Age-­specific template head models for EEG source analysis
Creators: name: Lanfer, Benjamin
name: Spangler, Robert
name: Richards, John E.
name: Paul-Jordanov, Isabella
Publisher: name: F1000Research
Year: 2016

description: <b>Introduction</b><br>Incorporating the realistic anatomy of the head into the solution of the EEG forward problem improves the accuracy and reliability of EEG source analysis (Vorwerk et al., 2014). Individual realistic head models can be derived from the subject's MRI. In infants and children MRI data are often difficult to collect. However, source analysis accuracy would particularly benefit from realistic head models in children, as head tissue structure differs vastly from adults. A solution might be to use age-specific template models representing the child-typical anatomy.<br>In this work, we describe a pipeline for EEG source analysis with age-specific head model templates. In addition, a study is presented demonstrating the feasibility of the template models.<br><br><b>Methods</b><br>The age-specific templates were constructed based on MRIs of subjects in different age groups that were non-linearly aligned and averaged (Sanchez et al., 2012a, 2012b). For the ages from 7.5 months to 4 years a four compartment segmentation existed (Sanchez et al., 2012a). For the age groups from 6 years upwards a tissue segmentation was computed from the averaged T1- and T2-MRIs using a Markov Random Field-based segmentation approach facilitated by a custom tissue atlas (Lanfer, 2014). Geometry-adapted hexahedral meshes (Wolters et al., 2007) at 1mm resolution were constructed from the segmented images. Each element in the mesh was labeled as scalp, skull, CSF, or brain. Electrode positions were coregistered to the scalp surface reconstructed from the averaged T1-MRI. Finally, for the given mesh and electrode configuration the leadfield was pre-computed on a regular 2mm cubic grid covering the whole brain. During source analysis the scalp surface distribution could be interpolated from the pre-computed leadfield table using a second order Bezier interpolation.<br>For our feasibility study, an individual realistic head model – the reference model – was constructed from the T1-MRI of a 12 year old child. In the reference model the EEG was simulated for a set of representative sources at 12 locations and with radial and tangential orientations. Source analysis on the simulated reference data was then performed using an ellipsoidal model and the age-specific templates for the forward computation. Differences between the reconstructed and reference source positions in Talairach space were evaluated.<br>Furthermore, real EEG data from interictal spikes measured in the same subject were analyzed using the individual head model, an ellipsoidal head model, and the age-specific template models. Again, reconstructed Talairach source positions were compared.<br><br><b>Results</b><br>Dipole fitting was performed on the simulated reference data. Using the ellipsoidal model for the forward computation, the source position was reconstructed with a mean error of 13.2mm. Mean localization errors for the age-specific templates ranged between 5.0mm and 20.0mm with the minimum mean error observed for the age-appropriate model (12 years).<br>To estimate the origin of the interictal spike, a regional source was fitted to the interval from -35ms to 0ms relative to the peak of the spike. A second source was placed to account for alpha activity. Source analysis with the individual realistic head model led to a plausible source position close to a lesion visible in the subject's MRI. Computing the forward solution with the age-appropriate head model (12 years) yielded a very similar source reconstruction (Fig. 2).<br><br><b>Conclusion</b><br>Age-specific realistic template head models, as described here, allow us to effortlessly analyze EEG data, and taking into account at least the typical anatomical features of the head even when no individual MRI data are available. Using the age-appropriate template model substantially improves source reconstruction as compared to an ellipsoidal model. In conclusion, we recommend using the realistic template models when accurate source localization is desired but no individual MRI data are available.<br>