Influence of Head Tissue Conductivity in Forward and Inverse Magnetoencephalographic Simulations Using Realistic Head Models

Abstract

The influence of head tissue conductivity on magnetoencephalography (MEG) was investigated by comparing the normal component of the magnetic field calculated at 61 detectors and the localization accuracy of realistic head finite element method (FEM) models using dipolar sources and containing altered scalp, skull, cerebrospinal fluid, gray, and white matter conductivities to the results obtained using a FEM realistic head model with the same dipolar sources but containing published baseline conductivity values. In the models containing altered conductivity values, the tissue conductivity values were varied, one at a time, between 10% and 200% of their baseline values, and then varied simultaneously. Although changes in conductivity values for a single tissue layer often altered the calculated magnetic field and source localization accuracy only slightly, varying multiple conductivity layers simultaneously caused significant discrepancies in calculated results. The conductivity of scalp, and to a lesser extent that of white and gray matter, appears especially influential in determining the magnetic field. Comparing the results obtained from models containing the baseline conductivity values to the results obtained using other published conductivity values suggests that inaccuracies can occur depending upon which tissue conductivity values are employed. We show the importance of accurate head tissue conductivities for MEG source localization in human brain, especially for deep dipole sources or when an accuracy greater than 1.4 cm is needed.

Electrical Conductivities of the Freshly Excised Cerebral Cortex in Epilepsy Surgery Patients; Correlation with Pathology, Seizure Duration, and Diffusion Tensor Imaging

“…Also, the conductivity values of the bulk brain varied by about three fold in this limited patient population. The conductivity values were in the lower range and sometimes below the lowest values reported by other investigators (Schmid et al 2003a,b;Latikka et al 2001;Gabriel et al 1996;Foster and Schwann 1989;Geddes and Baker 1967 population, suggests that inclusion of accurate conductivity values for individual patients, and not nominal or average values, are necessary when source localization accuracy of better than a few cm is needed in EEG, MEG, and combined MEG/EEG analysis (Gutierrez et al 2004;Van Uitert et al 2004;Marin et al 1998;Ollikainen et al 1999;Pohlmeier et al 1997;Law 1993). Based on present studies, an average value of 0.092 S/m for brain conductivity with a standard deviation of 0.022 may be more realistic in computation of the forward solution.…”

“…Accurate knowledge of these tissue conductivities (termed σ , inverse of resistivities ρ) is necessary to reduce errors in calculating the forward solution of the extracranial magnetic fields and electric potentials produced by intracranial sources of activity. The misspecification of these tissue conductivities can affect the apparent magnitude (strength) of magnetic fields and electric surface potentials (Okada et al 1999;Haueisen et al 1997Haueisen et al , 1999Haueisen et al , 2002Awada et al 1998), and lead to source mislocalization (Gencer and Acar 2004;Van Uitert et al 2004;Marin et al 1998;Ollikainen et al 1999;Pohlmeier et al 1997). Although specification of the conductivity ratio of adjacent compartments may be sufficient in cases of multiple spheres and boundary element method (BEM) models, absolute knowledge of these values is needed for finite element and combination MEG/EEG models (Pohlmeier et al 1997;Haueisen et al 1997;Law 1993).…”

Influence of anisotropic electrical conductivity in white matter tissue on the EEG/MEG forward and inverse solution. A high-resolution whole head simulation study

“…Further studies (Awada et al, 1998;Gençer and Acar, 2004;Ramon et al, 2006;Uitert et al, 2004) investigated the effect of uncertainties of conductivity values on the EEG solution. If the conductivities of the brain and head tissues and the distribution of these tissues throughout the head were modeled accurately, localization was accurate to within a few millimeters.…”

Dynamic Imaging of Coherent Sources Reveals Different Network Connectivity Underlying the Generation and Perpetuation of Epileptic Seizures

“…The standard conductivity values were used for all the five layers and they were 0.33 S/m for scalp, CSF and gray matter. The conductivity value for skull was 0.0042 S/m and 0.31 S/m for the white matter [28]. The models used for the forward computation are multilayer anisotropic spheres in which the innermost shell is considered to be anisotropic.…”