Gajda Gregory B, Bly Stephen H
*775 Brookfield Road, Ottawa, ON, Canada, K1A 1C1.
Health Phys. 2017 Jun;112(6):501-511. doi: 10.1097/HP.0000000000000663.
Guidelines for prevention of peripheral nerve stimulation from exposure to low frequency magnetic fields have been developed by standard-setting bodies. Exposure limits or reference levels (RLs) are typically set in terms of the maximum root-mean-square amplitude of a sinusoidal waveform; however, environmental flux densities are often periodic, non-sinusoidal waveforms. This work presents a procedure for deriving RLs for any generalized periodic waveform using the empirical nerve-stimulation threshold data obtained from human volunteer MRI experiments. For this purpose, the "Law of Electrostimulation" (LOE), which sets forth conditions of a waveform necessary to trigger the action potential required to depolarize cell membranes, is applied to various waveforms. The results of the LOE analysis are waveform-specific, amplitude thresholds of stimulation that are found in terms of the empirically-derived rheobase threshold time-rate-of-change flux density and chronaxie from trapezoidal pulse MRI experiments. The thresholds are converted to amplitude RLs in two asymptotic frequency regimes as per the usual practice in standard setting. The resulting RLs have the same frequency dependence as in existing standards (i.e., inverse-frequency below a transition frequency and flat above). It is shown that the transition frequency is dependent only on the shape of the waveform. Both sinusoidal and non-sinusoidal waveforms have identical peak-to-peak amplitude RLs above their respective transition frequencies. Below these frequencies, all peak-to-peak amplitude RLs have the same functional dependence on frequency when the frequency is normalized to the waveform-specific transition frequency. This results in simple criteria for testing the amplitude of any arbitrary periodic waveform against potential for stimulation. These criteria are compared to guidance given for non-sinusoidal waveforms in the ICNIRP 1 Hz-100 kHz exposure standard.
标准制定机构已经制定了预防因暴露于低频磁场而导致外周神经刺激的指南。暴露限值或参考水平（RLs）通常根据正弦波形的最大均方根幅度来设定；然而，环境通量密度往往是周期性的非正弦波形。这项工作提出了一种程序，可利用从人体志愿者MRI实验中获得的经验性神经刺激阈值数据，推导出任何广义周期性波形的RLs。为此，将阐述触发使细胞膜去极化所需动作电位的波形条件的“电刺激定律”（LOE）应用于各种波形。LOE分析的结果是特定于波形的刺激幅度阈值，这些阈值是根据梯形脉冲MRI实验中经验得出的基强度阈值时间变化率通量密度和时值确定的。按照标准制定中的常规做法，在两个渐近频率范围内将这些阈值转换为幅度RLs。所得的RLs具有与现有标准相同的频率依赖性（即低于转变频率时与频率成反比，高于转变频率时保持不变）。结果表明，转变频率仅取决于波形的形状。正弦波形和非正弦波形在各自的转变频率以上具有相同的峰峰值幅度RLs。在这些频率以下，当频率归一化为特定于波形的转变频率时，所有峰峰值幅度RLs对频率具有相同的函数依赖性。这就产生了用于测试任何任意周期性波形的幅度是否具有刺激可能性的简单标准。将这些标准与ICNIRP 1 Hz - 100 kHz暴露标准中针对非正弦波形给出的指南进行了比较。
