Measurement and Modelling of Electric Fields Induced in the Human Body by Temporally Changing Magnetic Fields

在向临床环境过渡的25年中，MRI已成为一种宝贵的临床工具。然而，MRI有许多公认的潜在不良健康影响，涉及对铁磁性物体的力，射频加热和时变场引起的周围神经刺激。这些影响对病人和工作人员都得到了很好的控制，世界各地的监管机构制定了若干标准来处理磁场和电场及其危害。然而，尽管没有证据表明它们对健康有有害影响，但关于核磁共振成像中涉及的电磁场的其他生物效应的数据却很缺乏。在高场核磁共振装置周围会产生金属味和头晕等急性效应，但没有证据表明这些对健康有任何影响。最近，欧盟同意推迟其2004年关于工作场所电磁场的指令，该指令将损害核磁共振成像的临床和研究用途。立法中提出的限制很少或没有科学依据，特别是在MRI使用的频率范围内。因此，进行新的研究是至关重要的，这将有助于制定监管水平，以便患者在不危及设备操作人员的情况下获得满意的护理水平。这项拟议的研究建立在申请人最近在模拟和测量人体中感应的电场（由于磁场）以及了解对整个身体的影响方面的工作基础上。到目前为止，还没有通过实验测量来验证用于计算身体电流的数值模型。这将通过模拟在磁场运动和梯度切换期间在特定对象中产生的场，然后通过实验测量新开发的偶极子探针的表面电场来实现。在验证了模型之后，将进行四个具体的实验。这些是：1)测量和模拟周围神经刺激部位的电场。这种测量将导致更好地理解PNS的过程，并允许未来的扫描仪操作员优化扫描参数的成像速度，同时避免PNS。目前，通过设置保守的限制来避免PNS，这可能会损害成像质量。2)模拟由前庭电刺激（GVS）和磁场运动产生的头部电流。直接比较感应电流和直流电的大小和方向将支持眩晕效应是由感应电场介导的假设。3)测量受试者在磁场运动中对眩晕的敏感性，并将其与平衡系统对GVS的敏感性联系起来。正相关将间接支持眩晕效应是由周围前庭系统的刺激产生的假设。4)比较GVS与磁场运动产生的前庭诱发动眼肌反应，直接验证眩晕效应是由外周前庭系统刺激产生的假说。

In the 25 years since its transition to the clinical environment, MRI has become an invaluable clinical tool. However there are a number of well recognised potential adverse health effects of MRI related to forces on ferromagnetic objects, radio-frequency heating and peripheral nerve stimulation due to time varying fields. These effects are well controlled for both patients and staff, and there are several standards from regulatory bodies worldwide which deal with magnetic and electric fields and their hazards. However there is a paucity of data on other biological effects of the EMFs involved in MRI, although is no evidence that they have a detrimental effect on health. There are acute effects such as metallic taste and dizziness around high field MRI magnet installations, but there is no evidence that these have any consequence for health. Recently the European Union agreed to postpone its 2004 directive on electromagnetic fields in the workplace, which would have compromised clinical and research uses of MRI. The limits proposed in the legislation had little or no scientific basis particularly in the frequency ranges employed in MRI. It is therefore essential that new research is carried out which will assist in setting regulatory levels so that patients receive a satisfactory level of care without endangering operators of the equipment.This proposed research builds on the applicants' recent work in modelling and measuring the electric fields induced in the human body (due to magnetic fields) and understanding the effects on the whole body. So far there has been no verification of the numerical models used to calculate body currents by experimental measurement. This will be carried out by simulating the fields induced in a particular subject during movements in the magnetic field and during gradient switching and then measuring, by experiment, the surface electric fields using newly developed dipole probes. Having verified the models, four specific experiments will be carried out. These are: 1) Measure and model electric fields at the site of peripheral nerve stimulation, PNS. This measurement will lead to a better understanding of the process of PNS and allow a future scanner operator to optimise scan parameters for imaging speed whilst avoiding PNS. Currently PNS is avoided by setting conservative limits which can compromise imaging quality. 2) Modelling of head electric currents produced by galvanic vestibular stimulation (GVS) and by movements in magnetic fields. Direct comparison of the magnitudes and direction of induced and direct currents will support the hypothesis that the vertigo effect is mediated by an induced electric field.3) Measure subjects' sensitivity to vertigo during movements in magnetic fields and correlate it with the sensitivity of their balance system to GVS. A positive correlation will indirectly support the hypothesis that the vertigo effect is produced by stimulation of the peripheral vestibular system.4) Comparison of vestibular-evoked oculomotor responses produced by GVS with those produced by movements in magnetic fields to test directly the hypothesis that the vertigo effect is produced by stimulation of the peripheral vestibular system.