Katrina Wendel

EEG/MEG source imaging: methods, challenges, and open issues

We present the four key areas of research—preprocessing, the volume conductor, the forward problem, and the inverse problem—that affect the performance of EEG and MEG source imaging. In each key area we identify prominent approaches and methodologies that have open issues warranting further investigation within the community, challenges associated with certain techniques, and algorithms necessitating clarification of their implications. More than providing definitive answers we aim to identify important open issues in the quest of source localization.

The Influence of CSF on EEG Sensitivity Distributions of Multilayered Head Models

We examined how the cerebrospinal fluid (CSF) affects the distribution of electroencephalogram (EEG) measurement sensitivity. We used concentric spheres and realistic head models to investigate the difference between computed-tomography (CT) and magnetic resonance image (MRI) models that exclude the CSF layer. The cortical EEG sensitivity distributions support these phenomena and show that the CSF layer significantly influences them, thus identifying the importance of including the CSF layer inside the head volume conductor models. The results show that the highly conductive CSF channels the current, thus decreasing the maximum cortical current density relative to models that do not include the CSF. We found that the MRI and CT models yielded HSV results 20% and 45%, respectively, too small when compared with CSF-inclusive models.

The influence of age and skull conductivity on surface and subdermal bipolar EEG leads

Bioelectric source measurements are influenced by the measurement location as well as the conductive properties of the tissues. Volume conductor effects such as the poorly conducting bones or the moderately conducting skin are known to affect the measurement precision and accuracy of the surface electroencephalography (EEG) measurements. This paper investigates the influence of age via skull conductivity upon surface and subdermal bipolar EEG measurement sensitivity conducted on two realistic head models from the Visible Human Project. Subdermal electrodes (a.k.a. subcutaneous electrodes) are implanted on the skull beneath the skin, fat, and muscles. We studied the effect of age upon these two electrode types according to the scalp-to-skull conductivity ratios of 5, 8, 15, and 30 : 1. The effects on the measurement sensitivity were studied by means of the half-sensitivity volume (HSV) and the region of interest sensitivity ratio (ROISR). The results indicate that the subdermal implantation notably enhances the precision and accuracy of EEG measurements by a factor of eight compared to the scalp surface measurements. In summary, the evidence indicates that both surface and subdermal EEG measurements benefit better recordings in terms of precision and accuracy on younger patients.

Correlation between Live and Post Mortem Skull Conductivity Measurements

The skull is a tissue with a widely controversial range of conductivity values. This article correlates live skull conductivity measurements with post mortem conductivity measurements with a scaling factor ranging between 2.5 and 4. The scaling factor is validated by a mathematical model that determines the skull conductivity using saline and cerebrospinal fluid (CSF) conductivities and correlated with published physical live and post mortem skull conductivity measurements which show support for this live-to-post mortem scale factor

Investigating the measurement capability of densely-distributed subdermal EEG electrodes

This paper studies the effect of bipolar subdermal EEG lead placement on measurement sensitivity distributions. The electrodes were subdermally located on the skull of a realistic human head and arranged in a 5×5 matrix of electrodes located at the apex of the cranial vault. The effects on the measurement sensitivity were studied by means of the half-sensitivity volume (HSV). The results indicate that subdermal measurements focus the accuracy and specificity of EEG measurement; however, the size of the half sensitivity volume varies due to electrode location across one or more gyri, gray matter and cerebrospinal fluid (CSF) thickness. The results of the study suggest that subdermal needle electrodes could provide specific and accurate measurements of cortical activation but warrant further studies to understand how much the measurement sensitivity is influenced by placement of the subdermal electrodes over the gyri versus sulci.

Cortical potential imaging using L-curve and GCV method to choose the regularisation parameter.

Background The electroencephalography (EEG) is an attractive and a simple technique to measure the brain activity. It is attractive due its excellent temporal resolution and simple due to its non-invasiveness and sensor design. However, the spatial resolution of EEG is reduced due to the low conducting skull. In this paper, we compute the potential distribution over the closed surface covering the brain (cortex) from the EEG scalp potential. We compare two methods – L-curve and generalised cross validation (GCV) used to obtain the regularisation parameter and also investigate the feasibility in applying such techniques to N170 component of the visually evoked potential (VEP) data. Methods Using the image data set of the visible human man (VHM), a finite difference method (FDM) model of the head was constructed. The EEG dataset (256-channel) used was the N170 component of the VEP. A forward transfer matrix relating the cortical potential to the scalp potential was obtained. Using Tikhonov regularisation, the potential distribution over the cortex was obtained. Results The cortical potential distribution for three subjects was solved using both L-curve and GCV method. A total of 18 cortical potential distributions were obtained (3 subjects with three stimuli each – fearful face, neutral face, control objects). Conclusions The GCV method is a more robust method compared to L-curve to find the optimal regularisation parameter. Cortical potential imaging is a reliable method to obtain the potential distribution over cortex for VEP data.

Sensitivities of Bipolar Subcutaneous and Cortical EEG Leads

An ideal bioelectric measurement should be focused on and specific to the target region. Volume conductor effects such as the poorly conducting bones or the moderately conducting skin are known to affect the specificity and accuracy of the surface electroencephalography (EEG) measurements. This paper introduces a modeling study on the effect of bipolar EEG lead implantation on measurement sensitivity. The electrodes were implanted at two depths in the realistic human head, one on the skull and the other on the cortex. The effects on the measurement sensitivity were studied by means of the half-sensitivity volume (HSV) and the region of interest sensitivity ratio (ROISR). The results indicate that subcutaneous implantation notably enhances the accuracy and specificity of EEG measurement compared to the surface measurement. Deeper measurements i.e. implantation on the cortex enables specific monitoring of a small source volume in contrast to partial and whole regions such as the visual cortex. The results of the study implicate that in clinical practice the subcutaneous needle electrodes would provide more specific and accurate measurements of cortical activation than scalp measurements.

The Effect of Electrode Size on Cortical EEG Sensitivity Distributions

We investigated the influence of electrode contact size on cortical electroencephalogram (EEG) sensitivity distributions. We focused this investigation on the fifth sensitivity volume (FSV) of mapping the lead field in the cortex. We evaluated the cortical sensitivity in concentric spheres and realistic models, using electrode diameters of 1 mm, 5 mm, 10 mm, and 15 mm. We found that when electrodes were spaced farther apart than the bipolar pair of the 128-channel montage, real dimensional electrodes should be used rather than point electrodes in the simulation studies.

Incorporating Craniofacial Anthropometry into Realistically-Shaped Head Models

Borrowing from the fields of paleoanthropology and cranial anthropology we mathematically transform a universal head model to reflect the size of an average female of other ethnic groups. Our goal is to make deformable generic head models readily available for individuals who have not been medically imaged. We concluded that elliptical and exponential functions must be implemented to reshape the template in the frontal, occipital and parietal lobes. We have identified a few parametric values, namely the exponents and major-to-minor axis ratios to generate realistic human shapes.

Recording cortical EEG subcortically — Improved EEG monitoring from depth-stimulation electrodes

The electroencephalogram (EEG) generated by cerebral cortex can be recorded far away from the cortex, analogous to the electrocardiogram (ECG) that can be recorded far from the heart. ECG is often seen as an artifact in EEG recordings. In this paper we demonstrate that the burst suppression pattern of EEG, which is generated by the cerebral cortex, can be recorded at a distance from the cortex with a pair of electrodes in the subthalamic nucleus and also with an electrode pair on the masseter muscle below the zygomatic arch. We then present a fundamental theoretical model which explains the currents inside and outside the cranium, which produce the EEG at these locations.