Numerical estimation of skull and cortex boundaries from scalp geometry

One of the most sophisticated means of dipole localization is based on a multi-compartment head model in which the inhomogeneity of the electric conductivity of the head is approximated by compartments of uniform conductors. Compartments usually refer to scalp, skull, cerebrospinal fluid (CSF) and brain cortex. The CSF layer in normal subjects is thin and can be neglected in dipole localization to good approximation. The authors aim at constructing the so-called SSB head model in which the CSF layer is neglected. To construct this model 3D MR images or 3D X-ray CT images are necessary. These images, however, are not easy to get, and the authors tried to estimate geometrical shapes of these compartments from a head geometry which is easily measured with a 3D digitizer. The authors have examined the statistical properties of the head structure through 3D MR images of 12 normal subjects, and the estimation error in their method is 1.5 mm for the skull and 2.2 mm for the cortex.