Evaluation of exposure to (ultra) high static magnetic fields during activities around human MRI scanners.

Abstract

To assess the individual exposure to the static magnetic field (SMF) and the motion-induced time-varying magnetic field (TVMF) generated by activities in an inhomogeneous SMF near high and ultra-high field magnetic resonance imaging (MRI) scanners. The study provides information on the level of exposure to high and ultra-high field MRI scanners during research activities. A three-axis Hall magnetometer was used to determine the SMF and TVMF around human 3- and 7-Tesla (T) MRI systems. The 7-T MRI scanner used in this study was passively shielded and the 3-T scanner was actively shielded and both were from the same manufacturer. The results were compared with the exposure restrictions given by the International Commission on Non-Ionizing Radiation Protection (ICNIRP). The recorded exposure was highly variable between individuals, although they followed the same instructions for moving near the scanners. Maximum exposure values of B=2057mT and dB/dt=4347mT/s for the 3-T scanner and B=2890mT, dB/dt=3900mT/s for 7T were recorded. No correlation was found between reporting the MRI-related sensory effects and exceeding the reference values. According to the results of our single-center study with five subjects, violation of the ICNIRP restrictions for max B in MRI research environments was quite unlikely at 3 and 7T. Occasions of exceeding the dB/dt limit at 3 and 7T were almost similar (30% of 60 exposure scenarios) and highly variable among the individuals.