Anesthetic Management for MRI

Patient acceptance of MRI examination is generally high. Most adults and small babies (if recently fed and well wrapped up) tolerate the procedure without sedation/analgesia. Sedation/analgesia may be required in older children and adults who cannot cooperate. Sedation/analgesia is usually administered by radiologists and nurses in the MRI suite. This practice generally works well, as long as it conforms to the ASA recommendations for sedation by nonanesthesiologists.^[6] The likelihood that a child can undergo the examination successfully under sedation/analgesia is enhanced if the child is sleep deprived for several hours the previous night. All patients receiving sedation/analgesia need supplemental oxygen and standard monitoring. Anesthesia personnel become involved in these patients' care in the MRI suite when sedation fails, when it is impossible to control patient movement without general anesthesia, and when it is necessary to protect the patient's airway and control ventilation.

Several articles have discussed anesthetic management of patients undergoing MRI.^{[52] [53] [54] [55] [56]} As might be inferred from the preceding discussion, provision of anesthesia in the MRI suite poses several unique problems, including the following:

1. Limited patient access and visibility, especially when the patient must be placed head first into the magnet ( Fig. 69-1 )
2. Absolute need to exclude ferromagnetic components
3. Interference/malfunction of monitoring equipment produced by the changing magnetic field and RF currents
4. Potential degradation of the imaging caused by the stray RF currents produced by the monitoring equipment and leads
5. The necessity to not move the anesthetic and monitoring equipment once the examination has started to prevent degradation of magnetic field homogeneity
6. Limited access to the MRI suite for emergency personnel in accordance with the recommended policies noted earlier

Figure 69-1 View of a magnetic resonance scanner through the window from the control desk. Note the limited patient access. The photograph appears grainy because of the radiofrequency shield embedded in the window. (From Mackenzie RA, Southorn PA, Stensrud PE: Anesthesia at remote locations. In Miller RD [ed]: Anesthesiology, 5th ed. Philadelphia, Churchill Livingstone, 2000, p 2248.)

Limiting these problems mandates involvement of the department of anesthesiology in planning and construction of MRI suites, as well as administration of the operation of these suites. Satisfactory performance of the anesthetic and monitoring equipment and ensuring that this equipment does not interfere with the scanning need to be confirmed before clinical use. An early solution adopted by some MRI units was to keep the anesthetic and monitoring equipment outside the MRI suite.^[54] Disadvantages of this approach included the need for long monitoring leads, anesthesia machine hoses, and other connections to the patient with the risk of disconnection and the need to have extra personnel resources simultaneously with the patient and with the monitoring equipment. The advent of MRI-compatible anesthesia machines and monitors has rendered this approach unnecessary and relatively unsafe. A common approach now is to induce anesthesia in an induction area adjacent to the MRI suite outside the magnetic field by using conventional equipment with the patient on a dedicated MRI transport table that is not ferromagnetic. This transport table is then used to bring the patient into the MRI suite, where anesthesia and monitoring are continued with MRI-compatible devices. The MRI transport table is also used to remove the patient rapidly from the scanner should an emergency arise. This is important because the ferromagnetic equipment used for patient resuscitation by the code team and others must never be brought into the MRI scanner room.^[42]

Several manufacturers make MRI-compatible anesthetic machines. ^[56] Various breathing circuits have been used successfully in the MRI suite. They can be used to deliver breaths generated by an MRI-compatible mechanical ventilator or by manual inflation of an anesthesia bag. Because the patient's airway is not easily accessed during the MRI scan and because patient assessment and communication are limited by both the magnet bore in which the patient is placed and the loud noise associated with MRI scanning, deep sedation/analgesia is not advisable. Patients requiring more than moderate sedation/analgesia are probably most safely administered a general anesthetic with airway control by either endotracheal intubation or a laryngeal mask airway (LMA). Endotracheal tubes and LMAs and their connections to the breathing circuit must contain no ferromagnetic material. Because conventional zinc batteries are strongly ferromagnetic, if a laryngoscope is needed in the MRI suite, it should be of plastic construction and equipped with lithium batteries and aluminum spacers. As noted, most emergency situations in the MRI scan room itself should be treated with basic life support techniques while the patient is rapidly moved out of the scanner vicinity for definitive treatment.

Given the patient's inaccessibility in the MRI scanner, patient observation by the anesthesiologist is limited

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1. Electrocardiography: Voltage induced by blood flow in the aorta when the patient is in a static magnetic field produces T- and ST-wave abnormalities. In addition, the rapidly changing magnetic fields can induce artifacts in the ECG trace. Furthermore, as noted previously, the patient may be subject to heating and thermal injury.
2. Pulse oximetry: The antenna effect in conventional pulse oximetry wires has resulted in thermal injury.^[57] Typically, MRI-compatible pulse oximeters use fiberoptic signal linking between the sensor and monitor.
3. Noninvasive blood pressure monitoring: Such monitoring can be successfully accomplished, provided that the connections of the blood pressure cuff and hoses are plastic.
4. Precordial and esophageal stethoscopes: These instruments do not function well because of the interference created by the noise of the scanner when it is in operation.
5. Capnography: This monitor usually functions satisfactorily, but the length of the sample tube may cause a significant time delay in signal transduction.
6. Temperature: Temperature monitoring is not generally a problem if temperature probes with RF filters are used.

Meeting all the ASA recommendations for standard monitors in the MRI suite is thus problematic. Pulse oximetry should be used on all patients receiving sedation/analgesia or anesthesia, with the probe placed as far from the scanning site as possible to limit the potential for thermal injury, which still exists despite the use of nonferromagnetic connections.^[56] Noninvasive blood pressure monitoring should likewise be used in all anesthetized patients. Capnography should be performed in all applicable cases. ECG monitoring in the MRI scanner should probably be limited to patients deemed to be at particularly high risk, if it is used at all, with precautions taken to limit the potential for thermal injury.^[42]

No specific anesthetic technique is required for MRI. Most cooperative patients, as noted, will tolerate MRI with either no sedation or light to moderate sedation/analgesia. MRI is not painful, and the medication regimen may thus be biased toward sedation unless the patient's underlying condition or the long period in the scanner in the supine position requires analgesia. Patients with critical neurologic and cardiac problems, including congenital cardiac conditions, are frequently and safely anesthetized for MRI. Should the patient undergo sedation/analgesia, the anesthesia provider should be present in the scanner room. Again, hearing protection is mandatory for anesthesia providers if they must remain in the scan room. Critically ill patients, uncooperative adults, and children older than 3 years, who may not respond favorably to chloral hydrate, may best be managed with general anesthesia. Because patient access is limited during the scan, our practice has been to secure the airway with either an endotracheal tube or LMA and control ventilation. General anesthesia is induced in an induction area adjacent to the MRI room after standard monitors are placed and the airway is secured. Particular care is taken to ensure removal of the ECG electrodes before transfer to the scan room, as well as all other non-MRI-compatible monitoring devices. The patient is then transferred to the adjacent scan room, ventilation resumed, and function of the airway device reassessed. Airway disconnection is typically limited to 15 to 30 seconds. Anesthesia is usually maintained with volatile anesthetics, although propofol is sometimes used. MRI-compatible monitors are placed, and the examination proceeds. It is not mandatory that the anesthesia provider remain in the scan room during general anesthesia, assuming that remote mirroring of the monitors and direct observation of the anesthesia machine are feasible from the immediately adjacent control room. If such observation is not feasible, the anesthesia provider will need to remain in the scanner room with the patient. At the end of the examination, the patient is transferred back to the induction area and awakened there, where conventional monitoring may be resumed and conventional management of any untoward event can take place. At any point in the examination, the patient may be emergently transferred to the induction area for management of an emergency situation. After emergence from anesthesia, the patient is transferred to the radiology recovery room and monitored. Patients are discharged when they are documented to have met conventional discharge criteria.