Hallez, Hans

Lemahieu, Ignace

Staelens, Steven

Van Hese, Peter

Vanrumste, Bart

English

Ghent University Academic Bibliography

Influence of anisotropic conductivities in EEG source estimation in patients with epilepsyHans Hallez1, Bart Vanrumste2, Peter Van Hese1, Steven Staelens1, Ignace Lemahieu11:MEDISIP – ELIS - IBBT – IBITECH, Ghent University, Ghent, Belgium2: BIOMED – SISTA – ESAT, K.U. Leuven, Leuven, BelgiumIntroduction: Epilepsy• Epilepsy– Neurological disorder– Seizure: abnormal synchronous brain activity– Prevalence: 0.5 – 1 %• Epileptic onset zone in partial epilepsy– One or multiple region(s) in the brainresponsible for the seizures17/03/2009 2Hans Hallez - Neuromath Workshop Leuven 2009Introduction: Epilepsy treatmentEpilepsyRefractory epilepsynervus vagus stimulationSurgeryDeep brain stimulation~ 75% ~ 25%30-40%~ 1%60-70%MedicationGoal of presurgical evaluationdetermining the epileptic onset zone17/03/2009 3Hans Hallez - Neuromath Workshop Leuven 2009Content• Introduction• Source localization– Forward problem– Inverse problem• Incorporating anisotropic conductivities• Influence of anisotropic conductivities• Future work17/03/2009 4Hans Hallez - Neuromath Workshop Leuven 2009Source Localization17/03/2009 5Hans Hallez - Neuromath Workshop Leuven 2009Source Localization: source model• Dipole model• Cortical patch of 5 by 5 mm generates a measurable potential• Currents have to be aligned orthogonally to the cortex: pyramidal neuronsActionpotential- Current sinkCurrent source+Location: r (x,y,z)Moment: dMicroscopic current flow (~5×10-5 nAm)17/03/2009 6Hans Hallez - Neuromath Workshop Leuven 2009Source Localization17/03/2009 7Hans Hallez - Neuromath Workshop Leuven 2009Source Localization: Head model• Spherical head models– Simple, but unrealistic• Realistic head models– Medical imaging– Requires segmentationScalpSkullCSFWhite matterGrey matter17/03/2009 8Hans Hallez - Neuromath Workshop Leuven 2009Source Localization17/03/2009 9Hans Hallez - Neuromath Workshop Leuven 2009Source Localization: Forward problem• Solving Poisson’s equation in head model due to a dipolesource• Spherical head models– Analytical solution• De Munck, Zhang• Realistic head models– Numerical methods• BEM, FEM, FDM( ( , , ) ( , , )) ( , , )x y z V x y z x y z  J17/03/2009 10Hans Hallez - Neuromath Workshop Leuven 2009Source Localization17/03/2009 11Hans Hallez - Neuromath Workshop Leuven 2009Source Localization: Inverse problem• Fits the dipole parameters to a measured set of potentials• Minimization of the Relative Residual energy:• Nelder-mead simplex method( , )electrodes modelelectrodesRREV V r dVexcitatorysynapseapicaldendritesoma axon-I+I17/03/2009 12Hans Hallez - Neuromath Workshop Leuven 2009Content• Introduction• Source localization• Incorporating anisotropic conductivities• Influence of anisotropic conductivities• Future work17/03/2009 13Hans Hallez - Neuromath Workshop Leuven 2009Anisotropic conductivities: skull• Layered structuretangentialnormal101Hard boneSoft bone17/03/2009 14Hans Hallez - Neuromath Workshop Leuven 2009[Akhtari M. et al., Brain Topography, 2002; Marin G. et al, Human Brain Mapping 1998]Anisotropic conductivities: skull• Derive normal vector of a skull segment using medicalimaging• Cartesian tensors andtransformations17/03/2009 15Hans Hallez - Neuromath Workshop Leuven 2009tangentialnormal1010 00 00 0nTttR R     ΣR = matrix indicating the rotation from local to global reference frameAnisotropic conductivities: white matter• Fiber structurelongitudinaltransversal91intracellularspaceintercellularspace17/03/2009 16Hans Hallez - Neuromath Workshop Leuven 2009[Nicholson, P.W., Experimental Nuerology, 1965]Anisotropic conductivities: white matter• Diffusion Tensor Imaging– The fiber direction in white matter– Conductivities can be derivedλ1λ2λ3 Fractional Anisotropy» mainly in white matter» Also in grey matter17/03/2009 17Hans Hallez - Neuromath Workshop Leuven 2009Anisotropic conducitivities• From the diffusion to conductivity17/03/2009 18Hans Hallez - Neuromath Workshop Leuven 2009Isotropic conductivitiesfrom literature0.330.02116isotropicbrainisotropicisotropic brainskullSmSm  Geddes L.A. and Baker, Med.Biol.Eng., 1967Anisotropic conducitivities: forward problem• Incorporating anisotropic conductivities in the forwardproblem– Finite Difference Method– For every center node:– System of equations– Head model (1mm resolution): approx. 4500000 elements => approx. 4500000 unknowns in the system[Saleheen et al., 1998 & Hallez et al., 2005]IAAiiiii  0181181IA 3D potential distribution withwith anisotropic conductivities17/03/2009 19Hans Hallez - Neuromath Workshop Leuven 2009Content• Introduction• Source localization• Incorporating anisotropic conductivities• Influence of anisotropic conductivities– Estimation errors due to different anisotropic conductivity models of white matter (method B vs method A)– Estimation errors due to neglecting anisotropic conductivities– Estimation errors due to neglecting anisotropic conductivities in the presence of noise• Future work17/03/2009 20Hans Hallez - Neuromath Workshop Leuven 2009Influence of anisotropic conducitivities• Estimation errors due to different anisotropic conductivitymodels of white matter (method B vs method A)ˆ r r ˆ, d dError on dipole locationError on dipole orientation17/03/2009 21Hans Hallez - Neuromath Workshop Leuven 2009Influence of anisotropic conducitivities• Estimation errors due to different anisotropic conductivitymodels (method B vs method A)– Dipole location error17/03/2009 22Hans Hallez - Neuromath Workshop Leuven 2009Influence of anisotropic conducitivities• Estimation errors due to different anisotropic conductivitymodels (method B vs method A)– Dipole orientation error17/03/2009 23Hans Hallez - Neuromath Workshop Leuven 2009Influence of anisotropic conducitivities• Estimation errors due to neglecting anisotropic conductivitiesˆ r r ˆ, d dError on dipole locationError on dipole orientation• Skull, white matter and grey matter• Skull only• White and grey matter17/03/2009 24Hans Hallez - Neuromath Workshop Leuven 2009Influence of anisotropic conductivities• Dipole location errors due to not incorporating anisotropic conductivities– Error is largest when skullanisotropy is neglected– Error is largest at the edgesthan in center– Error due to neglecting whitematter anisotropy is very small17/03/2009 25Hans Hallez - Neuromath Workshop Leuven 2009Influence of anisotropic conductivities• Estimation errors due to neglecting anisotropic conductivitiesin the presence of noise• Skull, white matter and grey matter• Skull only• White and grey matterˆ r r ˆ, d dError on dipole locationError on dipole orientation17/03/2009 26Hans Hallez - Neuromath Workshop Leuven 2009Influence of anisotropic conductivities• Estimation errors due to neglecting anisotropic conductivitiesin the presence of noiseDipole estimation over• 1 time sample• 20 time samples by firstcomponent of SVD17/03/2009 27Hans Hallez - Neuromath Workshop Leuven 2009RMSnoiseRMSdipoleVnoiselevelVInfluence of anisotropic conductivities• Average dipole location error due to noise in anisotropic and isotropic head model– Error due to noise only islarger in anisotropic head-model than in isotropic onenoiselevelDipolelocationerror(mm)17/03/2009 28Hans Hallez - Neuromath Workshop Leuven 2009Influence of anisotropic conductivities• Anisotropic conductivitymakes the problem moreillposed• Potentials are more attenuated and smearedout over the surface• Incorporating anisotropic conductivities will provide a more accurate estimation, but the illposedness will make it more difficultIsotropicAnisotropicPotentials (mV)17/03/2009 29Hans Hallez - Neuromath Workshop Leuven 2009Influence of anisotropic conductivities• Application of an averaged epileptic spike– left hippocampus (manually segmented)– Surgery outcome: seizure freeAnisotropic conducting head modelIsotropic conducting head modelLocalization difference: 9.8 mm17/03/2009 30Hans Hallez - Neuromath Workshop Leuven 2009Content• Introduction• Source localization• Incorporating anisotropic conductivities• Influence of anisotropic conductivities• Future work– Incorporation of functional imaging as a priori knowledge– Multi-level approaches on solving the inverse problem– Brain Connectivity17/03/2009 31Hans Hallez - Neuromath Workshop Leuven 2009Future work• Incorporate functional activation (SPECT or fMRI) boundaries as a priori knowledgeMR imageSPECTSPECT-MREEG source localizationColloboration with Maarten De Vos, prof. Van Huffel (ESAT), prof. Van Paesschen (Neurology, Gasthuisberg)17/03/2009 32Hans Hallez - Neuromath Workshop Leuven 2009Hallez et al., 2009Future work• Multilevel techniques to efficiently solve the inverse problemin a discrete non-convex search spaceCollaboration with prof. Luc Dupréand Guillaume Crevecoeur (EELAB)17/03/2009 33Hans Hallez - Neuromath Workshop Leuven 2009Future work• Multilevel techniques to efficiently solve the inverse problemin a discrete non-convex search spaceCollaboration with prof. Luc Dupréand Guillaume Crevecoeur (EELAB)17/03/2009 34Hans Hallez - Neuromath Workshop Leuven 2009Crevecoeur et al, 2008Brain Connectivity• Use EEG source localization to estimate location and time series of dipoles• Use connectivity to estimate causalrelationship17/03/2009 35Hans Hallez - Neuromath Workshop Leuven 2009Brain Connectivity• Intracranial recordings during seizure17/03/2009 Hans Hallez - Neuromath Workshop Leuven 2009 36The functional connectivity pattern between the brain regions through energy weighted  Adaptive Directed Transfer Function:Between LH contact points:The information flow spreads from LH4 and LH5 symmetrically to the other contact pointsvan Mierlo et al. 2009ReferencesM. Akhtari, H.C. Bryant, A.N. Marmelak, E.R. Flynn, L. Heller, J.J. Shih, M. Mandelkern, A. Matlachov, D.M. Ranken, E.D. Best, M.A. DiMauro, R.R. Lee and W.W. Sutherling, Conductivities of Three-Layer Live Human Skull, Brain Topography, Vol. 14 (3), 2002, pp. 151-167P.W. Nicholson, Specific impedance of cerebral white matter, Exp.Neurol., 13, 1965, pp.386-401L.A. Geddes and L.E. Baker, The specific resistance of biological material. A compendium of data for the biomedical engineer andphysiologist, Med.Biol.Eng., 5, 1967, pp. 271—293Saleheen, H.I.; Ng, K.T., A new three-dimensional finite-difference bidomain formulation for inhomogeneous anisotropic cardiac tissues, IEEE Transactions in Biomedical Engineering, Volume 45, Issue 1, Jan. 1998 Page(s):15 – 25Hallez, H.; Vanrumste, B.; Van Hese, P.; D'Asseler, Y.; Lemahieu, I. & Van de Walle, R. A finite difference method with reciprocity used to incorporate anisotropy in electroencephalogram dipole source localization Physics in Medicine and Biology, 2005, 50, 3787-3806Crevecoeur, G.; Hallez, H.; Van Hese, P.; D'Asseler, Y.; Dupré, L. & Van de Walle, R. A hybrid algorithm for solving the EEG inverse problem from spatio-temporal EEG data Medical and Biological Engineering and Computing, 2008Hallez, H.; De Vos, M.; Vanrumste, B.; Van Hese, P.; Van Laere, K.; Dupont, P.; Van Paesschen, W.; Van Huffel, S. & Lemahieu, I. Removing muscle and eye artifacts using blind source separation techniques in ictal EEG source imaging, Clinical Neurophysiology, 2008van Mierlo P., Hallez H., Assecondi S., Staelens S., Carrette E., Lemahieu I., Boon P., “Feasibility study of the time-variant functional connectivity pattern during an epileptic seizure”, submitted to NFSI Rome 200917/03/2009 Hans Hallez - Neuromath Workshop Leuven 2009 37Acknowledgements• Ghent UniversityMEDISIPProf. dr. Steven StaelensProf. dr. Ignace Lemahieudr. Peter Van HeseSara AssecondiPieter Van MierloEELABProf. dr. Luc DupréGuillaume Crevecoeur• Ghent University HospitalProf. Paul BoonProf. Kristl VonckEvelien Carrette• KU LeuvenESATProf. dr. Sabine Van Huffeldr. Bart VanrumsteMaarten De VosKatrien VanderperrenWouter Deburchgraeve• University Hospital GasthuisbergProf. dr. Wim Van PaesschenProf. dr. Koen Van LaereProf. dr. Patrick Dupont17/03/2009 38Hans Hallez - Neuromath Workshop Leuven 2009Future Work: Event related potentials• Functionality of the brain– How does the brain work when doingcertain tasks? StimulusStimulus• Diagnosis of Dyslexia• Which brain regions are involved?17/03/2009 39Hans Hallez - Neuromath Workshop Leuven 2009

Influence of anisotropic conductivities in EEG source estimation in patients with epilepsy

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Influence of anisotropic conductivities in EEG source estimation in patients with epilepsy

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