In a few cases, the large magnetic field could be a health hazard to the patient,78 necessitating the use of alternative diagnostic methods such as ultrasound or CT. Internal cardiac pacemakers may be converted to an abnormal asynchronous mode by the magnetic field. Certain types of steel cerebral aneurysm clips (ferromagnetic as opposed to nonmagnetic stainless steel) may experience strong forces, with the potential of harming the brain. Small steel slivers imbedded in the eye (occasionally seen in asymptomatic sheet metal workers or welders) could injure the retina and cause blindness. Life-support equipment containing magnetic steel will be strongly attracted into the magnetic field, threatening both the patient and the system. Cochlear implants may be damaged or cause unacceptable injury due to eddy current heating effects. Patients in any of the aforementioned categories cannot be scanned with MRI. There are other devices, such as implantable cardiac defibrillators, neurostimulators, and bone growth stimulators, that may malfunction in the presence of high magnetic fields. Certain prosthetic heart valves contain nearly magnetic stainless-steel components that are subject to strong forces when placed in powerful magnetic fields. However, it has been pointed out that the forces on the valve from the heart exceed those generated by even high-field magnets, and hence this is a relative contraindication for MRI scanning.
The pulsed radio waves are a source of heat energy deposited within the body. Software programs built into the computers restrict the frequency of pulsing so that the maximum allowable power deposition averaged over the whole patient is never exceeded. Occasionally, skin burns have been reported when a patient's skin has come into direct contact with uninsulated rf leads, but proper precautions prevent this from happening.
The complete examination takes from 30 to 60 min, is painless, and is well tolerated by most patients. It does require suspension of all motion, except for breathing, for periods ranging from a few seconds to 15 min at a time, depending on the particular pulse sequence. Some patients are claustrophobic and have difficulty with the examination. Most problems of this nature are satisfactorily treated with minor tranquilizers administered orally. Infants, younger pediatric patients, and agitated adults need to be sedated, as in CT, because any motion degrades the MRI scan.
Some minor precautions are necessary. Magnetically encoded plastic cards, such as credit, cash, and parking cards, may be damaged when they come within a certain range of the magnetic field. Some watches with steel parts and hearing aids (and their batteries) are vulnerable to damage. Any ferromagnetic steel objects are potentially lethal missiles if carried into the magnet room. Patients need to leave such objects outside the scanning room.
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