Dosimetry of Exposure of Patients to Pulsed Gradient Magnetic Fields in MRI

In magnetic resonance imaging, time-varied gradient magnetic fields may stimulate nerves and muscles by inducing electric fields and currents in patients, which may potentially cause health problem. In this study, realistic transverse x-, y-gradient coils and longitudinal z-gradient coil were numerically designed. The magnetic flux density, current density and electric fields in a 3-D real human man model were calculated by employing impedance method. The exposure levels have been compared with the guidelines of the International Commission on Non-Ionizing Radiation Protection (ICNIRP). It was found that the spatially averaged magnetic flux density greatly exceeds the ICNIRP reference level, and the maximum value of the 1-cm2-averaged induced current density in central nervous system greatly exceeds the ICNIRP basic restriction. The strongest levels of exposure are observed when all three gradient coils are operated simultaneously.