Another component of the MR environment is a pulsed gradient magnetic field that is used for signal localization. When this gradient magnetic field is applied, the magnetic field intensity changes rapidly, giving rise to a time-varying magnetic field. During the rise time of the magnetic field, a voltage is induced in an electrical conductor, even when it is stationary in the field. However, in most MRI systems, the currents induced by the pulsed magnetic gradient field are about 1,000 times smaller than those induced by the pulsed RF component and are therefore not of great concern with regard to thermal injuries.1 Major concerns with the pulsed gradient fields are biological effects including electrical nerve stimulation and the generation of light flashes (magnetophosphenes) that may result from a slight torque exerted on the retinal cones.
Current FDA guidance limits the Time Rate of Change of Magnetic Field (dB/dt) to levels which do not result in painful peripheral nerve stimulation.
Ref.¨A Primer on Medical Device Interactions with Magnetic Resonance Imaging Systems¨, Food and Drug Administration, Feb. 27, 1997.