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Anesthesia Concerns in the Magnetic Resonance Imaging Environment
Providing anesthesia in the magnetic resonance imaging (MRI) environment differs from that in the operating room. The static magnetic field necessitates that all monitors, devices, and machines be nonferrous. The gradient magnetic field can interfere with electrocardiographic (ECG) tracing and is the main source of acoustic noise. Radiofrequency field energy is transferred into heat, so the patient usually becomes warm when more energy is absorbed. If too much heat is transferred to the patient, burns may occur.
Because of the desire to minimize ionizing radiation and the ability of MRI to provide better imaging of some tissues and tissue planes, a gradual increase has occurred in the use of MRI not only for diagnosis but also as a part of treatment procedures. In a tertiary care hospital in which a sick patient population often has multiple comorbidities, patients are frequently unable to lie flat, still, or comfortably. Thus a corollary increase is seen in the request for anesthetic support.
This chapter presents some of the issues faced when anesthetizing a patient in the MRI suite. Training and preparation are needed before caring for a patient in the MRI environment. The MRI room should be inspected to make sure it is suitably prepared for the emergency and contingency situations, such as airway difficulty and cardiorespiratory collapse requiring resuscitation. Primarily because of acoustic noise during MRI, staff must be prepared to monitor the patient remotely after the onset of the anesthetic. Constant vigilance is required before bringing anything into the MRI room, because several commonly used anesthetic devices have not been tested to determine if they are even conditionally MRI safe.
With the increase in the number and type of interventional procedures done in the MRI suite, a corresponding rise has occurred in the amount of anesthetic support needed. The MRI suite should be viewed as a non–operating room anesthesia location that requires not only an anesthetic machine, monitors, cart, and drugs but also constant vigilance for the safety of both patients and staff.
Training and Personnel
As a requirement to practice anesthesia in MRI in our institution, individuals must attend an orientation lecture and video about MRI safety. This orientation includes a tour of the facility to highlight areas of concern, such as the magnetic zone and corresponding safety zones. The operating room safety checklist has been expanded for MRI to include not only the patient’s MRI checklist but also a personal pat-down before entering zone 4 with the patient. Table 19-1 lists the zones in the MRI area.
Table 19-1
Magnetic Resonance Imaging Zones
| Zone | Description |
|---|---|
| Zone 1 | Zone 1 consists of all areas freely accessible to the general public. This zone includes the entrance to the MRI facility, and the magnet poses no hazards in these areas. |
| Zone 2 | Zone 2 acts as a buffer between zone 1 and the more restrictive zone 3. Here, patients are under the general supervision of MRI personnel. Normally, these areas are also safe from the magnet. Zone 2 may include the reception area, dressing room, and interview room. |
| Zone 3 | Access to zone 3 should be restricted by a physical barrier. Only approved MRI personnel and patients who have answered a medical questionnaire and interview are allowed inside zone 3. The MRI control room and/or computer room are located within zone 3. |
| Zone 4 | Zone 4 is strictly the area within the walls of the MRI scanner room, sometimes called the magnet room. Access into the MRI scanner room should be available only by passing through zone 3. Zone 4 is sometimes considered to be inside zone 3 because it does not have a direct entrance to unrestricted areas. Zones 3 and 4 are sometimes collectively referred to as the MRI suite. |
In addition to the MRI technologist, our institution requires that a nurse be present whenever an anesthetic is administered. This policy applies to both diagnostic scans and procedures. Having a nurse assist during the anesthetic administration acknowledges the fact that the patient who requires an anesthetic for a diagnostic scan often has comorbidities. This practice began after a situation in which a patient needed a preoperative MRI for surgical planning. Administration of a general anesthetic began with the anesthesiologist and an MRI technologist present in the room. The patient developed cardiopulmonary arrest after the induction of general anesthesia. The ensuing resuscitation was unsuccessful. A review of the events led to the requirement to treat such cases as they would be handled in the main operating room. Whenever a therapeutic procedure is performed, the interventionalist must also be present in the MRI suite.
The Magnetic Resonance Imaging Suite
Although the operating room table is the center of the operating room, the magnet is the central, space-occupying mass in the MRI suite. The two issues that must be addressed are whether the patient bed is fixed or can be docked and undocked and whether access to the patient exists for emergent exit.
If the bed is fixed to the MRI scanner, a general anesthetic will require induction of the patient on an MRI-compatible stretcher before being transferred to the bed. The stretcher is left in MRI zone 4 in case the patent must be quickly evacuated. A bed that docks and undocks allows for mobility to position the patient as needed for induction of an anesthetic and quick transfer from the bore of the magnet.
Emergency access always should be assessed when a patient enters zone 4. An area in zone 3 should be available with adequate space, oxygen, suction, and outlets for resuscitation of a patient if needed. An MRI bay must meet basic criteria that allow for an anesthetic to be provided safely. For example, if an MRI bay does not allow adequate space for entry and exit of zone 4 or fails to provide an adequate number of electrical outlets for standard anesthesia equipment necessary in zone 3 for a resuscitation, that bay should considered unsafe and not used as an anesthetic site. In some institutions, older MRI bays do not fit these criteria. For all new construction, a member of the anesthesia staff should be involved in the planning stages to ensure there is no compromise for providing a safe anesthetic.
MRI zone 3 is the buffer zone where resuscitation should be done. Most of the emergency scenarios such as cardiopulmonary collapse, difficult airway, and allergic or anaphylactic reactions require instruments that might not be MRI safe, as well as a variety of support personnel, not all of whom may be cognizant of MRI safety, and the patient should be moved as quickly as possible to zone 3. Although zone 2 is preferred for resuscitation, the urgency of the situation might dictate a short transit to begin emergent resuscitative maneuvers.
Patient Selection
A poor quality of continuous ECG tracing is one the most serious limitations in monitoring an anesthetized patient in the MRI suite. If a patient is at risk for cardiac ischemia or further injury in the setting of recent ischemic stress, the benefit of anesthetizing the patient for MRI should be weighed against that risk. Technological advances will eventually overcome this limitation. Cardiac MRI does allow for visualization of global cardiac function. This type of MRI requires frequent breath-holding, but techniques that allow for free breathing have been reported.
Known difficult intubation or prior history of allergic reaction to medication or contrast dye must be considered before entering zone 4. However, unanticipated difficult intubation or an anaphylactic reaction to contrast dye requires quick action and exit from zone 4. Plans to deal with contingency situations should be in place before a patient is brought into the MRI suite.
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