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Similarities between simulated spatial spectra of scalp EEG, MEG and structural MRI.

Ramon C, Freeman WJ, Holmes M, Ishimaru A, Haueisen J, Schimpf PH, Rezvanian E - Brain Topogr (2009)

Bottom Line: These peaks are definitely due to the gyri structures and associated larger patterns on the cortical surface.Smaller peaks in the range of 1-3 cycles/cm were also observed which are possibly due to sulci structures.These results suggest that the spatial information was present in the EEG and MEG at the spatial frequencies of gyri.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical Engineering, University of Washington, Seattle, WA 98195, USA. ceon@u.washington.edu

ABSTRACT
Electrical dipoles oriented perpendicular to the cortical surface are the primary source of the scalp EEGs and MEGs. Thus, in particular, gyri and sulci structures on the cortical surface have a definite possibility to influence the EEGs and MEGs. This was examined by comparing the spatial power spectral density (PSD) of the upper portion of the human cortex in MRI slices to that of simulated scalp EEGs and MEGs. The electrical activity was modeled with 2,650 dipolar sources oriented normal to the local cortical surface. The resulting scalp potentials were calculated with a finite element model of the head constructed from 51 segmented sagittal MR images. The PSD was computed after taking the fast Fourier transform of scalp potentials. The PSD of the cortical contour in each slice was also computed. The PSD was then averaged over all the slices. This was done for sagittal and coronal view both. The PSD of EEG and MEG showed two broad peaks, one from 0.05 to 0.22 cycles/cm (wavelength 20-4.545 cm) and the other from 0.22 to 1.2 cycles/cm (wavelength 4.545-0.834 cm). The PSD of the cortex showed a broad peak from 0.08 to 0.32 cycles/cm (wavelength 12.5-3.125 cm) and other two peaks within the range of 0.32 to 0.9 cycles/cm (wavelength 3.125-1.11 cm). These peaks are definitely due to the gyri structures and associated larger patterns on the cortical surface. Smaller peaks in the range of 1-3 cycles/cm were also observed which are possibly due to sulci structures. These results suggest that the spatial information was present in the EEG and MEG at the spatial frequencies of gyri. This also implies that the practical Nyquist frequency for sampling scalp EEGs should be 3.0 cycles/cm and an optimal interelectrode spacing of about 3 mm is needed for extraction of cortical patterns from scalp EEGs in humans.

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A segmented image slice with major tissues identified in it. The cortical contour formed by the boundary of CSF and the gray matter was used for dipole locations to model the electrical activity. The same contour was used for computing the PSD of the cortical contours. Only the top portion of the contour, above 9 cm, was used for modeling and analysis work
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Fig1: A segmented image slice with major tissues identified in it. The cortical contour formed by the boundary of CSF and the gray matter was used for dipole locations to model the electrical activity. The same contour was used for computing the PSD of the cortical contours. Only the top portion of the contour, above 9 cm, was used for modeling and analysis work

Mentions: For computation of scalp EEGs and MEGs, a finite element method (FEM) model of the head was used. Our model building details have been described earlier (Haueisen et al. 2002; Ramon et al. 2004, 2006a, 2006b). For the sake of completeness, a summary is provided here. The T1 weighted sagittal MRI slices of an adult male subject with 3.2 mm thickness were collected with a 1.5 Tesla GE Signa scanner. The original MR slices were of 256 × 256 resolution with 1.0 mm size pixels (Haueisen et al. 2002; Ramon et al. 2006a, 2006b). A total of 51 contiguous slices was used. The MR images were segmented using a semiautomatic tissue classification program developed by us. The identified tissues were: scalp, fat, muscle, hard skull bone, soft skull bone, gray matter, white matter, eyes, spinal cord and cerebellum, cerebrospinal fluid (CSF) and soft tissue. A detailed structure of the eye sockets, sinus and oral cavities, and occipital hole was also included in the segmentation. The segmented images were sub-sampled to a 2 × 2 mm resolution for computational work. One of the segmented slices, marked as slice number 30 is shown in Fig. 1. This is the 30th slice starting from the left side of the subject. It is 1.7 cm to the right from the midline of the brain. The x coordinate increases from anterior (front) to the posterior (back) of the subject, the y coordinate increases from superior (top of the head) to the inferior (bottom) and the z coordinate increases from left to the right side of the subject.Fig. 1


Similarities between simulated spatial spectra of scalp EEG, MEG and structural MRI.

Ramon C, Freeman WJ, Holmes M, Ishimaru A, Haueisen J, Schimpf PH, Rezvanian E - Brain Topogr (2009)

A segmented image slice with major tissues identified in it. The cortical contour formed by the boundary of CSF and the gray matter was used for dipole locations to model the electrical activity. The same contour was used for computing the PSD of the cortical contours. Only the top portion of the contour, above 9 cm, was used for modeling and analysis work
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2749166&req=5

Fig1: A segmented image slice with major tissues identified in it. The cortical contour formed by the boundary of CSF and the gray matter was used for dipole locations to model the electrical activity. The same contour was used for computing the PSD of the cortical contours. Only the top portion of the contour, above 9 cm, was used for modeling and analysis work
Mentions: For computation of scalp EEGs and MEGs, a finite element method (FEM) model of the head was used. Our model building details have been described earlier (Haueisen et al. 2002; Ramon et al. 2004, 2006a, 2006b). For the sake of completeness, a summary is provided here. The T1 weighted sagittal MRI slices of an adult male subject with 3.2 mm thickness were collected with a 1.5 Tesla GE Signa scanner. The original MR slices were of 256 × 256 resolution with 1.0 mm size pixels (Haueisen et al. 2002; Ramon et al. 2006a, 2006b). A total of 51 contiguous slices was used. The MR images were segmented using a semiautomatic tissue classification program developed by us. The identified tissues were: scalp, fat, muscle, hard skull bone, soft skull bone, gray matter, white matter, eyes, spinal cord and cerebellum, cerebrospinal fluid (CSF) and soft tissue. A detailed structure of the eye sockets, sinus and oral cavities, and occipital hole was also included in the segmentation. The segmented images were sub-sampled to a 2 × 2 mm resolution for computational work. One of the segmented slices, marked as slice number 30 is shown in Fig. 1. This is the 30th slice starting from the left side of the subject. It is 1.7 cm to the right from the midline of the brain. The x coordinate increases from anterior (front) to the posterior (back) of the subject, the y coordinate increases from superior (top of the head) to the inferior (bottom) and the z coordinate increases from left to the right side of the subject.Fig. 1

Bottom Line: These peaks are definitely due to the gyri structures and associated larger patterns on the cortical surface.Smaller peaks in the range of 1-3 cycles/cm were also observed which are possibly due to sulci structures.These results suggest that the spatial information was present in the EEG and MEG at the spatial frequencies of gyri.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical Engineering, University of Washington, Seattle, WA 98195, USA. ceon@u.washington.edu

ABSTRACT
Electrical dipoles oriented perpendicular to the cortical surface are the primary source of the scalp EEGs and MEGs. Thus, in particular, gyri and sulci structures on the cortical surface have a definite possibility to influence the EEGs and MEGs. This was examined by comparing the spatial power spectral density (PSD) of the upper portion of the human cortex in MRI slices to that of simulated scalp EEGs and MEGs. The electrical activity was modeled with 2,650 dipolar sources oriented normal to the local cortical surface. The resulting scalp potentials were calculated with a finite element model of the head constructed from 51 segmented sagittal MR images. The PSD was computed after taking the fast Fourier transform of scalp potentials. The PSD of the cortical contour in each slice was also computed. The PSD was then averaged over all the slices. This was done for sagittal and coronal view both. The PSD of EEG and MEG showed two broad peaks, one from 0.05 to 0.22 cycles/cm (wavelength 20-4.545 cm) and the other from 0.22 to 1.2 cycles/cm (wavelength 4.545-0.834 cm). The PSD of the cortex showed a broad peak from 0.08 to 0.32 cycles/cm (wavelength 12.5-3.125 cm) and other two peaks within the range of 0.32 to 0.9 cycles/cm (wavelength 3.125-1.11 cm). These peaks are definitely due to the gyri structures and associated larger patterns on the cortical surface. Smaller peaks in the range of 1-3 cycles/cm were also observed which are possibly due to sulci structures. These results suggest that the spatial information was present in the EEG and MEG at the spatial frequencies of gyri. This also implies that the practical Nyquist frequency for sampling scalp EEGs should be 3.0 cycles/cm and an optimal interelectrode spacing of about 3 mm is needed for extraction of cortical patterns from scalp EEGs in humans.

Show MeSH