Foster K R, Schepps J L, Stoy R D, Schwan H P
Phys Med Biol. 1979 Nov;24(6):1177-87. doi: 10.1088/0031-9155/24/6/008.
Dielectric permittivity and conductivity are reported for grey and white matter from dog brain tissue between 0.01 and 10 GHz. Between 0.01 and approximately 1 GHz, the permittivity decreases and conductivity increases as a power law of frequency. Above 1 GHz, the conductivity increases quadratically with frequency due to dipolar reorientation of free water molecules in tissue; the apparent rotational relaxation frequency at 37 degrees C is 21--25 GHz, slightly below the 25 GHz characteristic frequency of pure water at that temperature. The microwave data are analysed using the Maxwell mixture theory applicable for a suspension of nonconducting, low permittivity spheres in bulk water. From the increase in conductivity above 1 GHz, and the tissue permittivity at 2--4 GHz, the apparent volume fraction of water is approximately 0.70 and 0.55 for grey and white matter, respectively, about 10--15% lower than respective values from the literature. This discrepancy is apparently due to a small fraction of water which does not contribute to the tissue permittivity above 1 GHz. Empirical equations are given to summarise the dielectric properties of 'average' brain tissue at 37 degrees C for future theoretical studies of microwave absorption in the head.
报道了犬脑组织灰质和白质在0.01至10 GHz之间的介电常数和电导率。在0.01至约1 GHz之间，介电常数随频率的幂律下降，电导率随频率的幂律增加。在1 GHz以上，由于组织中自由水分子的偶极重排，电导率随频率呈二次方增加；37℃时的表观旋转弛豫频率为21 - 25 GHz，略低于该温度下纯水的25 GHz特征频率。使用适用于非导电、低介电常数球体在大量水中悬浮液的麦克斯韦混合理论对微波数据进行分析。从1 GHz以上电导率的增加以及2 - 4 GHz时的组织介电常数来看，灰质和白质的表观水体积分数分别约为0.70和0.55，比文献中的相应值低约10 - 15%。这种差异显然是由于一小部分水对1 GHz以上的组织介电常数没有贡献。给出了经验方程，以总结37℃时“平均”脑组织的介电特性，供未来头部微波吸收的理论研究使用。
