Magnetic stimulation of excitable tissue: calculation of induced eddy-currents with a three-dimensional finite-element model

Abstract

The eddy-current distribution in a 3-D inhomogeneous model of a dog's thorax due to time varying fields in magnetic resonance imaging (MRI) x, y, and z-gradient coils and a pair of coplanar coils are calculated. From experimental data of cardiac threshold from a z-gradient coil for a 540 mu s rectangular pulse, a current density of 21.5 A/m/sup 2/ was calculated. For a co-planar coil pair, the threshold current density for cardiac stimulation is 19.2 A/m/sup 2/ at a duration of 640 mu s. The different conductivities of tissues in the thorax have a significant influence on the eddy-current flow. Calculations combined with measurements will help identify safe levels of pulsed gradient fields in future MRI systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>