Objectives Workers’ exposure to static magnetic stray fields from MRI scanners was evaluated in a large cross-sectional study in The Netherlands. Factors determining the exposure to static magnetic fields (SMF) and time-varying magnetic fields (TVMF) like scanner characteristics, work patterns and individual practices could be quantified.

Methods Occupational exposure to static magnetic fields (B) and rate of change of field due to movement through a static magnetic field (dB/dt) were measured using a Magnetic Field Dosimeter (University of Queensland). About 480 predominantly full-shift measurements were collected from more than 300 employees working at 14 clinical or research MRI facilities. During measurement days, participants kept a log of the tasks they performed and the scanner (s) at which they worked.

Results Highest peak B and dB/dt exposures were observed among MRI radiographers and research staff. Peak exposures were highest in academic hospitals (mean [range]: 814 mT [36 - 4928]; 1291 mT/s [27 - 5057]) and lowest for people working in experimental animal imaging facilities (mean [range]: 227 mT [31 - 625]; 395 mT/s [32 - 1329]). Scanner field strength showed a strong association with peak B and dB/dt exposure when subjects worked near a closed bore scanner. However, for small bore scanners this association appeared to be negative. This could be partially explained by variability in shielding (active vs. passive) of the small-bore magnets.

Conclusions For assessment of exposure for epidemiological studies classification solely based on scanner field strength is insufficient. The type of scanner (open/closed/extremity scanner; large/small bore) and type of shielding of the static magnetic field (active/passive) should be taken into account as well.