Electromagnetic inverse solutions in anatomically constrained spherical head models

Two classes of functional neuroimaging methods exist: hemodynamic technique s such as PET and fMRI, and electromagnetic techniques such as EEG/ERP and MEG. In order to fusion these images with anatomical information, co-regist ration with volumetric MRT is needed. While such co-registration techniques are well established for hemodynamic images, additional steps are needed f or electromagnetic recordings, because the activity is only recorded on the scalp surface and inverse solutions based on specific head models have to be used to estimate the 3-dimensional current distribution. To date most of the experimental and clinical studies use multi-shell concentric sphere mo dels of the head, solve the inverse problem on this simplistic model, and t hen co-register the solution with the MRT using homogeneous transform opera tions. Contrary to this standard method, we here propose to map the MRI to the spherical system by defining transformation operations that transform t he MRI to a best-fitting sphere. Once done so, the solution points are defi ned in the cerebral tissue of this deformed MRI and the lead field for the distributed linear inverse solutions is calculated for this solution space. The method, that we call SMAC (Spherical Model with Anatomical Constrains) is tested with simulations, as well as with the following real data: 1) es timation of the sources of visual evoked potentials to unilateral stimulati on from data averaged over subjects, and 2) localization of interictal disc harges of two epileptic patients, one with a temporal, the other with an oc cipital focus, both confirmed by seizure freedom after resection of the epi leptogenic region.