Effects of Cavities in the Human Skull on the Inverse Moving Dipole Solutions

The effects of empty cavities in the human skull on the inverse estimation of dipolar sources in the brain have been investigated by computer simulation, in which an eccentric spherical model has been used. The head is simplified as a homogeneous spherical conductor with an eccentric spherical cavity in it. 17 electrodes are arranged on the upper hemisphere in a similar fashion to the international 10-20 electrode system, where the X-axis crosses the Inion-Nasion axis at about 20°. The scalp potential distribution generated by a current dipole is calculated by using the eccentric sphere model mentioned above, the inverse dipole solution is determinated with a homogeneous sphere model, and a deviation of the estimated dipole from the original one has been investigated; a 162-electrode configuration was also tried for the sake of comparison. The present study shows that the empty cavity in the skull is one of the causes which affect the accuracy of the dipole localization. Indeed, the electric current path coming from the dipole will be disturbed by the cavities, and accordingly the potential distribution on the scalp generated by the dipole source will be affected. Then the inverse dipole solution estimated from the scalp potential distribution is also affected. When the original dipole is located at the thalamus which is near the cavity in the present head model under the electrode configuration proposed in the present study, the estimated location error will be less than 5 mm. The present electrode configuration is suggested for the cavity-free head models. On the other hand, the cavities in the skull should be taken into consideration for estimating dipole sources in the brain stem, if high precision is required.