MRI and Anesthesia

The technique of MRI under general anesthesia.

M.P. Boidin MD, PhD, G.R. Wolff MD, C. Doelman MD
Afdeling Anesthesiologie
Amphia Ziekenhuis Breda
Breda, The Netherlands

Email: mpboidin@wanadoo.nl

The article also available in PDF: 90 KB

Introduction

In this issue of CWWJ, we discuss the MRI topic in three articles, this one describing the technique and equipment. In our clinical practise, there is an impression that anesthetists tend to prefer general anesthesia instead of sedation for diagnostic procedures. However, pediatricians and radiologists may often hope that sedation would be sufficient for MRI investigations. In fact, sedation is sometimes insufficient [1 – 3], and there is some morbidity and even mortality connected with MRI investigations under sedation [4 – 5]. On the other hand, general anesthesia is not available for everybody at any time, and specially designed ventilators and monitors are needed to anesthetize patients in the MRI room [6]. More detailed information on MRI and general anesthesia may be obtained from various consensus articles and guidelines [7– 11]. In our opinion, the excuse to select sedation only because it is cheaper - is no longer valid, as trained personnel and proper equipment must also be available in sedation cases.

Magnetic resonance imaging

Magnetic resonance imaging (MRI) investigations result in high contrast images in pre-set planes. MRI is the product of radio waves applied to an object in a strong permanent magnetic field. The protons of the hydrogen atom nucleus (Nuclear), in polar molecules, as for example water or ammonia, are uni-directed by a strong permanent magnetic field (Magnetic). Radio waves are applied to the object by a coiled antenna around the body, and a one sort of radio-echo (Resonance) of the radio waves can be measured when the application of the radio waves is interrupted. A powerful computer calculates the results and transforms these data into an image (Imaging).

Magnetism

Tesla is the unit which indicates the force of a magnet, the stronger the magnet the higher the Tesla -rating. MRI equipment ranges from 0.2 to 9.0 Tesla. Gauss -rating indicates the force of a magnetic field in a certain place. So a 0.2 Tesla magnet can generate a maximal magnetic force of approximately 200 Guass. Typically, batteries are discharged in magnetic fields. That implicates that watches, laryngoscopes and other equipment (including batteries) may malfunction near the strong MRI magnet. Particularly important is that equipment with ferro-metals may become polarized and attracted by the magnet. Hence, oxygen cylinders, scissors, coins, and keys can be drawn into the magnet with great force. This is potentially dangerous for patients and personnel. When ferro-metals are used in instruments, those objects must be fixed to the wall or to the ground. Magnetic strips in credit cards or other magnetic media can be damaged in the proximity of a MRI magnet. Strong magnetic fields can also mechanically damage sensitive instruments like watches.

Resonance

In the MRI technique, most of the resonance is obtained from water molecules of the human body. Contrast agents and fluids can be used, particularly when visualizing vascular structures, and they are based on molecules having a different resonance pattern in the same radio waves.

Typically, bone tissue is shown as black, because it contains very little water per volume, resulting to low polarization and resonance. Other tissues are shown on the grey scale, according to their water content. With a MRI-scan it is possible to visualize inter-vertebral disks, cartilage of menisci, joint surfaces, and it may visualise differences in brain structures. Clearly, MRI is based on totally different technique than the more traditional X-rays that are based on ionizing radiation. Hence, one of the advantages in MRI is that no protection for ionizing radiation is needed.

Magnetism, radio waves and anesthesia

The permanent magnetic field, the antenna and weak radio signals present in the MRI environment, all oppose the use of normal anesthesia equipment. Until recently, it was not necessary to involve magnetism and the emission of radio waves into the considerations, designing anaesthesia equipment.

Today, anesthesiologists are involved in MRI procedures more and more often, and new questions arise. Ordinary ventilators or monitors, which we may use in the operating room are strictly forbidden in the shielded MRI room. Only equipment that is anti-magnetic or equipment that is shielded for radio waves in the correct way, (i.e. MRI compatible instruments) are allowed near the MRI magnet in use.

Other physical phenomena

Coils in electric wiring and other electronic instruments may emit radio frequency (RF) signals disturbing MRI measurement.

During measurement, the audible noise is monotonous and intense, and it is impossible to concentrate on patient physiology and monitoring, as long as measurement is being made. Ear protection is necessary for those who are in the MRI room at the time of measurement.

Increased temperatures may interfere quality of imaging. Cooling of the magnet can result in increased convection, and it is often necessary to cover the patient with blankets.

Anesthetic management

Pre-operative anesthesia assessment takes place in the morning of MRI examination. Patients have no pre-medication, but they are allowed to use their own daily medication. Parents come to the preparation room with their children, and remain there until the children are asleep.

Many of the pediatric patients in our clinic may stay in the hospital for long periods of time, or be mentally retarded. Hence, for practical reasons, other painful investigations (punctures) are combined in the same session if that is necessary. When the patients are stable after induction, they are transferred to the MRI room, and connected to the ventilator and monitor. Usually, maintenance of anesthesia is on sevoflurane, oxygen and air. Patients are mechanically ventilated and their physiology carefully monitored throughout the procedure.

First generation of technology for MRI anaesthesia

In the early 90's, MRI safe instruments were not available. A battery-operated pulse-oximeter was the first to be introduced. When the magnetic force was less than 2000 Gauss, we found it possible to use the Servo C/D ventilator (Siemens). A battery powered pulse-oximeter (InViVo4500 MRI, InViVoResearch Inc.) was used for hearth rate and saturation, but it was positioned to a distance of over 200 centimeters to save the batteries. As the MRI investigation program continued, more and proper physiologic and ventilatory parameters were required by anesthesiologists.

In our department, assisted ventilation was mandatory during general anesthesia MRI, to have a predictable, stable, and controllable situation. Hence, our group of anesthesiologists chose for general anesthesia with intubation and ventilation. Infact, we designed a dual stage treatment protocol for MRI under general anesthesia.

Induction outside MRI room (stage 1)

The induction of anaesthesia is performed in an induction unit, outside the shielded MRI room. Adult patients are placed on the MRI cart; children remain in their beds for induction. Patients are connected to an ordinary anesthesia monitor for ECG, pulse oximetry, temperature, and NIBP measurement. Anesthesia is administered according to a standard operating procedure with propofol, atropine, and rocuronium, a muscle relaxant. Patients are intubated, connected to the Servo ventilator (Siemens) and stabilized. For children bag and mask induction on Sevoflurane with air, in the sitting position opposite the mother is good. After induction of anesthesia, the mother and the accompanying nurse leave the induction room, and muscle relaxant is administered to facilitate intubation.

Anesthesia during MRI measurements (stage 2)

Routine anesthesia carts with ventilator and monitor and laryngoscopes, for example, can not be used near the MRI magnet. Therefore, anesthesiologists are not completely free to work as they normally do in the standard operating room.

When induction is complete the patients are transferred to the shielded MRI room, and connected to the ventilator, Aestiva MRI (Datex-Ohmeda) and MRI compatible monitor. That monitor has special carbon cables for ECG and fibre optic cables for saturation measurement. The ventilatory parameters are sampled through narrow-bore tubing connected to the ventilator tubing. ECG electrodes should be positioned near each other because of thermo-induction. There are even special carbon ECG electrodes available for monitoring near the MRI magnet. According to our own protocol, maintenance of anesthesia is on sevoflurane, oxygen, and air. At the end of the procedure, the Sevoflurane is stopped and ventilation continued until the patients are awake. After MRI, the awake patient is transferred to the recovery area in the central operation suite. When children wake up, their mothers are at the bedside.

Comments and discussion

Our hospital's dual-stage anesthesia protocol serves the best interest of the anesthesiologist and the patient. Outside the shielded MRI room, every anesthesiologist is completely free to serve the patient as he or she wishes. In the MRI room near the magnet, unauthorized personnel can cause dangerous situations. For example, dangerous situations may occur when oxygen cylinders are accidentally taken to the MRI room, or when someone places a patient monitor too close to the magnet, so that it becomes attracted to the magnet.

The Aestiva-MRI ventilator can deliver pressure-controlled ventilation. Hence, it is possible to deliver very low tidal volumes, and children may be ventilated with great accuracy. The Aestiva can also produce volume-controlled ventilation which seems to be the most used technique in adults. Volume and pressure parameters are displayed on the ventilator screen and the volume/pressure -loop can be seen on the MRI monitor display. The large size of the patients MRI monitor screen is a benefit. In the future, when more intensive care patients are scheduled for MRI, pressure measurement may be added.

Working the dual stage anesthesia set-up enhances the presence of parents during the induction. Because these parents are very close to their child, they can attend to the child while they "are put to sleep". Medical consultants, hematology assistants, and other personnel can freely enter the induction area. When the patient is in the MRI instrument, it is hardly possible to even see the patient. As a practical example, iv access for the injection of contrast dye is only possible when the needle is in the back of the hand.

Patients must remain well covered as there is a lot of cooling by convection in the patient compartment. So patients under general anesthesia easily cool down because of their skin dilatation. The MRI monitor used in general anesthesia can also be used for sedated children. Easy logistics and planning should enable all personnel involved in MRI to get access to this instrument. Not only should the sedated patient be connected to the monitor, there should also be a trained nurse be available to observe the vital parameters. Training in the operation theater to handle the monitor is a prerequisite.

Literature:

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7. Royal Colleges of Anaesthetists and Radiologists. Report of a joint working party. Sedation and anaesthesia in radiology. London: Royal Colleges of Anaesthetists and Radiologists, 1992.
8. Royal Collegeof Surgeons of England. Commission on the provision of surgical services. Report of the working party on guidelines for sedation by non-anaesthetists. London: Royal College of Surgeons, 1993.
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11. Committee on Drugs: Guidelines for Monitoring and Management of Pediatric Patients During and After Sedation for Diagnostic and Therapeutic Procedures. Pediatrics 1989: 1110-1115, 1992.
    

Last updated: 1 August 2002Created
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