Biomedical subjects

G J Huiskamp

Publications and source records attributed to G J Huiskamp.

11 recordsLinked to original sources

PubMed2003

Magnetic source imaging contributes to the presurgical identification of sensorimotor cortex in patients with frontal lobe epilepsy.

OBJECTIVE: One of the primary goals of preoperative evaluation of patients considered to be candidates for epilepsy surgery is the delineation of eloquent cortex adjacent to the area of resection. The aim of this study is the functional localization of the sensorimotor cortex in relation to an epileptogenic frontal lobe lesion, thus enabling a more complete resection in these patients while minimizing the risk of postoperative neurological deficits. METHODS: Participating in this study were patients with epilepsy, diagnosed as being related to a left or right frontal lobe lesion. Magnetoencephalographic responses evoked by electrical stimulation of the left and right hand median nerve were localized using single time-point equivalent dipole (ED) modeling, taking into account the realistic shape of the head. Instead of relying on the primary component (N/P 20) of the somatosensory evoked magnetic fields (SEFs) in this study ED fits were obtained for each time-point of the somatosensory evoked responses. On a cortical rendering, the reconstructed dipoles were depicted relative to the anatomy obtained from 3D-magnetic resonance imaging. RESULTS: The results of single time-point ED analysis including all the components of the responses indicated that the sources underlying the SEFs are located at the borders of the central sulcus (CS). The opposite direction of the sources underlying, respectively, the primary and subsequent late component of the SEFs indicated distinct sources located at the opposite banks of the CS. These sources, therefore, might correspond to the sensory hand projection area and the primary motor area of the sensorimotor cortex. It appeared that the location of the EDs obtained for the SEFs of 4 of the 7 patients studied were asymmetric for the left and right hemisphere, probably because of a displacement of the sensorimotor areas relative to the CS. The systematic assessment of the dipole fits compared to brain anatomy confirmed that volume conduction changes due to the lesion were not responsible for these observed deviations, thus leaving as explanation space-occupying and neurophysiological changes due to the lesion.

Adolescent↗

PubMed2001

The existence of two sources in rolandic epilepsy: confirmation with high resolution EEG, MEG and fMRI.

In benign rolandic epilepsy seizure semiology suggests that the epileptic focus resides in the lower sensorimotor cortex. Previous studies involving dipole modeling based on 32 channel EEG have confirmed this localization. These studies have also suggested that two distinct dipole sources are required to adequately describe the typical interictal spikes. Since in benign epilepsy invasive validation is prohibited, this study tries to further establish these results using a multi-modal approach, involving 32 channel EEG, high resolution 84 channel EEG, 151 channel MEG and fMRI. From one patient interictal spikes were recorded and analyzed using the MUSIC algorithm in a realistic volume conductor model. In an fMRI experiment the same patient performed voluntary tongue movements, thus mimicking a typical seizure. Results show that EEC, MEG and fMRI localization converge on the same area in the lower part of the sensorimotor cortex, and that high resolution EEG clearly reveals two distinct sources, one in the post- and one in the pre-central cortex.

Brain Mapping↗

PubMed2000

A bidomain model based BEM-FEM coupling formulation for anisotropic cardiac tissue.

A hybrid boundary element method (BEM)/finite element method (FEM) approach is proposed in order to properly consider the anisotropic properties of the cardiac muscle in the magneto- and electrocardiographic forward problem. Within the anisotropic myocardium a bidomain model based FEM formulation is applied. In the surrounding isotropic volume conductor the BEM is adopted. Coupling is enabled by requesting continuity of the electric potential and the normal of the current density across the boundary of the heart. Here, the BEM part is coupled as an equivalent finite element to the finite element stiffness matrix, thus preserving in part its sparse property. First, continuous convergence of the coupling scheme is shown for a spherical model comparing the computed results to an analytic reference solution. Then, the method is extended to the depolarization phase in a fibrous model of a dog ventricle. A precomputed activation sequence obtained using a fine mesh of the heart was downsampled and used to calculate body surface potentials and extracorporal magnetic fields considering the anisotropic bidomain conductivities. Results are compared to those obtained by neglecting in part or totally (oblique or uniform dipole layer model) anisotropic properties. The relatively large errors computed indicate that the cardiac muscle is one of the major torso inhomogeneities.

Animals↗

PubMed1997

The inverse problem in electroretinography: a study based on skin potentials and a realistic geometry model.

The problem of obtaining the retinal source distribution that generates the electroretinogram (ERG) from measured skin potentials is addressed. A realistic three-dimensional (3-D) volume conductor model of the head is constructed from magnetic resonance image (MRI) data sets. The skin potential distribution generated in this model by a dipole layer source at the retina is computed by using the boundary element method (BEM). The influence of the various compartments of the complete model on the results was investigated, and a simplified model was defined. An inverse procedure for estimating the source distribution at the retina from ERG's obtained from skin electrodes was developed. The procedure was tested on simulated potentials. A fair correspondence between the original and estimated source distribution was found. Furthermore, the ERG's measured at seven skin electrodes were used to estimate the source distribution at the retina. The ERG potential waveform at an additional skin electrode was computed from this source distribution and compared to the measured potential at this electrode. Again a fair correspondence was obtained. It is concluded that the methods may become a useful tool for clinical applications, i.e., for the assessment of localized defects in retinal function.

Analog-Digital Conversion↗

PubMed1996

The uniform double layer model and myocardial infarction: forward solution consideration.

The Uniform Double Layer (UDL) model of the cardiac generator is often used for forward simulation of body surface potentials (BSPs). The model also proved to be very useful for the inverse computation of heart activation. However, for the purposes of Myocardial Infarction (MI) modelling mostly the Multiple Dipole (MD) models are used. In our study, the ability of UDL model to represent the activation of the heart with an old MI was examined. The finite element model of the heart was used to simulate electrical activation of the heart with an old MI. Different locations of endocardial MI were used. For each of them three cases were considered according to the scale of the infarcted area: small and medium endocardial and large transmural. For the further computation of the electric field within the torso volume conductor two types of UDL representation of the cardiac generator were used. For the first UDL model, supposing the scared tissue to be unexcitable, an "infarcted" surface (different from the "healthy" surface) of activated myocardium was generated for each case of MI. Times when activation wavefront reached particular nodes on the surface served as an input for the forward computation of BSPs. To be able to understand the behaviour of the UDL, we also created the second UDL model, where the "infarcted activation sequence" was approximated on the original "healthy" heart surface. The BSPs were computed for each case of MI using both UDL cardiac generators. The boundary element method with the inhomogeneous volume conductor was used for computations. The BSPs generated by both models for the same case of MI were compared using the correlation coefficient. The results show, that it is possible to find an approximation of the "infarcted activation sequence" on the "healthy" heart generator surface in a way that BSPs generated by both models have a correlation coefficient higher than 0.96 for the entire period of depolarisation. Visualisation of the epicardial isochrones might help to understand the UDL model behaviour under the MI conditions. It would be useful for the correct interpretation of the results when using the UDL model for inverse solution. (Fig. 7, Ref. 5.)

Body Surface Potential Mapping↗

PubMed1995

Lead system transformation for pooling of body surface map data: a surface Laplacian approach.

In this paper, a method is described to transform Body Surface Map (BSM) data from one lead system to that of another. This enables pooling of BSM data between different centres. The transformation tool is based upon Laplacian interpolation. It is evaluated by inspecting transformations from lead systems having few leads to one having many leads.

Body Surface Potential Mapping↗

PubMed1992

Heart position and orientation in forward and inverse electrocardiography.

A study has been made of the influence of the position and orientation of the heart within the thorax on computed ECG waveforms (forward model) and on computed activation sequences (inverse model) in three normal cases. Results show that differences in heart position and orientation, associated with shifts relative to the precordium of the order of 0.5 cm, may result in amplitude differences or QRS waveforms of typically tenths or millivolts, which constitute part of the observed interindividual variability of the ECG. The inverse study shows that, in spite of similar errors in estimated heart position and orientation, stable solutions of the ventricular activation sequence can still be found. However, in the case where the heart is very close to precordial leads, the stability of the inverse solution is found to be intrinsically poor.

Electrocardiography↗

PubMed1992

Implicit and explicit constraints in inverse electrocardiography.

This paper reviews the major distributed source models that have been postulated over the years to support the interpretation of observed body surface potentials: double-layer models, the source description in terms of epicardial (ie, pericardial) potentials, and its equivalent: the distributed monolayer. This includes a presentation and discussion of a source model that has been developed over the past decade: the uniform double-layer model. The properties of this model are contrasted to those of other distributed source models from the perspective of their inherent capacity for imposing the constraints that are essential for regularizing the involved inverse problem.

Electrocardiography↗

PubMed1991

A realistic torso model for magnetocardiography.

This paper contains the description of an inhomogeneous, multi-compartmental volume conductor model which is in use in our group. Although initially developed for the study of the ECG, it has been found to serve equally well for simulating the magnetocardiogram (MCG), the forward problem, and as a basis for source analysis of the MCG, the inverse problem. For both problems some illustrative examples are included demonstrating the necessity of using an inhomogeneous volume conductor model of the torso, having realistic--preferably tailored--geometry. A simple inverse procedure, based on correlation techniques, is included for the solution of the problem of source localization when an accurate description of the inhomogeneous volume conductor is available.

Adult↗

PubMed1990

The magnetocardiogram as derived from electrocardiographic data.

Magnetocardiographic signals, as present outside the thorax and generated by the depolarization process within the ventricles of the human heart, have been computed by using a model that incorporates the uniform double layer as the exclusive primary source. The volume conductor effects are treated by using an inhomogeneous, multicompartmental model of the thorax, based on "tailored" geometry derived from magnetic resonance imaging. The required activation function, specifying the timing of the ventricular depolarization process, was derived from an inverse procedure that uses as input data electric signals measured at the body surface. Next, the magnetic signals from the same subjects were measured. A close correspondence between computed and measured magnetic signals was observed (relative root mean square residual difference of 0.37). These results demonstrate that magnetocardiograms and electrocardiograms have a common basis and that it is unlikely that prominent sources exist that are electrically silent and yet active in the genesis of the magnetic fields associated with the depolarization process of the heart. Moreover, fresh support is implied for the usefulness of the classical uniform double layer as the electrical source model during ventricular depolarization. The contributions of the secondary sources have previously been found to be a major component of the electric signals; they are now also shown to be a major component of the magnetic signals.

Electrocardiography↗

PubMed1989

The effect of torso inhomogeneities on body surface potentials quantified using "tailored" geometry.

The effect of torso inhomogeneities on the electrocardiographic body surface potentials in general, and the waveform of the QRS complex in the standard leads in particular, were studied by using an inhomogeneous, numerical model of the torso including the (lower) conductivity of the lungs and the (higher) conductivity of the ventricular cavities. The relevant geometry was derived from actual data as measured from NMR scans of the subjects studied. The simulated potentials were compared to the body surface potentials recorded in the same individuals. In view of the observed high quality of the correspondence in the fully inhomogeneous case, the quantitative results of the study of the effects of the involved inhomogeneities attain a high level of credibility. To facilitate this analysis a new tool is introduced: the contribution maps.

Action Potentials↗