title: MR safety: simultaneous B0, dΦ/dt, and dB/dt measurements on MR-workers up to 7 T.
abstract: OBJECTIVE
  The EU directive on safety requirements (2004/40/EC) limits the exposure to time varying magnetic fields to dB /dt=200 mT/s. This action value is not clearly defined as it considers only the temporal change of the magnitude of B. Thus, only the translational motion in the magnet's fringe field is considered and rotations are neglected.
  
  
  METHODS
  A magnetic field probe was constructed to simultaneously record the magnetic flux density B(x, y, z) with a 3-axis Hall sensor and the induced voltage due to movements with a set of three orthogonal coils. Voltages were converted into time-varying magnetic flux d Φ(x, y, z)/dt serving as an exposition parameter for both translations and rotations. To separate the two types of motion, d B/dt was additionally calculated on the basis of the Hall sensor's data. The calibrated probe was attached to the forehead of 8 healthcare workers and 17 MR physicists, and B and dΦ/dt were recorded during standard operating procedures at three different MR systems up to 7 T.
  
  
  RESULTS
  The maximum percentage of the translational motion referring the data including both translations and rotations amounts to 32%. During volunteer measurements, maximum exposure values of dΦ/dt=21 mWb/s, dB/dt=1.40 T/s and |B|=2.75 T were found.
  
  
  CONCLUSIONS
  The findings in this work indicate that both translations and rotations in the vicinity of an MR system should be taken into account, and that a single regulatory action level might not be sufficient.
authors.affiliation: Department of Medical Physics in Radiology (E020), German Cancer Research Center (DKFZ), Im Neuenheimer Feld 280, 69120, Heidelberg, Germany.
authors.forename: Jens
authors.initials: J
authors.lastname: Groebner
authors.forename: Reiner
authors.initials: R
authors.lastname: Umathum
authors.forename: Michael
authors.initials: M
authors.lastname: Bock
authors.forename: Axel J
authors.initials: AJ
authors.lastname: Krafft
authors.forename: Wolfhard
authors.initials: W
authors.lastname: Semmler
authors.forename: Jaane
authors.initials: J
authors.lastname: Rauschenberg
mesh_terms: D002138:Calibration; D004574:Electromagnetic Fields; D004869:Equipment Safety; D006801:Humans; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging / Q000379:methods*; D009038:Motion; D009068:Movement; D016273:Occupational Exposure / Q000517:prevention & control*; D011829:Radiation Dosage; D011834:Radiation Monitoring / Q000379:methods*; D011835:Radiation Protection; D012399:Rotation; D013997:Time Factors