Anesthesia Delivery in the MRI Environment

Overview

Magnetic resonance imaging (MRI) is a widely used diagnostic modality with 30 million scans done annually in the United States alone according to a U.S. Food and Drug Administration (FDA) report from 2016. During MRI, the patient must remain in a confined, noisy, sometimes hot environment for periods varying from 30 to 60 minutes or longer. Familiarity with anesthesia equipment, the environment, and basic monitoring is vital to the safe and successful administration of anesthesia in the MRI environment. Equipment design and knowledge of the principles of monitoring in the MRI environment require a thorough understanding of the effects of the intense magnetic field and the underlying technology behind the challenges, limitations, and even the location of MRI equipment.

In 1986, the American Society of Anesthesiologists (ASA) first established standards for monitoring during anesthesia and subsequently delineated specific standards for the delivery of anesthesia and sedation in sites distant to the operating room (OR). The most recent applicable update is the ASA practice advisory on anesthetic care for MRI. Recognizing that not all care in areas outside the OR requires anesthesia, the ASA and other nonanesthesia specialty societies followed with guidelines for sedation and analgesia. In the 1990s, the American Academy of Pediatrics (AAP) and American College of Emergency Physicians (ACEP) published sedation guidelines. In 1996, the ASA introduced their guidelines for the administration of sedation and analgesia by nonanesthesiologists; these guidelines were recently updated in 2018. ^, The 2018 update provides practice guidelines for moderate procedural sedation and analgesia (previously called conscious sedation). Mild sedation, deep sedation, and general anesthesia are excluded from the guidelines. The ASA advisory on granting privileges for deep sedation to non-anesthesiologist physicians was last amended on October 25, 2017. The ASA believes that the best means to achieve the safest care is the participation of a physician anesthesiologist in all deep sedation cases. However, in view of Medicare regulations that allow for the participation of non-anesthesiologist physicians in providing deep sedation, that document offers a framework to identify those physicians, dentists, oral surgeons, and podiatrists who may potentially qualify to administer or supervise administration of deep sedation.

The Joint Commission has published guidelines to encourage uniformity and documentation of monitoring at locations throughout the hospital. The delivery of anesthesia and sedation in the MRI environment would fall under the guidelines and regulations not only of the ASA but also of The Joint Commission, other specialty organizations, and the Centers for Medicaid and Medicare and Medicaid Services (CMS). In 2009 the CMS published its Revised Hospital Anesthesia Services Interpretive Guidelines—State Operations Manual (SOM); these were subsequently updated in 2010 with an amendment in 2011. ^, In this amendment, CMS more clearly acknowledges that the boundary between “anesthesia and analgesia” is a continuum. According to CMS the anesthesia services must be organized under a single anesthesia service, that must be directed by a qualified physician. There is a hospital requirement to monitor quality and safety indicators for all “anesthesia” and “analgesia” services. “Anesthesia” means general anesthesia, regional anesthesia, deep sedation/analgesia, or monitored anesthesia care. “Analgesia/Sedation” means local/topical anesthesia, minimal sedation, and moderate sedation/analgesia (“conscious sedation”).

Accordingly, the CMS guidelines indicate that the administration of “anesthesia” should be limited to a (1) qualified anesthesiologist; (2) doctor of medicine or osteopathy (other than an anesthesiologist); (3) dentist, oral surgeon, or podiatrist qualified to administer anesthesia under state law; (4) certified registered nurse anesthetist (CRNA); or (5) anesthesiologist’s assistant under the supervision of an anesthesiologist who is immediately available if needed. Coincident with this restructuring, anesthesiologists have now become more prevalent providers in the MRI environment with a coincident sensitivity to the importance of establishing guidelines for safe practices in the MRI environment.

Many of the challenges encountered with anesthesia delivery and physiologic monitoring in the MRI environment have now been addressed. Technologic advances over the past decade have yielded monitoring, equipment, and devices that are more consistent, reliable, and valuable in the magnetic environment. As anesthesiologists become more prevalent in the MRI environment, they have coincidentally taken a more active role in the design, layout, space allotment, and equipment acquisition of new MRI units.

This chapter outlines some of the basic principles of magnetic fields, as well as the challenges and limitations to delivering general anesthesia, deep sedation, or monitored anesthesia care in the MRI environment. It includes a detailed review of the latest monitors, equipment, and devices. We have also included information regarding the use of cardioverter-defibrillators in an MRI setting.

MRI Physics and Clinical Applications

Nuclear magnetic resonance, which is the basis for MRI, was first described by the American physicist Isadore Rabi in 1939. In 1944 he was awarded the Nobel Prize in Physics for his discovery. The first human images were produced in 1977, and the first commercial scanner was introduced in 1980. The physical principles of MRI are based on the integrity of the magnetic field. Atoms with an odd number of protons and/or neutrons are capable of acting as magnets. When they are aligned in a static magnetic field, they can be subjected to radiofrequency (RF) energy that alters their original orientation. MRI static magnetic fields mainly exert attractive forces on metallic objects within the field and produce translational force, torque, and projectile movement. With removal of the RF pulse, the nuclei rotate back to their original alignment (relaxation), and the energy released can be detected and transformed into an image. Hydrogen is the atom most often used for imaging, as it is present in most tissues as a component of water and long-chain triglycerides.

The U.S. FDA deems MRI a significant risk when used under an operating condition of more than 8 Tesla (T) for adults, children, and infants aged more than 1 month, and 4 T in neonates (infants aged 1 month or less). Tesla is defined as a unit of measurement for the strength of the magnetic field. Gauss (G) is defined as a unit of measurement of magnetic flux density. The other FDA criteria for significant risks are specific absorption rate (SAR >4 watts per kilogram [W/kg] for whole body ≥15 min or SAR >3.2 W/kg for head ≥10 min), gradient fields rate of change (any change sufficient to produce severe discomfort or painful nerve stimulation), and sound pressure level (>140 dB). The application of MRI studies is far reaching, and MRI is used for the evaluation of neoplasms, trauma, skeletal abnormalities, and vascular anatomy. Brain MRIs are frequently performed to evaluate developmental delay, behavioral disorders, seizures, failure to thrive, apnea/cyanosis, hypotonia, mitochondrial, metabolic disorders, stroke, autism, and infectious processes such as meningitis/encephalitis, osteomyelitis, and abscess formation. MR angiography (MRA) and MR venography (MRV) are especially helpful in evaluating vascular flow, and can sometimes replace invasive catheterization studies for follow-up or initial evaluations of vascular malformations, interventional treatment, or radiotherapy. The performance of fetal MRI has dramatically increased since 2010. As of 2020 there are no studies demonstrating definitive risks to the fetus, neonate, or the mother when MRI is operated under guidelines set forth by FDA and other regulatory organizations.

Functional MRI (fMRI) has been increasingly used since the early 2000’s. fMRI measures the hemodynamic or even metabolic response related to neural activity in the brain or spinal cord, and fMRI is often able to localize specific sites of brain activation. fMRI is being widely used because of its low invasiveness and lack of radiation exposure. fMRI is being increasingly used in the preoperative workup of brain tumors, medically refractory epilepsy, as well as depicting cognitive function, neuropsychiatric disorders, and response to pharmacological agents. Some fMRI studies require cognitive facility that demands a conscious and responsive patient. Recently, dynamic MRI airway studies have used three-dimensional (3D) reconstruction of the images to visualize areas of airway collapse, tracheomalacia, or other compromise. ^, Audio-visual stimulation technology is commercially available for fMRI studies (VisuaStim Digital, Resonance Technology Inc., Northridge, CA). The VisuaStim yields 3D images that appear to emerge from the screen. The images are high-resolution (500,000 pixels per 0.25 square inch) with a refresh rate of 85 Hz. This can be equipped with an optional eye tracker module to record ocular movement.

Historically, anesthetic management of children in the MRI suite has been highly dependent on and somewhat limited by the availability of monitors and anesthesia machines suitable for use in an MR suite. In 2007, the American College of Radiology (ACR) established guidelines to minimize the risk of MRI-related mishaps and updated them in 2013. These guidelines were written in response to fatalities that occurred when loose, ferromagnetic oxygen cylinders became projectiles when accidentally brought into the MRI suite with a patient in the bore of the magnet. The revised 2013 guidelines were developed with representation from ASA members. The guidelines state that adherence to standards of care mandates following sedation guidelines developed by the AAP, the ASA, The Joint Commission, federal/state law, and institutional protocols. In 2008 the ASA assembled a task force composed of anesthesiologists and a radiologist with MRI expertise. This Task Force on Anesthetic Care for Magnetic Resonance Imaging created a practice advisory on anesthetic care for MRI, which was subsequently updated in 2015, focusing on anesthetic care of patients in the MRI. In this practice advisory, the MRI environment was acknowledged as being hazardous due to the presence of a very strong static magnetic field, RF waves, and a pulsed magnetic field. Other hazards included high-level acoustic noise, systemic and localized heating, and accidental creation of projectiles. This document establishes important recommendations for safe practice and consistency of anesthesia care in the MRI environment. Most important, this practice advisory includes all sedation, monitored anesthesia care, general anesthesia, critical care, and ventilatory support. Conditions of high-risk imaging were defined as imaging in patients with risks related to health, equipment, procedure, or surgery. MRI-guided interventions, cardiac imaging, and airway imaging were all identified as high-risk procedures. The advisory was designed to promote patient and staff safety, prevent MRI mishaps, recognize limitations in physiologic monitoring, optimize patient management, and identify potential equipment and health risks. Also, in 2016, the AAP and the American Academy of Pediatric Dentistry published guidelines for monitoring and management of pediatric patients before, during, and after sedation for diagnostic and therapeutic procedures.

Anesthetic management depends on the availability of support personnel, equipment, and monitors; the personal style and comfort level of the anesthesiologist; and, of course, the patient’s medical history. Safe monitoring conditions include the use of MRI safe/conditional monitors (including capnometers), remote monitoring, and compliance with ASA standards. Requiring a general anesthetic solely to ensure immobility for a radiologic imaging study is often a frightening concept for parents to embrace. They frequently equate pain and surgical interventions with the need for anesthesia and are reluctant to expose their child to a general anesthetic for an MRI study. Children younger than 5 years of age most commonly require moderate to deep sedation or a general anesthetic to ensure immobility. Pentobarbital, propofol, and dexmedetomidine have all been described as successful alternatives to inhalation anesthesia. Dexmedetomidine, a selective alpha-2 agonist, has become an adjunct and, at high doses, a sole anesthetic for MRI. Even at higher- than- recommended doses of dexmedetomidine (3 mcg/kg/h), airway patency and tone are maintained, making it an ideal agent for children with obstructive sleep apnea, dynamic airway imaging, and morbidities with potential for airway collapse such as mediastinal mass or other pediatric airway syndromes. ^{, ,} In addition to the intravenous (IV) route, dexmedetomidine is being increasingly administered intranasally with or without other adjuvants to achieve procedural sedation for MRI. The role of dexmedetomidine in anesthesia is the subject of a 2018 review article. Careful attention should be given to the bradycardia, hypotension, or transient hypertension related to concomitant use of anticholinergics with dexmedetomidine. The addition of a dexmedetomidine bolus (1–2 mcg/kg) to propofol can decrease propofol requirements and reduce airway complications.

In the presence of a magnetic field, anesthesiologists must be aware that many personal items, usually taken for granted, can be a hazard. These include clipboards, pens, watches, scissors, clamps, credit cards, eyeglasses, and paper clips. Specifically, medical equipment including oxygen tanks, medical cylinders, IV stands, wheelchairs, and beds can cause injury as projectiles. As early as 1985, some 24% of all MRI centers cited a projectile-related accident. Projectiles, however, are not the only concern. Conventional electrocardiographic (ECG) monitoring is not possible during MRI because when the lead wires traverse the magnetic field, image degradation occurs. Most importantly, however, the ECG leads may inductively heat and cause patient burns (the most common MRI-related adverse event). Fiberoptic or carbon fiber wiring ECG monitoring is necessary to minimize the risk of patient burns; but even with fiberoptic cables, it is important to recognize that the connections between the ECG pads and the telemetry box are still hard wired, and careful attention must be paid to prevent frays, overlap, exposed wires, and knots in the cables. It is also important to ensure that the equipment does not emit RF waves.

To prevent patient injury, care must be used to avoid creating a conductive loop between the patient and a conductor (ECG monitoring/gating leads, plethysmography gating wire, and fingertip attachment). Gating is a process synchronizing MRI image acquisition to the cardiac or respiratory cycles to reduce cardiorespiratory artifacts. During the scan, no exposed wires or conductors can be allowed to touch the patient’s skin, and the imaging coil should be connected to the magnet. Imaging coils are used in MRI to send and receive electromagnetic waves and are a type of radio frequency coil. Coils are designed for various body parts and are essential in obtaining images. All MRI images require placing the coil designed for that particular part of the body. If a coil meant for another part of the body other than what is being scanned is accidentally left on the patient, the coil that is not connected to the magnet could interrupt flow of electricity and predispose to burns (e.g., in a patient having MRI of the brain, if the coil meant for an MRI of the abdomen were left disconnected on the patient). During active scanning, all coils on the patient should be connected to the magnet to provide safe patient care. Conventional pulse oximeters are not used; rather they must be fiberoptic. Failure to remove conventional pulse oximeter probes and adhesives has resulted in second- and third-degree burns. ^,

Average noise levels of 95 dB have been measured in a 1.5 T MRI machine, comparable to noise levels of very heavy traffic (92 dB) or light road work (90 to 110 dB). Exposure to this level of noise has not been considered hazardous if limited to less than 2 hours per day, although both temporary and permanent hearing loss after an MRI scan have been reported. The 3-T magnet offers the advantage of less image degradation and improved neuroskeletal and musculoskeletal imaging. As the field strength increases, so does the noise. A 3-T magnet is twice as noisy as a 1.5-T magnet and can exceed 130 dB. In fact, if the peak sound-pressure level of a 3-T magnet exceeds 99 dB (the level approved by the International Electrotechnical Commission), then noise reduction is necessary to reduce sound to safe levels. Noise reduction did not differ between earplugs and headphones, although the combination of both was more effective at reducing sound. Earplugs or MRI-compatible headphones should be offered to all pediatric patients and are required for all patients imaged in the 3-T magnet, in order to reduce sound below 99 dB. Silent scan is a promising upcoming technology.

Additional challenges in the 3-T environment occur with fMRI: the noise of the magnet can interfere with the acoustic stimulation generated for purposes of obtaining the fMRI. ^, Video goggles compatible with the 1.5-T and 3-T environments (Resonance Technology, Los Angeles, CA) may be worn by the patient during MRI to provide a 3D virtual reality system complete with audio integration (see MRI Video Goggles section later). The introduction of this integrated audio-video headset has revolutionized the ability to offer distraction to patients, so that many more patients are now able to tolerate imaging without adjuvant sedation or anesthesia.

Although studies in mice and dogs suggest that exposure to magnetic fields may increase body temperature, it is unlikely that static magnetic fields up to 1.5 T have any effect on core body temperature in adult humans. However, this may be of greater concern in infants and small children, such as in cardiac MRI studies that require a long scan time. The SAR is measured in watts per kilogram (W/kg) and is used to follow the effects of RF heating. The FDA allows an SAR averaged over the whole body. Generally, the upper limit given by the FDA for the SAR is 4 W/kg for the whole body and 3.2 W/kg for the head. Ex vivo exposure of large metal prostheses to fields more than six times that experienced in MRI have not revealed any appreciable heating. To date, no conclusive evidence has shown that RF is a significant clinical issue in magnets up to 3 T.

In the field of fetal medicine, the evidence suggests that the best use for MRI is in the assessment of central nervous system anomalies, and the fetus with airway obstruction, both requiring decisions about therapy. Studies in amphibians demonstrate that exposure to magnetic fields as high as 4 T does not cause any defects in embryologic development, and most hospital MRI machines are 1.5 to 3 T. There are no known adverse effects of MR exposure on pregnant woman and the human fetus, but the potential teratogenicity of fever, tissue heating, and acoustic damage has not been considered. Although the FDA limits sound intensity to 140 dB in the MRI suite, the degree and frequency of sound attenuation as a function of gestational age is not known. The Society for Pediatric Radiology concluded that there is no evidence in the literature of harmful effects of MRI on the fetus. Fetal MRI has limited utility in the first trimester and should be done sparingly. The Joint Commission and the ACR consider MRI relatively risk-free during pregnancy, but the risk-benefit ratio should always be considered. Contrast MRI agents such as gadolinium should be used only if it is considered critical, and if the potential benefits justify the potential unknown risks to the fetus. Patients with acute or severe renal insufficiency who receive gadolinium carry a high risk of nephrogenic systemic fibrosis.

Important MRI safety issues include implanted objects (i.e., cardiac pacemakers), ferromagnetic attraction creating “missiles,” noise, biologic effects of the magnetic field, thermal effects, equipment issues, and claustrophobia. Some stainless steel alloys may contain ferritic, austenitic, and martensitic components. Martensitic alloys contain fractions of a crystal phase known as martensite, which has a body-centered cubic structure, is prone to stress corrosion failure, and is ferromagnetic. Austenite is formed in the hardening process of low-carbon and alloyed steels and has ferromagnetic properties. Iron, nickel, and cobalt are also ferromagnetic. For this reason, the components of any implanted device should be carefully researched prior to the patient entering the magnet.

In addition, an external magnetic field may exert translational (attractive) and rotational (torque) forces on stainless steel or surgical stainless objects. Intracranial aneurysm clips, cochlear and stapes implants, retained shrapnel, orthodontic devices (braces), intraorbital metallic bodies, and prosthetic limbs may move and potentially dislodge. Intracranial aneurysm clips manufactured after 1995 contain less ferromagnetic material and are labelled MR conditional by the ACR. Cardiac and vascular stents are stable for MRI 6 weeks post-implant when the stents are securely embedded in the vessels. Special precautions should be taken with cochlear implants in the 3-T environment, because those nonremovable magnets may suffer demagnetization in the scanner. Patients implanted with such devices should undergo MRI imaging if the benefits of scanning outweigh the risks. The radiology team should be aware that the patient has a cochlear implantation in situ. There are many varieties of cochlear implants and the manufacturer’s guidelines and recommendations must be followed.

It is important to note that some eye makeup and tattoos may contain ferromagnetic pigments (certain red colors) and may therefore cause ocular, periorbital, and mild skin reactions or even second-degree burns. ^, In addition, some tissue expanders used in reconstructive surgery have a magnetic port to help identify the location for intermittent injections of saline. Bivona tracheostomy tubes (Smith’s Medical, Gary, IN) usually contain ferrous material, although this is not specified in the package insert; they are not considered MR safe by the U.S. FDA. These tubes should be replaced with a Shiley tracheostomy (Medtronic, Minneapolis, MN) or polyvinyl chloride (PVC) endotracheal tube prior to the patient entering the MRI environment.

In the presence of an external magnetic field, ferromagnetic objects can develop their own magnetic field and become projectiles. The attractive forces created between the intrinsic and extrinsic magnetic fields can propel the ferromagnetic object toward the MRI scanner. Special note should be made of the magnet strength. Over the past few years, 1.5-T magnets have been supplanted by 3-T magnets. The field strength and magnetic force generated by a 3-T scanner is unforgiving to the careless or accidental introduction of a ferrous object into the environment. Placing a magnet outside the MRI scanner can be a crude, helpful, and sometimes inaccurate way to test an object. If the object is not attracted to the magnet, this is not an absolute indication that no ferrous material is present. More sophisticated and sensitive detectors of ferrous material have been introduced that include hand-held detectors and walk-through detectors, similar to those at airports. None of these methods of detection should supplant or eliminate the careful, methodical, face-to-face screening that should be performed on everyone and everything prior to entry into the MRI suite. Some unusual objects that have found their way into the MRI suite to become projectiles include a metal fan, pulse oximeter, shrapnel, wheelchair, patient stretcher, furniture moving dolly, cigarette lighter, stethoscope, pager, hearing aid, vacuum cleaner, calculator, hair pin, oxygen tank, prosthetic limb, pencil, insulin infusion pump, keys, watches, hand-held intercoms, and steel-tipped and steel-heeled shoes.

Small objects can usually be easily removed from the magnet, but large objects may be subject to so much attractive force from the magnet that it is impossible to remove them by manual force. In these circumstances, the only way to release an object attached to the scanner is by quenching the magnet, which will eliminate the magnetic field over a matter of minutes. This process is not without substantial risk, as helium gas is vented, condensation occurs, and considerable noise fills the suite. All personnel are required to vacate the suite during a quench because of a risk of hypoxic conditions, should the helium enter the room. Although vaporized helium is not an asphyxiant, it may act like one when large amounts displace oxygen from the MRI room. The specific indications for quenching a magnet were detailed in 1997 in the ACR’s guidance document for safe MR practices: “Quenching the magnet (for superconducting systems only) is not routinely advised for cardiac or respiratory arrest or other medical emergencies, since quenching the magnet and having the magnetic field dissipate could easily take more than a minute. Because of the risks to personnel, equipment, and physical facilities, manual magnet quenches are to be initiated only after careful consideration and preparation.” The basic guide to install quench pipes and its specifications are provided in the Medicine and Healthcare Products Regulatory Agency’s safety guidelines for MRI equipment.

The relationship between cardiac pacemakers and MRI has been constantly evolving. Cardiac pacemakers historically have presented a special hazard in and around the MRI scanner, especially in patients who are pacemaker-dependent.

Most pacemakers have a reed relay switch that can be activated when exposed to a magnet of sufficient strength, which could convert the pacemaker to the asynchronous mode. This is an extremely dangerous situation: At least two cases are known of patients with pacemakers who died from cardiac arrest in an MRI scanner. The autopsy of one patient determined that the death was the result of an interruption of the pacemaker in the magnetic environment.

In cardiac pacemakers, static magnetic fields exert translational forces and torque on their ferromagnetic components, leading to movement of the pulse generator or dislodgement of the leads. The RF pulse can heat the tip of the lead, potentially causing thermal cardiac damage. Legacy pacemakers have been replaced by modern MR conditional pacemakers. Patients who have legacy pacemakers cannot undergo MRI scanning due to unpredictable behavior of the magnetic sensors within the pacemaker. Patients who have a modern MR conditional pacemaker can be scanned as per the guidelines and recommendations of the manufacturer. The internal circuits are modified to reduce the potential for cardiac stimulation. In addition, enhanced circuit protection strategies are designed to remove the danger of electrical reset.

More than half of patients with an implanted pacemaker may be faced with an indication for MRI later in life. The fear of some institutions and physicians of scanning patients with MR conditional pacemaker systems is due more to unfamiliarity than to concerns about the safety of the examination.

MR conditional Cardiac Implantable Electronic Devices (CIEDs) labeling requires testing sufficient to characterize behavior in the MRI environment including measuring magnetically induced force and torque, current induction, RF heating, and modeling of potential interference from MR with the CIED. Conditional labelling requires generators and leads that were tested together. MR nonconditional CIEDs, on the other hand, include those that do not meet the MR conditional criteria and MR conditional generators combined with nonconditional lead systems. Also, epicardial leads or MR conditional systems implanted in patients who do not meet all specified conditions of use (e.g., patients with abandoned leads) are labelled as MR nonconditional. Actually, none of the marketed CIEDs have an MR safe label.

In addition to the risk of pacemaker malfunction, the risk exists for torque on the pacemaker generator or pacing leads to create a disconnect or microshock. With careful preparation, select patients with permanent pacemakers and implantable cardioverter-defibrillators (ICDs) may safely undergo imaging in the 1.5-T environment without any inhibition or activation of their device. Most of the publications on MR conditional CIEDs use 1.5-T scans in “normal operating mode” and some systems are FDA-approved for a 3-T MRI.

Although the presence of a CIED has long been a contraindication for the performance of a MRI study, this has been recently challenged. Patients who had “non-MR-conditional” pacemakers (n = 1,000) or ICDs (n = 500) were studied for clinically indicated nonthoracic MRI at 1.5 T. No device or lead failures were noted in any patient. It is important to remember that these patients were appropriately screened and the devices were reprogrammed in accordance with the prespecified protocol. Another study assessed the safety of MRI in 1509 patients who had a legacy (not MR-conditional) pacemaker or a legacy ICD system and found no long-term clinically significant adverse events. With careful screening, the performance of an MRI study in patients implanted with “non conditional” devices will likely increase in the future.

The ACR formed a blue-ribbon panel on MRI Safety in 2001 to review existing safety practices and issue new guidelines. ACR guidance documents were initially published in 2002 and updated in 2004, 2007, and 2013, and the most recent revision in 2015 is an update on safe and optimal performance of fetal MRI. ^, Pediatric safety concerns were specifically addressed in the 2007 document, with special emphasis on patient screening, sedation, and monitoring issues. Implanted cardiac pacemakers or ICDs were considered a relative contraindication to MRI in the past. Caution should be exercised in patients implanted with such devices should only be scanned in locations staffed by radiologists and cardiologists who have appropriate expertise. The recommendation was for radiology and cardiology personnel, along with a fully stocked emergency cart, to be readily available with a programmer to adjust the device if necessary. Following the MRI, a cardiologist should confirm correct function of the device and recheck it within 1 to 6 weeks. In general, artificial heart valves are not ferromagnetic and are not a contraindication to MRI. It is critical that everyone entering the vicinity of the MRI scanner fills out a screening form that specifically lists every possible implantable device, alerting the MRI staff to any potential hazards. The 2015 fetal MRI update is a practice guideline that highlights the indications, safety guidelines, and possible contraindications. It also provides specifications for the examination and equipment, quality improvement, safety, and documentation.

It is important to note that the MRI magnetic field may affect the ECG. Changes in the T wave are not due to biologic effects of the magnetic field but rather they are due to superimposed induced voltages. This effect of the magnetic field on the T wave is not related to cardiac depolarization because no changes to the P, Q, R, or S waves have been observed in patients exposed to fields up to 2 T, and no reports have been made of an MRI scan affecting heart rate, ECG recording, cardiac contractility, or blood pressure. One study, however, found that humans exposed to a 2-T magnet for 10 minutes developed a 17% increase in the cardiac cycle length (CCL), which represented the duration of the R-R interval. The CCL reverted to preexposure length within 10 minutes of removing the patient from the magnetic field. The implications of this finding are unclear, and this change in CCL in patients with normal hearts may be of no consequence; however, the implications of this finding for patients with fragile dysrhythmias or sick sinus syndrome have yet to be determined. There are many developments needed in software and hardware to make it possible to safely acquire ECG in the MRI. For interventional MRI procedures, more work is required to further refine the technique.

In 2008, The Joint Commission recognized the potential and existent hazards of the MRI environment when they published a Sentinel Event Alert that identified five MRI-related incidents and four MRI-related deaths (one from a projectile and three from cardiac events). The alert specifically identified eight types of possible injury and was designed to prevent accidents and injuries in the MRI suite ( Box 20.1 ).

The ACR has designated different zones as they pertain to the proximity of the MRI scanner. In 2002, four zones were defined relative to the magnet that were important in identifying and demarcating the safety precautions that should be taken for each zone ( Table 20.1 ). Zone I is freely accessible to the general public. Zone II is where the patients are greeted, histories are obtained, and screening for MRI safety issues is done. A metal detector is needed in zone II. Zone III is the area of potential MRI-related adverse events and also where the imaging control room is located. Finally, zone IV is the MRI unit magnet room and the most dangerous area.

The FDA has developed a series of posters addressing MRI safety aspects for MR technologists and other MR personnel. The MRI safety labelling has three terminologies: MR safe, MR conditional, and MR unsafe.

MR safe indicates a device that does not pose a patient safety risk in the MR environment, but it does not indicate whether the image quality will be affected. Patients with MR safe devices have no scanning restrictions; MR compatible items may safely enter MRI zone IV only under the specific conditions provided in the labeling. Patients should not be scanned unless the device is positively identified as MR conditional and the conditions of safe use are met. For items to enter the MRI bore, the safety labeling should be matched with the MRI for the following: static field strength, maximum spatial field gradient, db/dt limitations for active implants, SAR limits, and any other condition needed for safe use of the device. The term db/dt is defined as the ratio between the change in the magnetic field amplitude (db) and the time it takes to achieve that change (dt). It is measured as Tesla per second (T/s). Items not intended for the MRI bore have gauss line positioning restrictions or other requirements to secure the device to an unmovable part of the room. MR-unsafe items should not enter the MRI zone IV and patients implanted with such devices should not be scanned. The terminologies are associated with color-coded and identifiable icons that could be used to identify all equipment and materials as MR safe, MR conditional, or MR unsafe. These icons are consistent with international standards for colors and shapes of safety signs ( Fig. 20.1 ). The terms are clear: MR safe items pose no known hazards in any MRI environment, regardless of field strength; MR conditional items pose no known hazards, but this depends on the specified MR environment; and MR unsafe items represent a hazard in any MR environment and should be kept out. All equipment monitors and materials should be clearly labeled with one of these identifying icons to ensure the safety of the patient, health care worker, and equipment.

MRI Anesthesia Equipment and Physiologic Monitors

A variety of anesthesia equipment, supplementary devices, and physiologic monitors are currently available. Although most are designated as MR conditional or MR safe, differences among the options may have important implications for the delivery of anesthesia. The more substantive and commonly utilized choices available are reviewed later. It is important to recognize that despite the technologic advances in monitors and equipment, some limitations remain. For example, a continued limitation in the MRI environment is the ability to monitor a pulmonary artery catheter. To date, the FDA has not approved any pulmonary artery catheter for use in the MR environment. Also, no monitor or device with MR conditional labeling is yet equipped with electronic record-keeping, which therefore still needs an interface to download data. With the advent of cardiovascular magnetic fluoroscopy for simultaneous measurement of cardiac function, flow, and chamber pressure, use of right heart catheterization using metallic guidewires and low SAR MR fluoroscopy at 1.5 T has recently been described.

Physiologic Monitors

Invivo

Invivo (Philips Electronics, Cambridge, MA) is a branch of Philips Electronics that produces the Expression, an MR conditional physiologic monitor approved by the FDA in 2009 ( Fig. 20.2 ). The Expression is approved to function at the 5000 gauss (G) line and 4 W/kg SAR. It is MR conditional with 1.5- and 3-T magnets to 5000 G. In general, for an actively shielded 1.5-T scanner, this monitor could be adjacent to the bore of the magnet and approximately 1 foot from the bore of a 3-T scanner.

The main monitor weighs 16 lb inclusive of the battery and has up to 8 hours of battery life. In addition to the main monitor is a remote monitor that communicates and interacts with the main monitor. This monitor is usually placed in zone III, which enables the health care provider to monitor the patient during MRI without being exposed to the noise and magnetic field of zone IV.

Invivo monitors offer wireless Quadtrode (Philips) ECG electrodes and pulse oximetry monitoring. These wireless modules are powered by batteries that hold a charge for up to 8 hours. The batteries of the monitor display remaining power in time and not percentages, facilitating the planning of battery replacement and charge. Two ECG leads may be monitored simultaneously, and the company claims to have developed algorithms to minimize artifact that can occur during advanced imaging sequences. A pneumatic respiration monitor encircles the chest circumferentially and is able to detect respiratory rate, and the pulse oximeter offers a variety of different size probes: two alligator-style clips for older children and adults and four adhesive disposable probes for neonates and infants. The disposable probes offer an advantage, particularly in those children who are immunocompromised or infectious. The monitor offers both disposable and reusable noninvasive blood pressure (NIBP) cuffs for the limbs and thigh, as well as invasive blood pressure capability that can display pressures from two locations simultaneously, such as arterial and intracranial pressure.

The monitor displays adaptive trend arrows to indicate the direction of change in blood pressure, temperature, and heart rate. The main monitor is equipped with a large blinking red light visible up to 360 degrees to facilitate visualization of any triggered alarms, which include physiologic thresholds and low battery.

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