Variable Anisotropic Brain Electrical Conductivities in Epileptogenic Foci

Source localization models assume brain electrical conductivities are isotropic at about 0.33 S/m. These assumptions have not been confirmed ex vivo in humans. This study determined bidirectional electrical conductivities from pediatric epilepsy surgery patients. Electrical conductivities perpendicular and parallel to the pial surface of neocortex and subcortical white matter (n = 15) were measured using the 4-electrode technique and compared with clinical variables. Mean (±SD) electrical conductivities were 0.10 ± 0.01 S/m, and varied by 243% from patient to patient. Perpendicular and parallel conductivities differed by 45%, and the larger values were perpendicular to the pial surface in 47% and parallel in 40% of patients. A perpendicular principal axis was associated with normal, while isotropy and parallel principal axes were linked with epileptogenic lesions by MRI. Electrical conductivities were decreased in patients with cortical dysplasia compared with non-dysplasia etiologies. The electrical conductivity values of freshly excised human brain tissues were approximately 30% of assumed values, varied by over 200% from patient to patient, and had erratic anisotropic and isotropic shapes if the MRI showed a lesion. Understanding brain electrical conductivity and ways to non-invasively measure them are probably necessary to enhance the ability to localize EEG sources from epilepsy surgery patients.

[1]  D.R. Jackson,et al.  Effect of conductivity uncertainties and modeling errors on EEG source localization using a 2-D model , 1998, IEEE Transactions on Biomedical Engineering.

[2]  Bart Vanrumste,et al.  Dipole estimation errors due to differences in modeling anisotropic conductivities in realistic head models for EEG source analysis , 2008, Physics in medicine and biology.

[3]  C Gabriel,et al.  The dielectric properties of biological tissues: I. Literature survey. , 1996, Physics in medicine and biology.

[4]  S Ueno,et al.  Magnetic resonance imaging of mean values and anisotropy of electrical conductivity in the human brain. , 2004, Neurology & clinical neurophysiology : NCN.

[5]  Sylvain Baillet,et al.  Influence of skull anisotropy for the forward and inverse problem in EEG: Simulation studies using FEM on realistic head models , 1998, Human brain mapping.

[6]  M. Fuchs,et al.  An improved boundary element method for realistic volume-conductor modeling , 1998, IEEE Transactions on Biomedical Engineering.

[7]  C. E. Acar,et al.  Sensitivity of EEG and MEG measurements to tissue conductivity , 2004, Physics in medicine and biology.

[8]  Georg Neubauer,et al.  Dielectric properties of porcine brain tissue in the transition from life to death at frequencies from 800 to 1900 MHz , 2003, Bioelectromagnetics.

[9]  Simon K. Warfield,et al.  EEG source analysis of epileptiform activity using a 1 mm anisotropic hexahedra finite element head model , 2009, NeuroImage.

[10]  A. Anwander,et al.  The influence of volume conduction effects on the EEG/MEG reconstruction of the sources of the Early Left Anterior Negativity , 2004, The 26th Annual International Conference of the IEEE Engineering in Medicine and Biology Society.

[11]  Carlos H. Muravchik,et al.  Estimating brain conductivities and dipole source signals with EEG arrays , 2004, IEEE Transactions on Biomedical Engineering.

[12]  M. Hämäläinen,et al.  Realistic conductivity geometry model of the human head for interpretation of neuromagnetic data , 1989, IEEE Transactions on Biomedical Engineering.

[13]  M Wagner,et al.  Improving source reconstructions by combining bioelectric and biomagnetic data. , 1998, Electroencephalography and clinical neurophysiology.

[14]  C. Gabriel.  Dielectric properties of biological tissue: Variation with age , 2005, Bioelectromagnetics.

[15]  N. Salamon,et al.  Electrical Conductivities of the Freshly Excised Cerebral Cortex in Epilepsy Surgery Patients; Correlation with Pathology, Seizure Duration, and Diffusion Tensor Imaging , 2006, Brain Topography.

[16]  W. Drongelen,et al.  Estimation of in vivo human brain-to-skull conductivity ratio from simultaneous extra- and intra-cranial electrical potential recordings , 2005, Clinical Neurophysiology.

[17]  Y. Okada,et al.  Experimental analysis of distortion of magnetoencephalography signals by the skull , 1999, Clinical Neurophysiology.

[18]  Leonid Zhukov,et al.  Influence of head tissue conductivity in forward and inverse magnetoencephalographic Simulations using realistic head models , 2004, IEEE Transactions on Biomedical Engineering.

[19]  Paolo Inchingolo,et al.  Improving Lesion Conductivity Estimate by Means of EEG Source Localization Sensitivity to Model Parameter , 2002, Journal of clinical neurophysiology : official publication of the American Electroencephalographic Society.

[20]  J S Ebersole,et al.  Non-invasive pre-surgical evaluation with EEG/MEG source analysis. , 1997, Electroencephalography and clinical neurophysiology. Supplement.

[21]  Sergey M. Plis,et al.  Bayesian brain source imaging based on combined MEG/EEG and fMRI using MCMC , 2008, NeuroImage.

[22]  J. Gotman,et al.  Modeling of post-surgical brain and skull defects in the EEG inverse problem with the boundary element method , 2002, Clinical Neurophysiology.

[23]  W. Sutherling,et al.  Conductivities of Three-Layer Live Human Skull , 2004, Brain Topography.

[24]  P. Wen,et al.  Comparison study of different head model structures with homogeneous/inhomogeneous conductivity , 2009, Australasian Physics & Engineering Sciences in Medicine.

[25]  Paolo Inchingolo,et al.  Lesion type misidentification: EEG potential sampling and source reconstruction errors. , 2002, Biomedical sciences instrumentation.

[26]  David R. Wozny,et al.  The electrical conductivity of human cerebrospinal fluid at body temperature , 1997, IEEE Transactions on Biomedical Engineering.

[27]  H P Schwan,et al.  ELECTRODE POLARIZATION IMPEDANCE AND MEASUREMENTS IN BIOLOGICAL MATERIALS * , 1968, Annals of the New York Academy of Sciences.

[28]  Georg Neubauer,et al.  Dielectric properties of human brain tissue measured less than 10 h postmortem at frequencies from 800 to 2450 MHz , 2003, Bioelectromagnetics.

[29]  E. R. Flynn,et al.  A Model for Frequency Dependence of Conductivities of the Live Human Skull , 2004, Brain Topography.

[30]  L. Geddes,et al.  The specific resistance of biological material—A compendium of data for the biomedical engineer and physiologist , 1967, Medical and biological engineering.

[31]  Itzhak Fried,et al.  Assessment and surgical outcomes for mild type I and severe type II cortical dysplasia: A critical review and the UCLA experience , 2009, Epilepsia.

[32]  R Sankar,et al.  FDG-PET/MRI coregistration improves detection of cortical dysplasia in patients with epilepsy , 2008, Neurology.

[33]  P. Basser,et al.  Microstructural and physiological features of tissues elucidated by quantitative-diffusion-tensor MRI. , 1996, Journal of magnetic resonance. Series B.

[34]  A. Palmini,et al.  Terminology and classification of the cortical dysplasias , 2004, Neurology.

[35]  Hsiao-Fang Liang,et al.  Noninvasive detection of cuprizone induced axonal damage and demyelination in the mouse corpus callosum , 2006, Magnetic resonance in medicine.

[36]  S. K. Law,et al.  Thickness and resistivity variations over the upper surface of the human skull , 2005, Brain Topography.

[37]  J P Kaipio,et al.  Effects of local skull inhomogeneities on EEG source estimation. , 1999, Medical engineering & physics.

[38]  H. Buchner,et al.  The influence of skull-conductivity misspecification on inverse source localization in realistically shaped finite element head models , 2005, Brain Topography.

[39]  W. van Drongelen,et al.  Estimation of in vivo brain-to-skull conductivity ratio in humans. , 2006, Applied physics letters.

[40]  C. Beaulieu,et al.  The basis of anisotropic water diffusion in the nervous system – a technical review , 2002, NMR in biomedicine.

[41]  Elysa Widjaja,et al.  Advances in neuroimaging in patients with epilepsy. , 2008, Neurosurgical focus.

[42]  B He,et al.  Estimation of in vivo Human Brain-to-Skull Conductivity Ratio by means of Cortical Potential Imaging , 2005 .

[43]  J. Haueisen,et al.  Influence of tissue resistivities on neuromagnetic fields and electric potentials studied with a finite element model of the head , 1997, IEEE Transactions on Biomedical Engineering.

[44]  J. Haueisen,et al.  The Influence of Brain Tissue Anisotropy on Human EEG and MEG , 2002, NeuroImage.