Electric fields induced in the human body by time-varying magnetic field gradients in MRI: numerical calculations and correlation analysis

The spatial distributions of the electric fields induced in the human body by switched magnetic field gradients in MRI have been calculated numerically using the commercial software package, MAFIA, and the three- dimensional, HUGO body model that comprises 31 different tissue types. The variation of vertical bar J vertical bar, vertical bar E vertical bar and vertical bar B vertical bar resulting from exposure of the body model to magnetic fields generated by typical whole- body x-, y- and z- gradient coils has been analysed for three different body positions (head-, heart- and hips- centred). The magnetic field varied at 1 kHz, so as to produce a rate of change of gradient of 100 T m(-1) s(-1) at the centre of each coil. A highly heterogeneous pattern of induced electric field and current density was found to result from the smoothly varying magnetic field in all cases, with the largest induced electric fields resulting from application of the y- gradient, in agreement with previous studies. By applying simple statistical analysis to electromagnetic quantities within axial planes of the body model, it is shown that the induced electric field is strongly correlated to the local value of resistivity, and the induced current density exhibits even stronger correlation with the local conductivity. The local values of the switched magnetic field are however shown to bear little relation to the local values of the induced electric field or current density.