57.2.1 What Is MRI and Why Is It Done?
MRI has steadily increased in popularity as a noninvasive, painless diagnostic imaging procedure. MRI images are produced using a strong (typically, 1.5 tesla [15,000 gauss]) magnetic field into which radiofrequency (RF) pulses are injected. MRI is the imaging method of choice for examinations in which water content differences make it possible to differentiate tissue types.1 It offers distinct advantages over computed tomography, both in terms of the quality of the obtained images for certain types of tissue (like brain) and the lack of exposure to ionizing radiation. MRI scans are frequently ordered by neurologists and neurosurgeons for patients of all ages with neurological disorders. In addition to intra-axial pathology, orthopedic problems such as osteomyelitis, soft tissue muscle tumors, and damaged knee menisci can be assessed using MRI techniques.1
57.2.2 What Is Unique About the MRI Suite in Terms of Caring for a Patient?
The extreme strength of the magnetic field in an MRI scanner can be hazardous. For example, patients with implanted ferromagnetic objects like aneurysm clips have had these fatally pulled out of position by the magnetic field.2 Similarly, some authorities have expressed concerns about carrying out MRI scans in patients with pacemakers.3 Likewise, ferromagnetic objects like wrenches, scissors, IV poles, pens, stethoscopes, and even hair barrettes can become accidental projectiles. In one case, a pillow containing metal springs, not detectable using a handheld magnet, flew into the magnet during positioning of a patient, fortunately without causing injury,4 However, projectile oxygen and nitrous oxide tank cylinder accidents causing injuries and even death have been reported during the last decade.5-7
Zimmer et al8 relate the following interesting cautionary tale. A 2-year-old boy underwent abdominal MRI scanning under general anesthesia. During the procedure, an anesthesia practitioner carried a portable sevoflurane vaporizer into the MRI suite. When the vaporizer was placed on an examination table, it was vigorously attracted toward the scanner, and it was only by the strength of two people that the vaporizer was directed to strike against the gantry, instead of flying directly into the magnet where it might have hit the child. Quenching the magnet, that is, emergency release of liquid helium from the scanner to collapse the magnetic field was initially considered, but the vaporizer could be removed with the help of a third individual. Of interest, immediately after the mishap the portable vaporizer was tested for magnetism with a strong handheld magnet, and no attraction was apparent. A review of the event revealed that the vaporizer contained ferromagnetic material in the temperature compensation module.
In addition to the attractive forces of the magnetic field on ferromagnetic objects, the strong magnetic field and associated RF pulses can interfere with the operation of ordinary anesthesia machines, as well as with patient-monitoring equipment, sometimes resulting in patient injury.9
It should be emphasized that some anesthesia machines and patient monitors that are alleged to be MRI compatible may still contain ferromagnetic components and may pose risks when safety precautions (often described in fine print in the user's manual) are violated.
57.2.3 Why Might MRI Require Moderate or Deep Sedation, or General Anesthesia?
Patients must remain motionless during MRI scans. However, the long duration (up to 20 minutes or more) of some MRI scans and the loud noise of the MRI machine may eventually lead to significant discomfort for many patients. In addition to fidgeting, many MRI patients are fearful or claustrophobic. Moderate to deep sedation, and sometimes general anesthesia, may be required to immobilize these patients sufficiently to obtain good quality scan images, particularly in children and mentally challenged patients.
57.2.4 What Special Precautions Must Clinicians Take When Responding to or Working in the MRI Suite?
Clinicians caring for patients in MRI suites must be careful to rid themselves of all objects with possible ferromagnetic components, such as pagers, mobile phones, keys, pens, and stethoscopes. In addition, credit cards and ID badges may be demagnetized by the magnetic field.
There are serious concerns regarding the clinical monitoring modalities available in an MRI unit. In order to monitor the patients undergoing general anesthesia properly, it is necessary to have MRI-compatible systems that support automatic noninvasive blood pressure monitoring, electrocardiography, pulse oximetry, capnography, and even multichannel invasive pressure monitoring. To avoid burns10 and fires,11 electrocardiogram electrodes must be applied at a distance from the imaging area, or (in special cases) should be replaced with special MRI-compatible carbon electrodes.
57.2.5 Where Can One Obtain MRI-Compatible Anesthesia Equipment?
The list of MRI-compatible anesthesia equipment needed in an MRI suite can be extensive and includes, but is not limited to, anesthesia machines, patient monitors, oxygen cylinders, and laryngoscopes. Hospital purchasing departments should be able to provide useful information on the availability of this equipment. The author recommends getting information from the web site www.magmedix.com. Additional resources appear in Appendix 57.1.