Computational dosimetry of induced electric fields during realistic movements in the vicinity of a 3 T MRI scanner

Abstract

Medical staff working near magnetic resonance imaging (MRI) scanners are exposed both to the static magnetic field itself and also to electric currents that are induced in the body when the body moves in the magnetic field. However, there are currently limited data available on the induced electric field for realistic movements. This study computationally investigates the movement induced electric fields for realistic movements in the magnetic field of a 3 T MRI scanner. The path of movement near the MRI scanner is based on magnetic field measurements using a coil sensor attached to a human volunteer. Utilizing realistic models for both the motion of the head and the magnetic field of the MRI scanner, the induced fields are computationally determined using the finite-element method for five high-resolution numerical anatomical models. The results show that the time-derivative of the magnetic flux density (dB/dt) is approximately linearly proportional to the induced electric field in the head, independent of the position of the head with respect to the magnet. This supports the use of dB/dt measurements for occupational exposure assessment. For the path of movement considered herein, the spatial maximum of the induced electric field is close to the basic restriction for the peripheral nervous system and exceeds the basic restriction for the central nervous system in the international guidelines. The 99th percentile electric field is a considerably less restrictive metric for the exposure than the spatial maximum electric field; the former is typically 60-70% lower than the latter. However, the 99th percentile electric field may exceed the basic restriction for dB/dt values that can be encountered during tasks commonly performed by MRI workers. It is also shown that the movement-induced eddy currents may reach magnitudes that could electrically stimulate the vestibular system, which could play a significant role in the generation of vertigo-like sensations reported by people moving in a strong static magnetic field.

Occupational exposure in MR facilities due to movements in the static magnetic field

“…In the last decade, several studies have focused on characterizing work‐related exposure to static magnetic fields and motion‐induced time‐varying magnetic fields from MRI scanners, in order to identify possible related transient and long‐term effects and to assess the actual exposure levels during MRI examinations …”

“…Computational techniques have been used to investigate the exposure to movement‐induced electric fields, revealing that in some cases and for some of the considered movements in the simulated MR environment, the basic restrictions set by international guidelines can be exceeded in proximity of both 3 T scanners and 7 T scanners…”

Relative Magnetic Force Measures and Their Potential Role in MRI Safety Practice

“…It should be noted that the proposed interpolation method does not guarantee that the results obey Maxwell's laws (eg, that the divergence of the magnetic field is zero everywhere). For this reason, the method reported previously where a coil model was used to produce a physically realizable magnetic field may be more exact (albeit more complicated) than the present approach. On the other hand, the quality of the fits between our experimental and predicted results suggests that the present method was sufficiently accurate for the present purpose.…”

Influence of motion speed in static magnetic fields of MRI scanners on the density of induced electric currents in bodies of MRI operators

“…Laakso et al [7] computed dosimetry of induced electric fields during realistic movements in the vicinity of 3 T MRI scanner. The derivative of the magnetic flux density (dB/dt) was approximately linearly proportional to the induced electric field in the head, independent of the position of the head with respect to the magnet.…”