Gradient magnetic fields

Gradient magnetic fields produce rapidly changing magnetic fields during scanning. Gradients turn on and off very rapidly to spatially encode the MR signal during a scan. High frequency gradient amplitudes switched very quickly (high dB/dt) may cause nerve stimulation at periphery of body due to induced currents in nerves. Because a current can be induced in any conductive material lying on or near the patient’s body, a potential biological hazard exists. The greater the rate of change of the magnetic field (dB/dt, slew rate), the larger the induced current. The muscles, nerves, and blood vessels of the human body are all conductive materials.