Tinniswood A D, Furse C M, Gandhi O P
Department of Electrical Engineering, University of Utah, Salt Lake City 84112, USA.
Phys Med Biol. 1998 Aug;43(8):2361-78. doi: 10.1088/0031-9155/43/8/026.
At certain frequencies, when the human head becomes a resonant structure, the power absorbed by the head and neck, when the body is exposed to a vertically polarized plane wave propagating from front to back, becomes significantly larger than would ordinarily be expected from its shadow cross section. This has possible implications in the study of the biological effects of electromagnetic fields. Additionally the frequencies at which these resonances occur are not readily predicted by simple approximations of the head in isolation. In order to determine these resonant conditions an anatomically based model of the whole human body has been used, with the finite-difference time-domain (FDTD) algorithm to accurately determine field propagation, specific absorption rate (SAR) distributions and power absorption in both the whole body and the head region (head and neck). This paper shows that resonant frequencies can be determined using two methods. The first is by use of the accurate anatomically based model (with heterogeneous tissue properties) and secondly using a model built from parallelepiped sections (for the torso and legs), an ellipsoid for the head and a cylinder for the neck. This approximation to the human body is built from homogeneous tissue the equivalent of two-thirds the conductivity and dielectric constant of that of muscle. An IBM SP-2 supercomputer together with a parallel FDTD code has been used to accommodate the large problem size. We find resonant frequencies for the head and neck at 207 MHz and 193 MHz for the isolated and grounded conditions, with absorption cross sections that are respectively 3.27 and 2.62 times the shadow cross section.
在某些频率下，当人体头部成为一个谐振结构时，当身体暴露于从前向后传播的垂直极化平面波时，头部和颈部吸收的功率会显著大于根据其阴影横截面通常预期的值。这在电磁场生物效应的研究中可能具有重要意义。此外，这些谐振发生的频率不能通过孤立头部的简单近似轻易预测。为了确定这些谐振条件，使用了基于人体解剖结构的全身模型，并结合时域有限差分（FDTD）算法来准确确定场传播、比吸收率（SAR）分布以及全身和头部区域（头部和颈部）的功率吸收。本文表明，可以使用两种方法确定谐振频率。第一种方法是使用基于人体解剖结构的精确模型（具有非均匀组织特性），第二种方法是使用由平行六面体截面（用于躯干和腿部）、头部的椭球体和颈部的圆柱体构建的模型。对人体的这种近似是由均匀组织构建的，其电导率和介电常数相当于肌肉的三分之二。使用了一台IBM SP - 2超级计算机以及并行FDTD代码来处理大尺寸问题。我们发现，在孤立和接地条件下，头部和颈部的谐振频率分别为207 MHz和193 MHz，吸收横截面分别是阴影横截面的3.27倍和2.62倍。
