MRI and Anesthesia

Anesthesia in the MRI Suite

Charlotte Bell, MD
Associate Professor
Yale University School of Medicine

Published by permission of GASNet Inc.© 2002

The article also available in PDF: 84 KB

Introduction

Children, adults experiencing anxiety, and intensive care patients may require anesthesia or sedation in the MRI suite. This imaging tool for precision diagnosis presents unique and complex concerns. Specifically, providers must follow all standards for patient safety; standards for monitoring according to society, region, and governmental agencies; and the individual needs of the patient while being knowledgeable about the unique hazards of the energy sources in the MRI environment.

The Process of Magnetic Resonance Imaging

The creation of an image involves six sequential steps: (Edelman, et. al. 1990, Edelman, et. al. 1993, Menon et. al 1992)

1. First, a powerful uniform magnetic field is created, which aligns all randomly oriented nuclei. In medical imaging, field strengths are usually 1.5 Tesla (1 T = 10,000 gauss (G)); in human research they are up to 4.0 T. The earth's magnetic field is just under 1 G.
2. Radiofrequency pulses (RF) are then directed at the patient. In the presence of an external magnetic field, cell nuclei absorb more energy. "Magnetic Resonance" is created when these cells are super-energized by RF waves in a magnetic field.
3. Nuclei recover their original alignment within the magnetic field after they have been super-energized with RF. Each tissue emits a RF signal proportional to the difference between its energized magnetic resonance state and original alignment. Tissue contrast develops as a result of different rates of realignment.
4. The brief application of magnetic field gradients to spatially encode the RF signals emitted from the patient (time varied magnetic fields or TVMF) enables the rendering of 2- or 3-dimensional images.
5. Signal readout times must be determined for each patient prior to the MRI, but after the patient is anesthetized or sedated, so that the RF signal may be recorded after initial excitation.
6. The signal from the patient is collected by the radio frequency coil surrounding the patient and is transformed by computer into a 2- or 3-dimensional image.

Monitoring Difficulties

A) Magnetic Interference

Before selecting an installation site for a MRI center, a thorough survey must be performed, since all environmental iron is magnetized. It may be necessary to re-route pipes and electrical wiring and/or to remove all stationary environmental iron (i.e., structural steel, floor decking, and concrete reinforcing rods). Notwithstanding, there is always some magnetization of the environment immediately surrounding the magnet and projectile-related accidents often occur (even jewelry or a pen can present a hazard). (Pavlicek, 1985)

Newer MRI machines are shielded so that the effects of the magnetic field decrease significantly, albeit in a deceptively nonlinear fashion, a few feet from the magnet bore. At the 30-50 gauss line, some ferromagnetic equipment may be used safely. However, it remains critically important that no equipment be brought into the MRI vicinity without the approval of the biomedical engineer in order to assure successful imaging and the safety of the patient, personnel, and equipment. (Jorgensen, et.al. 1994)

In general, replacing machines' ferrous material with stainless steel, brass or aluminum can enable their placement within the imaging room. However, much of equipments’ smaller, more delicate instrumentation is ferromagnetic and subject to torque—the alignment of ferromagnetic material within a magnetic field—which can cause malfunction or serious damage to equipment.

Additional concerns include the erasure of all magnetic media—such as credit cards, pass keys, floppy discs, and videotapes. Quartz driven analog watches will stop running in a magnetic field, but will resume function once outside the field.

B) Radiomagnetic Interference

Challenges to monitoring are also presented by the two large radiofrequency (RF) coils surrounding the patient. The scanning room must be shielded from outside RF interference (i.e. television transmitters, beeper paging systems, two-way radios, and commercial radio stations), which may affect RF transmission and reception. The MRI suite can be RF shielded by lining the walls and windows with thin continuous sheets of copper, and by using wave guides in the walls.

In addition, any cables or leads can behave as antennae for RF and must be shielded, because RF pulses may induce electrical eddy currents short-circuiting electrical equipment. Early MRI sites used aluminum foil to wrap cables and small copper boxes to isolate electrical equipment. MR-specific monitoring systems, besides using non ferromagnetic materials, must also isolate the equipment from RF.

C) Time Varied Magnetic Field

TVMF can induce electrical eddy currents in both biologic tissue and electrical wiring. Although the biologic effects are usually minimal, very large TVMF (>10,000 T/sec) could interfere with nerve conduction, induce seizures, or cause ventricular fibrillation. For this reason, coils in any cables or leads should be avoided; however, the TVMF for currently available units is well within the range of biologic current densities (1-10 mA/m2). (Pavlicek, 1988)

Monitoring in the MRI Suite

1) Basic Set - Up

Systems for central wall gases (oxygen, nitrous oxide, and air) are now commercially available for MRI centers and should be installed in consultation with biomedical engineers and architects during construction of the MRI suite. (Holschouser, 1993)

Electrical power sources consisting of isolated duplex power circuits with filtered 120V (alternating current) to prevent electrical noise artifacts from interfering with the images should be available in the magnet room. (Karlik, et.al. 1988)

2) Wave Guides

Wave guides are necessary for pipes, cables, ducts and electrical wires entering an RF shielded room in order to prevent leakage of RF pulses from the magnet room, interference from outside RF, and the antennae effect of electrical wiring and cables.

A low pass filter—an RF shielded copper box covering the external passage into the room through which cables or wires pass and then zigzag into the wave guide—is necessary for passing electrical signals through an RF shielded wall. (Koskinen, 1990) Newer magnets are installed with wave guides in the wall control panels so that monitoring systems can be conveniently added after construction.

3) Electrocardiogram (ECG)

Electrocardiographic monitoring is particularly problematic. Maximum voltage charges are induced in any column of conducting fluid (blood within the transverse aorta) when the fluid flow is 90 degrees to the field (supine patient in MR scanner). (Peden, et.al. 1992) The superimposed potentials are greatest in ST segments and T waves of leads I, II, V1, V2 and increase with field strength. Also, spike artifacts that mimic R waves are often produced due to the changing magnetic fields of the imaging gradients. These changes in the ECG waveform are present to some degree even in filtered systems designed for MRI use and make it essentially impossible to reliably monitor for ischemia or to interpret arrhythmias. Plethysmography can be used as a heart tachometer, but is not useful for ischemia or arrhythmia detection. (Sellden, et.al. 1990, Volgyesi, et.al. 1991). In patients highly susceptible to ischemia, a 12-lead ECG pre- and post- MRI is recommended. If cardiac ischemia or arrhythmia is suspected during scanning, the patient should be removed from the magnet room for accurate monitoring.

Several MRI-compatible ECG systems are currently available, utilizing ECG electrodes made of carbon graphite to lower resistance, eliminate ferromagnetism, and minimize RF interference. Useful for cardiac gating, these systems use coaxialized cables to avoid any coils.

4) Blood Pressure

MR-approved monitoring systems use automated oscillometric blood pressure monitoring, which, because it is based on pneumatic principles, avoids electromagnetic interference. (Patteson et. al. 1992) For invasive blood pressure monitoring, conventional disposable transducers may function adequately outside the gauss line, but should be approved by a biomedical engineer. Disposable transducers have a predictably high natural frequency, so tubing added to distance them from the patient is unlikely to cause damping.

5) Ventilation

Because of the magnet depth, nearly 2m, it is often virtually impossible to visualize the patient's face and chest for adequacy of ventilation during scanning. Direct visualization of the airway may be possible by observing the scan. (Bell, et. al. 1996, Mathru, et. al. 1992) Respiratory capnography is available in both conventional systems placed beyond the gauss line and MR-compatible systems.

In spontaneously breathing patients, the Jackson Rees circuit can be attached to the endotracheal tube, laryngeal mask airway, or a tight-fitting mask (using a mask strap). When the circuit is placed on the chest, visualization of bag movement outside the magnet reinforces adequacy of ventilation. In adults receiving propofol who are not tracheally intubated, adding continuous positive airway pressure (CPAP) has been shown to maintain airway patency during MRI. (Mathru, et. al. 1996).

Noise levels of 95 decibels—equivalent to light road work—are frequently appreciated in 1.5 T scanners, making auscultation during scanning almost impossible. Ear plugs are recommended for anesthetized or sedated patients for noise protection.

6) Oxygenation

Although many pulse oximeters function well in the magnet, severe burns to extremities have been caused by the induction of current within a loop of wire. This may be avoided by placing the sensor on the extremity distal to the magnet, keeping its wires free of coils, and protecting the digits with clear plastic wrap. (Brown, et. al. 1993, Hughes, 1990, Shellock, et. al. 1989) Alternatively, MR-specific pulse oximeters use heavy fiberoptic cables, which do not overheat and cannot be looped.

The magnet superconductors are kept cool in liquid nitrogen. Should this coolant evaporate due to leaky housing ("quench"), the ambient oxygen supply of the room can drop precipitously, causing hypoxia and the potential for cryo injury. Quench monitors within each MRI suite recognize changes in room oxygen concentration. Providers should be aware of the emergency procedures in each location should a quench occur.

7) Anesthesia Equipment

Anesthesia machines can be modified for use in a magnetic field by replacing their ferromagnetic components to less than 2% of the total weight. (Karlik, et. al. 1988) Currently, manufacturers offer MRI-compatible machines made largely of stainless steel, brass, aluminum and plastic. The position of the machine in the suite should be determined by a biomedical engineer.

Alternatively, total intravenous anesthesia with a continuous infusion of propofol may be selected, necessitating use of infusions pumps. All commercially available infusion pumps contain ferromagnetic circuitry, which can be damaged and/or malfunction in the presence of a high magnetic field. However, several pumps have been tested and found to be accurate outside the gauss line. (Pope, 1993)

Plastic battery-operated laryngoscopes may be used for tracheal intubation. Batteries will last longer if shielded with a paper casing or plastic coating. The airway can be secured using conventional laryngoscopes before the patient is moved into the scanner. Further, only medical gas cylinders constructed from aluminum should be used in the MRI suite. Vaporizers, however, are affected little by the magnetic field and function accurately.

8) Temperature

During MRI, body temperature may increase from heating caused by RF within the magnetic field or decrease from the cool environment necessary to protect superconductors. While no biological damage has been documented to date, tissues that dissipate heat poorly are at risk (i.e., lens and scrotum). Thermistors are not practical because of the ferromagnetic content of the cables. Liquid crystal thermometers may be used, although their accuracy is limited. (Marsh et. al. 1996) Intermittent temperature monitoring has been recommended to reduce the possibility of skin burns from the thermistor for critical patients or during very long scanning procedures. (Hall, et. al. 1992)

Conclusion

When using anesthsia equipment and monitoring systems within the MRI suite, providers must assure that the equipment will function properly, provide no danger to the patient or personnel, and have no effect on imaging. Standards and guidelines for patient safety have been recommended by the American Society of Anesthesiologists, American College of Radiology, the Joint Commission on Accreditation of Healthcare Organizations, and the American Academy of Pediatrics (see links below).

Links:

www.asahq.org/Standards/02.html#2

www.asahq.org/Standards/14.html

www.acr.org/dyna/?doc=committees/mr_safety/safe_mri.html

www.jcaho.org/accredited+organizations/long+term+care/
standards/revisions/2001/sedation+and+anesthesia+care.htm

www.aap.org/policy/04789.html

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Last updated: 1 August 2002Created
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