Weighted-peak assessment of occupational exposure due to MRI gradient fields and movements in a nonhomogeneous static magnetic field

Purpose. A procedure for assessing occupational exposure due to MRI gradient magnetic fields and movement induced effects in the static magnetic field is proposed and tested. Methods. The procedure was based on the application of the weighted-peak method in time domain. It was tested in two 1.5 T total-body and one 3 T head-only scanner MRI facilities in Rome (Italy). Exposure due to switched gradient fields was evaluated in locations inside the magnet room where operators usually stay during particular medical procedures (e.g. cardiac examinations of anesthetized patients); MRI sequences were selected to approach as far as possible a representative worst case exposure scenario. Movement induced effects were evaluated considering the actual movements of volunteer operators during work activity, by measuring the perceived time-varying magnetic field by a head-worn probe. The analysis of results was based on ICNIRP 1998 and 2010 guidelines, following a weighted-peak approach and including an ad-hoc extension to the latter ones, needed to verify compliance in the frequency range 0 - 1 Hz. Results. Exposures due to switched gradient fields in 1.5 T MRI scanners mostly resulted non-compliant with ICNIRP 1998 occupational reference levels, being, at the same time, always compliant with ICNIRP 2010 ones. Gradient field levels and ICNIRP indexes were significantly lower for the 3 T unit, due to its small dimensions, as that unit was a head-only scanner. Movement induced effects resulted potentially non compliant only in the case the operator moved the head inside the bore of a 1.5 T scanner. Conclusions. The procedure had proven to be a sound approach to exposure assessment in MRI. Its testing allowed to draw some general considerations about exposures to gradient magnetic fields and movement-induced effects.