Ultrasonography use is preferred if it can provide equivalent diagnostic information, and consideration should be given to postponing the MRI until late in the pregnancy or until after delivery.