Static magnetic field probe
Ultra high field MRI
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 [Formula: see text] . Thus, only the translational motion in the magnet's fringe field is considered and rotations are neglected.
MATERIALS AND METHODS:
A magnetic field probe was constructed to simultaneously record the magnetic flux density [Formula: see text] (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 phi(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 [Formula: see text] and dphi/dt were recorded during standard operating procedures at three different MR systems up to 7 T.
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 dphi/dt = 21 mWb/s, dB/dt = 1.40 T/s and [Formula: see text] Twere found.
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.
Type of Publication:
Journal article published