Analysis of eddy currents originating from switched magnetic gradient fields in magnetic resonance imaging.

The fact that time-varying magnetic fields cause eddy currents in conductive objects is very well known. Switched magnetic gradient fields, used in echo planar imaging, were shown in the past to be able to elicit a stimulation process in peripheral nerves. We report about the distribution and values of induced current densities in the human torso, modeled by the Finite Element Method using isoparametric formulation. Since the applied numerical method is a frequency-domain one, the trapezoidal waveform is decomposed into a Fourier series. The simulation was made for four different exposures to switched magnetic gradient fields: transverse (x- and y-gradients) coil systems, longitudinal (z-gradients) coil system and for all the three coil systems working simultaneously. Special attention was paid to the region of the heart, since stimulation of the heart muscle could be extremely dangerous for human health. Therefore, the three components of the current density in the region of the heart muscle were spatially analyzed in all directions (x, y, and z), trying to find out the 'worst case' position of the heart muscle relative to the gradient coil system at which the highest current density is induced. Finally, the calculated values were compared to existing recommendations, showing that the simulated 'worst case' amplitude is relatively close to the limiting value for sinusoidal currents.