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Bioimpedance spectroscopy and dielectric properties of biological tissues

Aim: For accurate forward and inverse modelling in EEG and MEG head tissues dielectric properties are of utmost importance. The purpose of this presentation is: (i) to give an overview of impedance spectroscopy, a powerful method for characterizing dielectric properties and (ii) to present a survey of dielectric properties of various tissues. Methods: Biompedance spectroscopy, which is measurement of dielectric properties (electric conductivity and permittivity) as a function of frequency, requires special attention and careful calibration of measurement equipment. To interface the instrument (LCR bridge, impedance analyzer, network analyzer) to biological sample, various configurations of electrodes, impedance cells and open-end coaxial probes are used. When measuring dielectric properties at low frequencies (up to a few kHz) electrode polarization is the main problem since electric double layers that form at the electrode-electrolyte interface cause large capacitance which has to be corrected for. Electrochemical processes at electrodes are very complicated and depend on electrode material, state of electrode surface, current density, etc. On the other hand, biological tissue, as a measurement object, is very complex in terms of conduction/polarization mechanisms and is generally inhomogeneous (properties vary with space coordinates), anisotropic (properties are different for different directions of propagations), dispersive (properties depend on the frequency of the field) and nonlinear (properties depend on the intensity of the field). At low frequencies biological tissue is predominantly an electrolytic conductor since there are always free ions to migrate. At the same time, biological tissue, due to its complex structure, exhibits in the presence of electric field characteristics of dielectric materials such as polarization. Besides, tissue electrical properties are temperature dependent. Moreover, pathological processes (e.g. tumours) can alter tissue electrical properties. Additionally, as measurements cannot always be performed in vivo post-mortem tissue changes should also be considered. Results: Based on our experimental studies and literature data we illustrate all the above mentioned tissue characteristics (inhomogenity, anisotropy, dispersions, nonlinearity, post mortem changes etc.) and factors affecting accuracy of conductivity and permittivity measurements. We also present results from literature survey on various head tissue conductivities. Apart from results obtained by bioimpedance spectroscopy, which is the main focus of this presentation, we also cover recent magnetic resonance electrical impedance tomography (MR EIT) studies for imaging head tissues conductivity distribution. Conclusion: Results of this presentation may carry important information for EEG/MEG source imaging since accurate solution of EEG/MEG inverse problem depends on accurate values of head tissues conductivities.

dielectric properties; biological tissue; conductivity; permittivity; EEG/MEG inverse problem

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