STUK Radiation and Nuclear Safety Authority, Finland.
Health Phys. 2011 Jun;100(6):641-53. doi: 10.1097/HP.0b013e318202ec7e.
Movement in a strong static magnetic field induces electric fields in a human body, which may result in various sensory perceptions such as vertigo, nausea, magnetic phosphenes, and a metallic taste in the mouth. These sensory perceptions have been observed by patients and medical staff in the vicinity of modern diagnostic magnetic resonance (MR) equipment and may be distracting if they were to affect the balance and eye-hand coordination of, for example, a physician carrying out a medical operation during MR scanning. The stimulation of peripheral nerve tissue by a more intense induced electric field is also theoretically possible but has not been reported to result from such movement. The main objective of this study is to consider generic criteria for limiting the slowly varying broadband (<10 Hz) electric fields induced by the motion of the body in the static magnetic field. In order to find a link between the static magnetic flux density and the time-varying induced electric field, the static magnetic field is converted to the homogeneous equivalent transient and sinusoidal magnetic fields exposing a stationary body. Two cases are considered: a human head moving in a non-uniform magnetic field and a head rotating in a homogeneous magnetic field. Then the electric field is derived from the magnetic flux rate (dB/dt) of the equivalent field by using computational dosimetric data published in the literature for various models of the human body. This conversion allows the plotting of the threshold electric field as a function of frequency for vertigo, phosphenes, and stimulation of peripheral nerves. The main conclusions of the study are: The basic restrictions for limiting exposure to extremely low frequency magnetic fields recommended by the International Commission on Non-Ionizing Radiation Protection ICNIRP in 1998 will prevent most cases of vertigo and other sensory perceptions that result from induced electric fields above 1 Hz, while limiting the static magnetic field below 2 T, as recently recommended by ICNIRP, provides sufficient protection below 1 Hz. People can experience vertigo when moving in static magnetic fields of between 2 and 8 T, but this may be controlled to some extent by slowing down head and/or body movement. In addition, limiting the static magnetic field below 8 T provides good protection against peripheral nerve stimulation.
在强静磁场中运动时，人体内部会产生感生电场，可能导致各种感觉，如眩晕、恶心、磁幻视和口中金属味。在现代诊断磁共振（MR）设备附近，患者和医务人员已经观察到这些感觉，而在进行磁共振扫描时，如果这些感觉会影响到医生的平衡和手眼协调，就会产生干扰。此外，理论上也有可能刺激外周神经组织，但尚未有报道称这种运动导致了这种刺激。本研究的主要目的是考虑限制人体在静磁场中运动时产生的缓慢变化的宽带（<10 Hz）感生电场的一般准则。为了找到静磁场磁通密度与时变感生电场之间的联系，将静磁场转换为暴露于静止物体的均匀等效瞬态和正弦磁场。考虑了两种情况：人体头部在非均匀磁场中运动和头部在均匀磁场中旋转。然后，通过使用文献中公布的人体各种模型的计算剂量数据，从等效场的磁通率（dB/dt）导出电场。这种转换允许绘制作为频率函数的眩晕、幻视和外周神经刺激的阈值电场。本研究的主要结论是：1998 年国际非电离辐射防护委员会（ICNIRP）推荐的限制极低频磁场暴露的基本限制将防止大多数由感生电场引起的眩晕和其他感觉，超过 1 Hz，同时将静磁场限制在 2 T 以下，如 ICNIRP 最近建议的那样，在 1 Hz 以下提供足够的保护。人在 2 至 8 T 的静磁场中移动时可能会感到眩晕，但通过降低头部和/或身体的运动速度，可能会在一定程度上得到控制。此外，将静磁场限制在 8 T 以下可以很好地防止外周神经刺激。
