Inhomogeneous Head Model Research Articles

Distribution of energy absorption in an inhomogeneous head model at 900 MHz

An inhomogeneous model of the human head, which consists of a number of tissues, is compared with a homogeneous model, commonly used for the evaluation of electromagnetic (EM) field inside the head in the 900-MHz frequency band. For both models, the level of exposure in the head is evaluated by means of numerical simulation of the Specific Absorption Rate (SAR) in the whole head and in separated areas. It is shown that the homogeneous and inhomogeneous models lead approximately to the same total SAR, but the magnitudes of the maximal point SAR and the local SAR in the inhomogeneous model are significantly higher than in the homogeneous one.

The definite non-uniqueness results for deterministic EEG and MEG data

The solvability of the inverse problems of electroencephalography and magnetoencephalography has been studied extensively in the literature using a variety of models, including spherical and non-spherical geometries, homogeneous and inhomogeneous head models, and neuronal excitations involving the discrete and continuous distribution of dipoles. Among the important methods used are the methods based on spectral decompositions, physical arguments and integral representation techniques. Regarding the uniqueness of these inverse problems, a general result, independent of the geometry and the homogeneity of the conducting medium, has been obtained recently by the second author. This paper summarizes this result which appears to be mathematically definitive, in the sense that the geometry is arbitrary, the brain is surrounded by shells of varying conductivities, the neuronal current is arbitrary, the data are complete and the proofs are analytical. Furthermore, this paper includes a summary of the main steps of the proofs leading to the above result. In addition, it demonstrates the consistency of this general uniqueness result with all earlier results known in the literature.

A systematic study of head tissue inhomogeneity and anisotropy on EEG forward problem computing

In this study, we propose a stochastic method to analyze the effects of inhomogeneous anisotropic tissue conductivity on electroencephalogram (EEG) in forward computation. We apply this method to an inhomogeneous and anisotropic spherical human head model. We apply stochastic finite element method based on Legendre polynomials, Karhunen-Loeve expansion and stochastic Galerkin methods. We apply Volume and Wang's constraints to restrict the anisotropic conductivities for both the white matter (WM) and the skull tissue compartments. The EEGs resulting from deterministic and stochastic FEMs are compared using statistical measurement techniques. Based on these comparisons, we find that EEGs generated by incorporating WM and skull inhomogeneous anisotropic tissue properties individually result in an average of 56.5 and 57.5% relative errors, respectively. Incorporating these tissue properties for both layers together generate 43.5% average relative error. Inhomogeneous scalp tissue causes 27% average relative error and a full inhomogeneous anisotropic model brings in an average of 45.5% relative error. The study results demonstrate that the effects of inhomogeneous anisotropic tissue conductivity are significant on EEG.

Estimation of in vivo brain-to-skull conductivity ratio in humans

The electrical conductivity value of the human skull is important for biophysics research of the brain. In the present study, the human brain-to-skull conductivity ratio was estimated through in vivo experiments utilizing intra-cranial electrical stimulation in two epilepsy patients. A realistic geometry inhomogeneous head model including the implanted silastic grids was constructed with the aid of the finite element method, and used to estimate the conductivity ratio. Averaging over 49 sets of measurements, the mean value and standard deviation of the brain-to-skull conductivity ratio were found to be 18.7 and 2.1, respectively.

Temperature Increase in Human Heads Due to Different Models of Cellular Phones

The temperature increase in a realistic human head model exposed to five different cellular phone models is investigated by a bioheat equation and the finite-difference time-domain (FDTD) method at 900 and 1800 MHz, respectively. These five cellular phone models are greatly differ from each other in structure. The realistic human head model is used to investigate the temperature increase in the head affected by varying factors including; electric properties of human head tissues, hand absorption power effect, cellular phone models, homogeneous and inhomogeneous head models, cellular phone material, and the distance between the cellular phone and the head. Computed results show that the temperature increase in the head reaches the steady-state value for exposure time of about 20 minutes. For worst-case, the maximum temperature increase in the head is found to be 0.19° C at 900 MHz and 0.055° C at 1800 MHz for the inhomogeneous human head with six tissues exposed to a metallic handset held by a hand, with antenna output power of 0.6 W and 0.125 W, respectively. The maximum temperature increase in the head is far less than the threshold temperature increase of 4.5° C which can cause neuron damage.

Spatio-temporal EEG source localization using a three-dimensional subspace FINE approach in a realistic geometry inhomogeneous head model

The subspace source localization approach, i.e., first principle vectors (FINE), is able to enhance the spatial resolvability and localization accuracy for closely-spaced neural sources from EEG and MEG measurements. Computer simulations were conducted to evaluate the performance of the FINE algorithm in an inhomogeneous realistic geometry head model under a variety of conditions. The source localization abilities of FINE were examined at different cortical regions and at different depths. The present computer simulation results indicate that FINE has enhanced source localization capability, as compared with MUSIC and RAP-MUSIC, when sources are closely spaced, highly noise-contaminated, or inter-correlated. The source localization accuracy of FINE is better, for closely-spaced sources, than MUSIC at various noise levels, i.e., signal-to-noise ratio (SNR) from 6 dB to 16 dB, and RAP-MUSIC at relatively low noise levels, i.e., 6 dB to 12 dB. The FINE approach has been further applied to localize brain sources of motor potentials, obtained during the finger tapping tasks in a human subject. The experimental results suggest that the detailed neural activity distribution could be revealed by FINE. The present study suggests that FINE provides enhanced performance in localizing multiple closely spaced, and inter-correlated sources under low SNR, and may become an important alternative to brain source localization from EEG or MEG.

SAR affected by shapes and electrical properties of the human head exposed to a cellular phone

AbstractThe local maximum specific absorption rate (SAR) induced in spherical, cubical, coarse, and realistic human‐head models exposed to a cellular phone is investigated using the finite‐difference time‐domain (FDTD) method at 900 and 1800 MHz, respectively. These four human‐head models each have greatly different shapes but almost the same volume. Obtained local maximum SARs induced in these four human‐head models for homogeneous cases are found to have an average differences of about 15% and 12% at 900 and 1800 MHz, respectively. A comparison of SARs induced in the realistic human‐head model for homogeneous and inhomogeneous cases is also made. It is found that the local maximum SAR induced in the homogeneous human‐head model is larger than that induced in the inhomogeneous human head model. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 42: 1–4, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20189

High-resolution EEG: Cortical potential imaging of interictal spikes

It is of clinical importance to localize pathologic brain tissue in epilepsy. Noninvasive localization of cortical areas associated with interictal epileptiform spikes may provide important information to facilitate presurgical planning for intractable epilepsy patients. A cortical potential imaging (CPI) technique was used to deconvolve the smeared scalp potentials into the cortical potentials. A 3-spheres inhomogeneous head model was used to approximately represent the head volume conductor. Five pediatric epilepsy patients were studied. The estimated cortical potential distributions of interictal spikes were compared with the subsequent surgical resections of these same patients. The areas of negativity in the reconstructed cortical potentials of interictal spikes in 5 patients were consistent with the areas of surgical resections for these patients. The CPI technique may become a useful alternative for noninvasive mapping of cortical regions displaying epileptiform activity from scalp electroencephalogram recordings.

Boundary Element Method-Based Cortical Potential Imaging of Somatosensory Evoked Potentials Using Subjects' Magnetic Resonance Images

A boundary element method-based cortical potential imaging technique has been developed to directly link the scalp potentials with the cortical potentials with the aid of magnetic resonance images of the subjects. First, computer simulations were conducted to evaluate the new approach in a concentric three-sphere inhomogeneous head model. Second, the corresponding cortical potentials were estimated from the patients' preoperative scalp somatosensory evoked potentials (SEPs) based on the boundary element models constructed from subjects' magnetic resonance images and compared to the postoperative direct cortical potential recordings in the same patients. Simulation results demonstrated that the cortical potentials can be estimated from the scalp potentials using different scalp electrode configurations and are robust against measurement noise. The cortical imaging analysis of the preoperative scalp SEPs recorded from patients using the present approach showed high consistency in spatial pattern with the postoperative direct cortical potential recordings. Quantitative comparison between the estimated and the directly recorded subdural grid potentials resulted in reasonably high correlation coefficients in cases studied. Amplitude difference between the estimated and the recorded potentials was also observed as indexed by the relative error, and the possible underlying reasons are discussed. The present numerical and experimental results validate the boundary element method-based cortical potential imaging approach and demonstrate the feasibility of the new approach in noninvasive high-resolution imaging of brain electric activities from scalp potential measurement and magnetic resonance images.

An equivalent current source model and laplacian weighted minimum norm current estimates of brain electrical activity.

We have developed a method for estimating the three-dimensional distribution of equivalent current sources inside the brain from scalp potentials. Laplacian weighted minimum norm algorithm has been used in the present study to estimate the inverse solutions. A three-concentric-sphere inhomogeneous head model was used to represent the head volume conductor. A closed-form solution of the electrical potential over the scalp and inside the brain due to a point current source was developed for the three-concentric-sphere inhomogeneous head model. Computer simulation studies were conducted to validate the proposed equivalent current source imaging. Assuming source configurations as either multiple dipoles or point current sources/sinks, in computer simulations we used our method to reconstruct these sources, and compared with the equivalent dipole source imaging. Human experimental studies were also conducted and the equivalent current source imaging was performed on the visual evoked potential data. These results highlight the advantages of the equivalent current source imaging and suggest that it may become an alternative approach to imaging spatially distributed current sources-sinks in the brain and other organ systems.

Comparison study of different head model structures with homogeneous/inhomogeneous conductivity

Most of the human head models used in dipole localisation research, which have been reported in the literature to date, assume a simplified cranial structure wherein the head is modelled as a set of distinct homogenous tissue compartments. The inherent inhomogeneity of the tissues has so far been ignored in these models due to the difficulties involved in obtaining the conductivity characteristics with sufficiently high enough spatial resolution throughout the head. A technique for developing an inhomogeneous head model based on the generation of pseudo-conductivity values from the existing but sparse conductivity values is proposed in this paper. Comparative studies are conducted on different model structures and different mechanisms for generating the pseudo conductivities. An evaluation of the results of these studies as reported in this paper, shows that contrary to current simplifying assumptions, tissue inhomogeneity has a major influence on the computation of electrical potential distributions in the head.

FDTD computation of temperature rise in realistic head models simulating adult and infant for 1.5-GHz microwave exposure

The temperature rise of the human head exposed to 1.5-GHz microwaves at the safety standard level (1 mW/cm2) in the general environment is computed by the FDTD method for a realistic adult model developed by the authors and an infant size model obtained by reducing the adult model. The temperature rise for the head in thermal equilibrium is attained from a nonuniform initial temperature distribution in the head, taking into consideration the external temperature. It is found that the peak values in all tissues are higher in the adult model than in the infant model. In the adult model, the maximum is reached in the eyeballs (0.07 °C), while in the infant model, the maximum occurs in the muscle (0.055 °C). The tissue average of the temperature rise is higher in the infant model than in the adult model except for the eyeballs. In particular, the average value (0.0058 °C) of the temperature rise in the brain tissue is lower than the peak value by almost one order of magnitude and is three times higher than the average value in the adult. The latter result is caused by the hot spot of SAR as a heat source generated inside the head. It is found that a hot spot appears in the inhomogeneous realistic head model of infant size. © 2000 Scripta Technica, Electron Comm Jpn Pt 1, 84(4): 57–66, 2001

Estimating cortical potentials from scalp EEG's in a realistically shaped inhomogeneous head model by means of the boundary element method.

Cortical potentials are estimated from scalp potentials using a realistically shaped inhomogeneous head model, by means of the boundary element method (BEM). A new adaptive algorithm has been developed to achieve high accuracy to link directly the cortical potentials to the scalp potentials in a realistically shaped inhomogeneous head model including the thin low-conductivity skull layer. Computer simulations using a concentric three-spheres head model have tested this approach. The present study demonstrates that the cortical potentials can be directly estimated from the scalp potentials using the BEM in a realistically shaped inhomogeneous head model.

High-resolution source imaging of brain electrical activity.

The author considers the enhancement of spatial resolution with cortical imaging. The following topics are considered: cortical-potential imaging from scalp potentials; cortical-potential imaging from scalp Laplacians; equivalent surface-source imaging; cortical-potential imaging in a realistically shaped, inhomogeneous head model; inverse-imaging algorithms; other EEG-based cortical-imaging approaches.