In-vivo measurements of human brain tissue conductivity using focal electrical current injection through intracerebral multicontact electrodes

Abstract

In-vivo measurements of human brain tissue conductivity at body temperature were conducted using focal electrical currents injected through intracerebral multicontact electrodes. A total of 1,421 measurements in 15 epileptic patients (age: 28 ± 10) using a radiofrequency generator (50 kHz current injection) were analyzed. Each contact pair was classified as being from healthy (gray matter, n = 696; white matter, n = 530) or pathological (epileptogenic zone, n = 195) tissue using neuroimaging analysis of the local tissue environment and intracerebral EEG recordings. Brain tissue conductivities were obtained using numerical simulations based on conductivity estimates that accounted for the current flow in the local brain volume around the contact pairs (a cube with a side length of 13 mm). Conductivity values were 0.26 S/m for gray matter and 0.17 S/m for white matter. Healthy gray and white matter had statistically different median impedances (P < 0.0001). White matter conductivity was found to be homogeneous as normality tests did not find evidence of multiple subgroups. Gray matter had lower conductivity in healthy tissue than in the epileptogenic zone (0.26 vs. 0.29 S/m; P = 0.012), even when the epileptogenic zone was not visible in the magnetic resonance image (MRI) (P = 0.005). The present in-vivo conductivity values could serve to create more accurate volume conduction models and could help to refine the identification of relevant intracerebral contacts, especially when located within the epileptogenic zone of an MRI-invisible lesion. Hum Brain Mapp 38:974-986, 2017. © 2016 Wiley Periodicals, Inc.

Measurements and models of electric fields in the in vivo human brain during transcranial electric stimulation

“…To date the data for the human head measured in vivo under direct current (0 Hz) are rather scarce and date back over sixty years (Burger and Milaan, 1943; Freygang and Landau, 1955). Just very recently new in vivo measures became available (Koessler et al, 2016) largely confirming the literature values for brain tissue, although these were measured at 50 kHz. Whether these literature values provide correct estimates of electric field magnitude in computational models of the human head has never been directly evaluated in vivo .…”

“…Bangera et al (2010) performed in vivo intracranial recordings from human, but the data were used to better predict EEG surface recordings, not for calibrating models of electric field with transcranial current stimulation. The most recent studies (Opitz et al, 2016; Koessler et al, 2016) involve similar in vivo intracranial recordings on human subjects and monkeys, but they do not provide a comparison and validation of current-flow models.…”

“…Additionally, there have been debates as to how exactly the diffusion anisotropy (as measured with DTI) should be converted into anisotropy of the electrical conductivity (Shahid et al, 2013). In fact, the most recent in vivo recordings in humans suggest that white-matter electrical conductivity may actually be isotropic (Koessler et al, 2016), at least when measured at 50 kHz. Here we evaluated the various approaches proposed in the literature, and found no obvious performance difference between them.…”

Limitations of ex vivo measurements for in vivo neuroscience

“…However, cadavers are still regularly used to examine possible methodological artifacts in brain imaging studies because of the complete absence of neuronal activity with largely preserved anatomical structures (3,4). Numerous studies have relied on cadavers to establish the conductivity of differing tissue types, with notable discrepancies compared with in vivo measurements (5)(6)(7)(8). Similarly, in developing noninvasive transcranial electric stimulation (TES) procedures, some have suggested the use of cadavers for measuring electric fields generated in the brain during stimulation.…”

Quantitative sodium imaging and gliomas: a feasibility study

“…The long scan time is still a major obstacle for regular clinical use, and also facilitates the appearance of motions artifacts, degrading image quality and limiting coregistration accuracy. Another limitation regards the necessity of the model to assume that water fraction is constant, although differences between gray and white matter and between different subregions of the brain have been demonstrated[27–29]. This permits only the determination of apparent total and intracellular sodium concentrations and pseudo-intracellular sodium concentration.…”