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

J Magn Reson Imaging. 2007 Nov;26(5):1204-9. doi: 10.1002/jmri.21152.


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 1.5 T conventional and a 0.6 T 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.

MeSH terms

  • Body Burden*
  • Electromagnetic Fields*
  • Health Personnel / statistics & numerical data*
  • Humans
  • Magnetic Resonance Imaging / statistics & numerical data*
  • Occupational Exposure / analysis*
  • Occupational Exposure / statistics & numerical data*
  • Radiation Dosage
  • Radiation Monitoring / statistics & numerical data*
  • Relative Biological Effectiveness
  • Risk Assessment
  • United Kingdom