Melcher J R, Cohen D
Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge 02139.
Electroencephalogr Clin Neurophysiol. 1988 Nov;70(5):460-72. doi: 10.1016/0013-4694(88)90024-7.
In theory, a radial current dipole in a conducting sphere produces zero magnetic field outside the sphere, while a tangential dipole produces a non-zero field. Because the heads of humans and some animals resemble a conducting sphere, it follows that the magnetic field due to a radial dipole in these heads should be suppressed compared to that due to a tangential dipole. This hypothesis, which is important in the interpretation of the MEG, has never been experimentally tested. We here present a test performed in the rabbit. First, a radial dipole was placed in the rabbit head, and the magnetic field over the head due to this source was measured. Then, a similar but tangential dipole was placed in the head and again the resulting magnetic field was measured. The two magnetic fields were then compared to determine the suppression of the field due to the radial dipole. This suppression was expressed as the ratio R, the magnetic field over the head due to the radial dipole divided by that for the tangential dipole. It was found that R = 0.17 +/- 0.07, or that the magnetic field due to a radial dipole is suppressed by a factor of about 6. Therefore, to first order, the hypothesis is supported for the rabbit head. From a comparison of rabbit and human head geometry, a similar degree of suppression could be expected in the human.
理论上，导电球体内的径向电流偶极子在球体外部产生的磁场为零，而切向偶极子产生的磁场不为零。由于人类和一些动物的头部类似于导电球体，因此与切向偶极子相比，这些头部中径向偶极子产生的磁场应该受到抑制。这一假设在脑磁图（MEG）解释中很重要，但从未经过实验验证。我们在此展示在兔子身上进行的一项测试。首先，在兔子头部放置一个径向偶极子，并测量该源在头部上方产生的磁场。然后，在头部放置一个类似但为切向的偶极子，并再次测量产生的磁场。接着比较这两个磁场，以确定径向偶极子产生的磁场的抑制情况。这种抑制用比率R表示，即径向偶极子在头部上方产生的磁场除以切向偶极子产生的磁场。结果发现R = 0.17±0.07，也就是说径向偶极子产生的磁场被抑制了约6倍。因此，初步来看，该假设在兔子头部得到了支持。通过比较兔子和人类头部的几何形状，预计在人类中也会有类似程度的抑制。
