High Field MRI Safety

Magnetic resonance imaging (MRI) machines have the most powerful magnets in common use. A standard MRI has 1.5 tesla (T) of magnetic field strength, about 500 times more powerful than a small kitchen magnet. Recently, high field MRI systems have been introduced with 3T magnets or stronger, to make sharper pictures with more detail. MRI magnets are powerful enough to warrant special precautions. High-field MRIs also require these safeguards, though no physical harm results from normal use.

Screening

• Before entering an MRI room, patients must remove all loose metal objects that could be drawn into the powerful magnet, such as cell phones, belt buckles, and coins. In addition, staff must screen the patient for implanted devices like pacemakers and stents. According to a report in the Internet Journal of World Health and Societal Politics, some stents that were cleared for 1.5T MRIs are not considered safe for 3T (Reference 3, page 2) In addition to medical implants, any equipment accompanying the patient, such as ventilators or medicinal pumps, must also be cleared.

Support Equipment

• The MRI process uses a host of support equipment, from small tools to electronic monitors. As with checking the patient for metal objects, any support equipment must also be cleared. If equipment has been certified safe for 1.5T, you cannot assume it is also safe for 3T. In addition to equipment, staff must be careful to watch for small, innocuous objects such as pens and bobby pins.

Shielding

• The walls of an MRI room have shielding material that prevents the strong magnetic field from reaching past the room's confines. If the clinic upgrades its MRI machine from a 1.5T to a 3T model, the shielding must also be upgraded. The additional shielding not only adds cost to the installation, it adds tons of weight to the room.

Sensations

• Some patients report headaches, nausea, feeling dizzy (vertigo), and other sensations while inside the MRI. These have been attributed to tiny currents produced in the body as it moves in the magnetic field. Slower movements will create smaller currents, so you should instruct the patient to avoid sudden movements while inside the machine. In a stronger field, more patients will report these feelings, though they are not harmful. The sensations disappear when the patient leaves the MRI.

Heat

• In addition to the magnetic field, an MRI machine uses focused radio waves to take images of the body. The technician must be careful to monitor the patient's exposure to the radio waves, as these can warm tissues to the point of burning and injury. The Food and Drug Administration has established limits for specific absorption rate (SAR) of radio energy to 8 watts per kilogram of tissue for 5 minutes, or 4 watts/kilogram for 15 minutes. Doubling the MRI's field strength from 1.5T to 3T quadruples the SAR, reducing the allowable time the patient can spend in the MRI machine.