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. J. Magn. Reson. Imaging 2007;26:1204–1209. © 2007 Wiley‐Liss, Inc.