- Inductive heating. According to Faraday's Law, a changing magnetic field generates electrical currents, called eddy currents, in conductive materials. The human body is a reasonable conductor (owing to the presence ions in extracellular in intracellular fluids), and, of course, metallic foreign materials are excellent conductors. When electric currents are channeled into a small area of high resistance (such as a metal-skin interface), heat is generated that may result in burns.
- Heating of a resonant loop. Certain electrical circuits are known to resonantly absorb and release energy at a specific frequency if they form a loop that contains both capacitance and inductance elements. For example, a coiled wire lying on a patient's stomach might contribute inductance, while the skin-metal interface at radiofrequencies becomes capacitive. Although a relatively uncommon situation, resonant loops may generate the highest level of inductive heating with burns possible.
- Antenna effect. Wires and elongated conductive objects can act like antennas, capturing electromagnetic waves to extract power from them. Antennas produce standing-wave patterns of voltage and current that are concentrated near their tips, maximal when their length is one-half of the RF wavelength. For MRI, therefore, wires measuring about 26 cm at 1.5T or 13 cm at 3.0T are the most likely to create heating problems.
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Additional Comments about Burns and MR Safety
Tattoos. The first degree burn associated with the dragon tattoo pictured above is a distinctly rare phenomenon. Today something like 20% of people in Europe and America have at least one tattoo, so the incidence of a severe reaction is extremely small. We therefore scan everyone with tattoos, but do warn each patient to be aware of any burning or discomfort during the scan.
Cosmetic tattoos (such as permanent eyeliner) also carry some risk, but because of their small size will seldom cause a problem. Reported cases include transient eyelid swelling, itching, and the like with no lasting sequela.
Clothing. Various sportswear and underwear are now sometimes impregnated with invisible metallic microfibers that are undetectable to the eye. Supposedly these fibers (often silver impregnated) have an antimicrobial effect. This is just one more reason that everyone getting an MRI scan should be stripped down completely (including underwear) and provided a hospital-style gown prior to MRI.
Foreign conductive/metallic objects. Scanning a patient who still has a non-MRI-compatible EKG pad, pulse oximeter, on the like should be considered a "never event" -- an egregious medical error for which no excuse is acceptable. The situations where I have seen this occur are usually in critically ill patients who are being rushed to MRI with a whole team of health care providers urging the tech to begin scanning immediately. It is good practice never to use any sheets, blankets, bedding from the hospital floor to be placed with the patient on the MR gurney, but to provide those items from the MR inventory itself.
Touching coils. Specialized MR foam padding should be used (not sheets or towels) to prevent patients from touching the walls of the scanner or the local coils. This may be difficult in very large patients but must be done.
Avoiding skin-to-skin contact.The "kissing burns" pictured above are what can happen if this rule is not observed. Typical scenarios include crossing the legs or holding ones hands.
Wires and cables. Of course, skin contact must be avoided by padding and positioning. The wires should not be allowed to cross. They should be run down the center of the MR system out between the patient's legs (not along the side of the system close to the RF coil). If more than one wire is exiting, try to leave a little space between them (to prevent capacitive coupling).
Wound dressings and patches. Some topical ointments like silver sulfadiazine (Silvadene) and Zinc Oxide have been known to become hot during MR imaging. A large number of transdermal patches for drug delivery have aluminum backing which has resulted in minor burns, so these should be recognized as well.
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How do radiofrequency fields affect biological tissues?