Minimizing the induced fields in MRI occupational workers by lowering the imager

Abstract

In magnetic resonance imaging (MRI), healthcare workers can be exposed to strong static and time-varying magnetic fields outside the imager. Body motion through the large, spatially nonuniform static magnetic field generated by the main magnet and exposure to low-frequency magnetic fields from pulsed gradient coils can lead to the stimulation of electric fields in the body. Tissue of the central and peripheral nervous system (CPNS) in the head and torso is particularly susceptible. To protect occupational workers, the member states of the European Union are required to incorporate the physical agents directive (PAD) 2004/40/EC into their legislation, which could restrict the use of MRI in some cases. This article reports a rather simple and general method/solution that can notably reduce the head/trunk exposure of MRI operators to both static and (low-frequency) time-dependant magnetic fields in MRI. It is numerically shown that the upper body CPNS exposure can be reduced by factors of up to 50 or more, when the MRI machine is simply lowered by 1 m in height relative to the ground floor. The proposed approach can lead to significantly reduced in situ electric fields/current in the upper body and thus allow faster worker motions around the lowered imager.

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

“…Moreover, the number of situations where the absolute maximum EI falls within the CNS reduces passing from model SM to TM. These data confirm the results of a previous study (30), where it was shown that the increase of the vertical distance between the head of the operator and the axis of a tubular scanner (eg, by using elevation platforms or scanners embedded into the ground floor) can lead to significant reduction of the exposure levels.…”

Reverse‐engineering of gradient coil designs based on experimentally measured magnetic fields and approximate knowledge of coil geometry‐application in exposure evaluations

“…However, operating the z-gradient coil at above $26 mT/m could potentially result in the excitation of peripheral nerves for the herein assumed model conditions. It is noted that these preliminary findings are quite similar to those of recent studies (14,24), which in effect to a certain extent validate the earlier models that relied on theoretical rather than realistic gradient sets.…”

Model forB1Imaging in MRI Using the Rotating RF Field

“…Gradient coil equipment is expensive/intricate, noisy, power and space demanding, and can induce complex spatiotemporal eddy currents in nearby conducting structures, including the patient [ 2 – 4 ]. Gradient switching rates are limited by regulatory agencies to avoid peripheral nerve stimulation [ 5 , 6 ].…”