The conductivity of brain tissues: comparison of results in vivo and in vitro measurements

Abstract

Abstract-Electric properties of tissues depend on many factors, including measurement frequency and temperature. Properties differ also in vivo and in vitro situations. We have collected conductivity values from several studies and compared the values measured from living tissue and tissue samples. The results show that the resistivity ratio of grey and white matter increases 36% after death, and the resistivity values increase over 100%. Keywords -Conductivity, brain tissue, source location I. INTRODUCTIONThe electric properties of tissues have a very important role in biomedical engineering. These properties determine the electrical current pathways through human body. If these properties are known, electrical models can be constructed, for example, to represent the electrical activation of the heart or the conduction of the brain activity to the scalp surface.With resistive model of the head, the information given by electroencephalography (EEG) can be effectively processed [1], [2], [3]. Models can be applied to the simulation of electric fields inside the head. For example, an electric source (dipole or set of dipoles) can be inserted inside the model and thereafter the electric field distribution can be computed. Further, the measured EEG signal can be used for obtaining the source location in the volume conductor. For example, epileptic loci can be located. In principal, accuracy of these computations is dependent on the accuracy of the volume conductor i.e. the number of compartments and their conductivities [4]. In [5] the results indicated that a 10% decrease in tissue resisitivity cause 3.0 -4.1% differences in the sensitivity distributions of the selected 3 EEG leads. In modeling the important factor is the ratio between various conductivities. The ratio of skull and brain resistivites is 15:1 rather than the commonly used ratio of 80:1 [6]. In [7] the estimated resistivity ratio of skull and brain is 14:1.There have been recent advances in source localization techniques. The amount of electrodes in EEG studies has been increased. Instead of the traditional 21 electrode 10-20 system, 64 or more electrodes are usually used. In some studies even 512 electrodes are utilized. This improves the spatial accuracy, thus giving more information about brain functions. However, most researchers continue to take conductivity parameters from standard references [8], [9]. The standard reference values are usually measured from tissue samples. An increase in tissue resistivity with time after death has been reported in [8], [10]. Literature values are also measured at much higher frequencies than EEG frequencies. II. METHODOLOGYPreviously we have made in vivo resistivity measurements with needle electrode from 9 patients with brain tumors [11]. Due to the location of tumors and selected surgical paths, it was not possible to measure both grey and white matters with every patient. The number of measurements ranged from 1 to 13 for each tissue measured.In addition to our own in vivo measurements [11], resistivity val...

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