As the push toward stronger and faster MRI scanners continues, so does concern over magnet safety, according to Filiz Yetisir, who discussed the potential effects MRI has on patients at the recent International Society for Magnetic Resonance in Medicine virtual meeting (ISMRM 2021).
An MRI device creates a magnetic field having a strength of several tesla. Any magnetic objects near the device can be sucked into the main field, becoming dangerous projectiles. For instance, in 2001 a six-year-old was killed by an oxygen cylinder. Yetisir warns: “Remember, the magnet’s always on.”
The gradient coils used during imaging produce magnetic fields much weaker than the dc main field, but they are turned on and off throughout the imaging pulse sequence. This causes two safety concerns. 1) The changing magnetic field induces eddy currents in the patient, which can stimulate nerves-an effect similar to transcranial magnetic simulation. 2) The switching of current in the gradient coils creates mechanical vibrations, leading to noise so loud that ear plugs may be needed to prevent hearing loss.
A radiofrequency magnetic field-which rotates the spins into the plane perpendicular to the main magnet-is an essential part of any MRI pulse sequence. This field can induce eddy currents that heat the tissue. Generally the field isn’t strong enough to cause significant heating, but if a person has metal implants or tattoos, the heating may be increased locally. Any implanted medical device, such as a pacemaker, can interact with all three types of magnetic fields.
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