Numerical field evaluation of healthcare workers when bending towards high‐field MRI magnets

Abstract

In MRI, healthcare workers may be exposed to strong static and dynamic magnetic fields outside of the imager. Body motion through the strong, non-uniform static magnetic field generated by the main superconducting magnet and exposure to gradient-pulsed magnetic fields can result in the induction of electric fields and current densities in the tissue. The interaction of these fields and occupational workers has attracted an increasing awareness. To protect occupational workers from overexposure, the member states of the European Union are required to incorporate the Physical Agents Directive (PAD) 2004/40/EC into their legislation. This study presents numerical evaluations of electric fields and current densities in anatomically equivalent male and female human models (healthcare workers) as they lean towards the bores of three superconducting magnet models (1.5, 4, and 7 T) and x-, y-, and z-gradient coils. The combined effect of the 1.5 T superconducting magnet and the three gradient coils on the body models is compared with the contributions of the magnet and gradient coils in sep With the latest developments in MRI technology, the interaction of electromagnetic fields generated by the newgeneration imagers and healthcare workers has recently attracted substantial awareness and consideration. The recent progress in high-field superconducting magnets has resulted in a significant increase in occupational worker exposure to very large static magnetic fields of up to several Tesla (1-5). Apart from exerting hydrodynamic translational forces on the moving charge carriers of the blood stream (6 -8), strong static magnetic fields could also lead to substantial induction of in situ currents during body motion through the non-uniform 3D static field pattern. In a computational study involving a whole-body human voxel model, Crozier and Liu (9) have shown that body motions around a realistic 4 T superconducting main magnet can induce field quantities in the tissue near the levels of physiological significance.A number of physiological responses have been reported, such as headaches, nausea, vertigo, phosphenes (light flashes), numbness and tingling, loss of proprioreception and balance, and a metallic taste in the mouth associated with vigorous head movement (10,11). Whether these reactions are directly related to the motion-induced electric fields in the tissue is presently under debate and investigation (12). Some of these responses might be explained in terms of synapse excitations in the central nervous system (CNS). The CNS consists of the brain, the spinal cord, and the cerebrospinal fluid (CSF), where the CSF contains no nerve synapses. Also, it is still not understood how different static field strengths and patterns produced by new-generation imagers relate to the distribution and magnitudes of induced field quantities in both male and female MRI healthcare workers, particularly as they lean towards the imager bore for routine attendance of patients.Another safety concern in MRI is related to the human exposur...

Assessment of exposure to MRI motion‐induced fields based on the International Commission on Non‐Ionizing Radiation Protection (ICNIRP) guidelines

“…Some studies have quantified the magnetic field experienced by the operators experimentally (11), and the risk related to the exposure has been analyzed statistically (12). Because in vivo measurements of induced quantities are unfeasible, most of the studies aimed at quantifying motion-induced fields rely on computations (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24). However, none of these studies included the computation of the exposure indexes recently recommended by ICNIRP, which could become a legal requirement in the next future.…”

A GPU accelerated modeling of bio-effects associated with magnetic resonance imaging

“…The 5-mm resolution HUGO model is adopted for this study [6]. In this particular scenario, the body model was facing the MRI entrance and was placed a certain distance away from the device end so that there would be a 10-mm gap between the head and the device end at the bend angle of 90 degrees.…”

The physics of MRI safety

“…For example, near the entry to the bore of a 3T MRI where j$B o j tends to be greatest, it can be shown that current densities over 0.1 A/m 2 may be produced in conductive tissues due to the voltage induced by normal movement. 38,39 This is more than twice the 0.04 A/m 2 (low frequency) limit for exposure of workers recommended by the International Commission for Non-Ionizing Radiation Protection (ICNIRP). 40 Whether or not one considers the ICNIRP limit as overly conservative, currents induced in tissue deserve serious attention as a potential MRI safety concern, especially as new high-field MRI systems are introduced.…”

“…It is well known that movement in a magnetic field can induce a voltage (due to Faraday's law of induction, discussed in more detail later) in electrically conductive materials, including biological tissues, especially when the motion is through regions of space where the magnetic field changes steeply. For example, near the entry to the bore of a 3T MRI where tends to be greatest, it can be shown that current densities over 0.1 A/m may be produced in conductive tissues due to the voltage induced by normal movement . This is more than twice the 0.04 A/m (low frequency) limit for exposure of workers recommended by the International Commission for Non‐Ionizing Radiation Protection (ICNIRP) .…”