Occupational exposure to static and time-varying gradient magnetic fields in MR units

Purpose: To assess exposure of MR staff to static and time-varying gradient magnetic fields by direct measurement and questionnaire and to compare these to the action values and exposure limits stated in published guidance. Materials and Methods: Instantaneous and time-averaged exposure to static magnetic fields were measured on clinical and research MR scanners. A questionnaire was sent to 100 clinical MR centers to estimate the average static field exposure. Instantaneous measurements of the time-varying magnetic field were conducted on a 1.5T conventional and a 0.6T open-bore MR system. Results: The 24-hour time-averaged static field exposure was measured to be 1.8 +/- 1 mT and the questionnaire produced a 24-hour time-average of 2.3 mT. The maximum static field strength recorded during patient set up was 554 mT. Time-varying gradient field exposure was found to exceed the action values up to 30 cm from the bore entrance or magnet rim. Conclusion: The time-varying gradient field exposure limit is the only limit to be exceeded. The time-averaged static field exposure to staff was found to be 100 times lower than the recommended guidelines.