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Variable anisotropic brain electrical conductivities in epileptogenic foci.

Akhtari M, Mandelkern M, Bui D, Salamon N, Vinters HV, Mathern GW - Brain Topogr (2010)

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Neuropsychiatric Institutes, David Geffen School of Medicine, University of California, Los Angeles, CA 90015, USA. Akhtarim@ucla.edu

ABSTRACT
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.

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Schematic diagram of the conductivity measuring instrumentation. a A function generator (FG) is used to modulate the current output of a stimulus isolator (SI) through the sample. The potential drops and phase changes are measured through the lock-in amplifiers. The resistor R4 is used to monitor the current and the phase change due to instrumentation. All instrumentation is automated through LabView software and controlled through the laptop computer, which also records and analyzes all data. b Photograph of the instrumentation in the operating room
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Fig2: Schematic diagram of the conductivity measuring instrumentation. a A function generator (FG) is used to modulate the current output of a stimulus isolator (SI) through the sample. The potential drops and phase changes are measured through the lock-in amplifiers. The resistor R4 is used to monitor the current and the phase change due to instrumentation. All instrumentation is automated through LabView software and controlled through the laptop computer, which also records and analyzes all data. b Photograph of the instrumentation in the operating room

Mentions: A function generator (Stanford Research Systems, SR360) was used to modulate the current output of a linear stimulus isolator (World Precision Instruments, A350) at several frequency settings (Fig. 2). A resistor (505 Ω) was placed in series with the brain tissue samples. The potential drop across this resistor was measured with a lock-in-amplifier (Stanford Research Systems, SR830 DSP Lock-in Amplifier) to obtain the applied current passing through the sample and the phase of the potential (outside the sample) with respect to the modulation signal of the function generator. The potential drop across lead wires and the phase between this potential and applied current were measured with a second identical lock-in amplifier. LabView software (LabView, National Instruments) was written to control the function generator and the lock-in amplifiers through GPIB card (PCMCIA-GPIB, National Instruments). This software was also used to acquire data, which were stored in a laptop computer (Gateway Solo 9300, Gateway Inc.). The absolute phase change across the sample was obtained by subtracting the phase angles measured with the first and second lock-in amplifiers, respectively. The frequency of the applied current was varied from 6 to 96 Hz in steps of 10 Hz and from 105 to 1005 Hz in steps of 100 Hz. Each set of data was obtained at current amplitude of 60 μA. The overall errors in potential and phase measurements were determined by replacing the sample with a known resistor (R = 988 Ω) and a R//C circuit respectively (Akhtari et al. 2003).Fig. 2


Variable anisotropic brain electrical conductivities in epileptogenic foci.

Akhtari M, Mandelkern M, Bui D, Salamon N, Vinters HV, Mathern GW - Brain Topogr (2010)

Schematic diagram of the conductivity measuring instrumentation. a A function generator (FG) is used to modulate the current output of a stimulus isolator (SI) through the sample. The potential drops and phase changes are measured through the lock-in amplifiers. The resistor R4 is used to monitor the current and the phase change due to instrumentation. All instrumentation is automated through LabView software and controlled through the laptop computer, which also records and analyzes all data. b Photograph of the instrumentation in the operating room
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2914871&req=5

Fig2: Schematic diagram of the conductivity measuring instrumentation. a A function generator (FG) is used to modulate the current output of a stimulus isolator (SI) through the sample. The potential drops and phase changes are measured through the lock-in amplifiers. The resistor R4 is used to monitor the current and the phase change due to instrumentation. All instrumentation is automated through LabView software and controlled through the laptop computer, which also records and analyzes all data. b Photograph of the instrumentation in the operating room
Mentions: A function generator (Stanford Research Systems, SR360) was used to modulate the current output of a linear stimulus isolator (World Precision Instruments, A350) at several frequency settings (Fig. 2). A resistor (505 Ω) was placed in series with the brain tissue samples. The potential drop across this resistor was measured with a lock-in-amplifier (Stanford Research Systems, SR830 DSP Lock-in Amplifier) to obtain the applied current passing through the sample and the phase of the potential (outside the sample) with respect to the modulation signal of the function generator. The potential drop across lead wires and the phase between this potential and applied current were measured with a second identical lock-in amplifier. LabView software (LabView, National Instruments) was written to control the function generator and the lock-in amplifiers through GPIB card (PCMCIA-GPIB, National Instruments). This software was also used to acquire data, which were stored in a laptop computer (Gateway Solo 9300, Gateway Inc.). The absolute phase change across the sample was obtained by subtracting the phase angles measured with the first and second lock-in amplifiers, respectively. The frequency of the applied current was varied from 6 to 96 Hz in steps of 10 Hz and from 105 to 1005 Hz in steps of 100 Hz. Each set of data was obtained at current amplitude of 60 μA. The overall errors in potential and phase measurements were determined by replacing the sample with a known resistor (R = 988 Ω) and a R//C circuit respectively (Akhtari et al. 2003).Fig. 2

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Neuropsychiatric Institutes, David Geffen School of Medicine, University of California, Los Angeles, CA 90015, USA. Akhtarim@ucla.edu

ABSTRACT
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.

Show MeSH
Related in: MedlinePlus