E Magnetic resonance imaging
MRI uses the dipole moment (the ability of the atomic nucleus to behave as a magnet) of the hydrogen atom. The patient is placed supine within the scanning gantry or bore of the magnet. The magnet used for MRI can be a permanent magnet or a powerful superconducting electromagnet cooled with liquid helium to 4° Kelvin. The quality of the MRI image is directly related to the strength of the magnetic field. Contrast media are also used in MRI studies to enhance the patient’s tissues and allow the scan to provide further diagnostic information. MRI contrast is most commonly gadopentetate dimeglumine (Magnevist) than CT scans, which contain the element gadolinium, bound as a chelated structure and administered primarily parenterally but rarely enterally.
2. Anesthetic technique
a) MRI can take up to 1 hour or longer. During this time, the patient should remain extremely still to reduce motion artifacts. These artifacts can cause unfaithful representations of the tissues being studied. The motions of breathing, the heart, blood flow, swallowing, and even cerebrospinal fluid flow produce artifacts in a highly sensitive MRI scan.
b) The patient is exposed to varying magnetic fields of up to 4 tesla (T), along with additional exposure to variable radiofrequency radiation. Blood flow is decreased by strong magnetic fields, and blood pressure compensates by rising. Patients also have reported symptoms of vertigo, nausea, headache, and visual sensations.
c) The MRI machine produces loud vibratory and knocking noises as coils are switched on and off during the course of the study.
d) Most patients are content with an explanation of what to expect during the procedure and with reassurance. Some patients need minimal or moderate sedation. Patients with claustrophobia and those who cannot or will not remain motionless during the study (children) as well as critically ill patients may require deep sedation or general endotracheal anesthesia.
e) MRI is not painful, so opioids are not usually required. Sedation has been performed with oral and IV midazolam, ketamine, pentobarbital, chloral hydrate, and propofol.
g) The LMA serves as an excellent, relatively noninvasive airway for MRI. Some anesthesia providers prefer general endotracheal intubation.
h) Because of the intense magnetic field always present in the MRI suite, anesthesia providers should be aware of every item on their persons and every item that is to be used in conjunction with anesthesia administered to the patient.
i) Ferromagnetic (iron-containing) substances are attracted at astonishing rates of speed into the bore of the magnet. Personal items such as pens, certain types of eyeglasses, jewelry, watches, pagers, personal computers, calculators, name badges, coins, audiotapes, videotapes, and credit cards are some of the items that should never enter the MRI suite, as well as any ferromagnetic anesthesia equipment, medication vials, and supplies. If a patient were present within the bore of the MRI, injury or death could be possible from the missile created.
j) Metals that are known to be safe within the proximity of the MRI bore are stainless steel, nonferrous alloys, nickel, and titanium. Materials and equipment constructed of plastic are safe.
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