Dependence of the MEG on dipole orientation in the rabbit head

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.