Biomedical subjects

H A Wischmann

Publications and source records attributed to H A Wischmann.

8 recordsLinked to original sources

PubMed1999

Linear and nonlinear current density reconstructions.

Minimum norm algorithms for EEG source reconstruction are studied in view of their spatial resolution, regularization, and lead-field normalization properties, and their computational efforts. Two classes of minimum norm solutions are examined: linear least squares methods and nonlinear L1-norm approaches. Two special cases of linear algorithms, the well known Minimum Norm Least Squares and an implementation with Laplacian smoothness constraints, are compared to two nonlinear algorithms comprising sparse and standard L1-norm methods. In a signal-to-noise-ratio framework, two of the methods allow automatic determination of the optimum regularization parameter. Compensation methods for the different depth dependencies of all approaches by lead-field normalization are discussed. Simulations with tangentially and radially oriented test dipoles at two different noise levels are performed to reveal and compare the properties of all approaches. Finally, cortically constrained versions of the algorithms are applied to two epileptic spike data sets and compared to results of single equivalent dipole fits and spatiotemporal source models.

Algorithms↗

PubMed1998

Improving source reconstructions by combining bioelectric and biomagnetic data.

OBJECTIVES: A framework for combining bioelectric and biomagnetic data is presented. The data are transformed to signal-to-noise ratios and reconstruction algorithms utilizing a new regularization approach are introduced. METHODS: Extensive simulations are carried out for 19 different EEG and MEG montages with radial and tangential test dipoles at different eccentricities and noise levels. The methods are verified by real SEP/SEF measurements. A common realistic volume conductor is used and the less well known in vivo conductivities are matched by calibration to the magnetic data. Single equivalent dipole fits as well as spatio-temporal source models are presented for single and combined modality evaluations and overlaid to anatomic MR images. RESULTS: Normalized sensitivity and dipole resolution profiles of the different EEG/MEG acquisition systems are derived from the simulated data. The methods and simulations are verified by simultaneously measured somatosensory data. CONCLUSIONS: Superior spatial resolution of the combined data studies is revealed, which is due to the complementary nature of both modalities and the increased number of sensors. A better understanding of the underlying neuronal processes can be achieved, since an improved differentiation between quasi-tangential and quasi-radial sources is possible.

Brain Mapping↗

PubMed1998

An improved boundary element method for realistic volume-conductor modeling.

An improved boundary element method (BEM) with a virtual triangle refinement using the vertex normals, an optimized auto solid angle approximation, and a weighted isolated problem approach is presented. The performance of this new approach is compared to analytically solvable spherical shell models and highly refined reference BEM models for tangentially and radially oriented dipoles at different eccentricities. The lead fields of several electroencephalography (EEG) and magnetoencephalography (MEG) setups are analyzed by singular-value decompositions for realistically shaped volume-conductor models. Dipole mislocalizations due to simplified volume-conductor models are investigated for EEG and MEG examinations for points on a three dimensional (3-D) grid with 10-mm spacing inside the conductor and all principal dipole orientations. The applicability of the BEM in view of the computational effort is tested with a standard workstation. Finally, an application of the new method to epileptic spike data is studied and the results are compared to the spherical-shells approximation.

Body Fluid Compartments↗

PubMed1995

Origin of P16 median nerve SEP component identified by dipole source analysis--subthalamic or within the thalamo-cortical radiation?

Following median nerve stimulation, several monophasic peaks were recorded at the scalp in the 15-18 ms time range. Source analysis, using three different methods, modelled a source near the centre of the head with an orientation towards the activated hemisphere and a peak activity at 16 ms post stimulus. Magnetic recordings detected no signal in this time range, which confirmed a subcortical location of the source. From dipole localization it was not possible to assign the exact origin of the P16 source to either the subthalamic level or the thalamo-cortical radiation, because of the limited spatial resolution at the centre of the spherical head model. An estimate of the conduction velocity of the medial lemniscus pointed towards a subthalamic origin. The P16 source was preserved in two patients with a lesion of the thalamo-cortical radiation and the ventral thalamus. Further evidence for a subthalamic location of P16 was derived from the physical mechanisms generating far-field potentials.

Adult↗

PubMed1995

Comparison of realistically shaped boundary-element and spherical head models in source localization of early somatosensory evoked potentials.

Source localizations of early somatosensory evoked potentials and electrical potentials produced by dipoles in the region of the central sulcus were computed using realistically shaped boundary-element head models (BEM) and compared to localizations obtained using 3-shell spherical models. Realistically shaped 3-shell boundary-element-models were constructed on the basis of the individual anatomy obtained from 3D-MR-tomography in 6 subjects. Spherical head models were fitted to the actual locations of the electrodes and to the surface of the heads, respectively. Source locations calculated within the spherical head models differed by an average of 4 mm (range: 2 to 7 mm) with respect to the 3-shell BEM, taking into account the limited accuracy of this model. This mislocation was most prominently due to deeper source locations predicted using a spherical head model and caused by incorrect modelling of the geometry of the heads, although sources were located in a favourable region of the heads.

Adult↗

PubMed1995

Coordinate system matching for neuromagnetic and morphological reconstruction overlay.

The overlay of functional and morphological images is an essential tool for advanced and improved functional diagnosis showing the correlation between spatial structures or lesions and functional areas. We present an improved coordinate system matching technique. The well known method of three orthogonal coils combined in one coilset for an angular-independent measure is validated for the use of first- or higher order gradiometers instead of magnetometers. The coilset localization procedure was modified with lock-in detection and current feedback for better long range sensitivity. Real measurements with the 31-channel Philips-MEG system have been carried out. A very good localization accuracy below the measuring area with deviations below 2 mm was found. For coordinate system matching, a 3-D cursor with surface images from segmented MR-data was implemented and an optimized, weighted least squares fit transformation algorithm between functional and morphological systems was developed. The resulting transformations consist of weighted shifts and best-fit rotations and lead to deviations of marker positions in the mm range, depending mainly on the spatial accuracy of the marker fixation.

Algorithms↗

PubMed1994

Source analysis of median nerve and finger stimulated somatosensory evoked potentials: multichannel simultaneous recording of electric and magnetic fields combined with 3D-MR tomography.

At the current state of technology, multichannel simultaneous recording of combined electric potentials and magnetic fields should constitute the most powerful tool for separation and localization of focal brain activity. We performed an explorative study of multichannel simultaneous electric SEPs and magnetically recorded SEFs. MEG only sees tangentially oriented sources, while EEG signals include the entire activity of the brain. These characteristics were found to be very useful in separating multiple sources with overlap of activity in time. The electrically recorded SEPs were adequately modelled by three equivalent dipoles located: (1) in the region of the brainstem, modelling the P14 peak at the scalp, (2) a tangentially oriented dipole, modelling the N20-P20 and N30-P30 peaks, and part of the P45, and (3) a radially oriented dipole, modelling the P22 peak and part of the P45, both located in the region of the somatosensory cortex. Magnetically recorded SEFs were adequately modelled by a single equivalent dipole, modelling the N20-P20 and N30-P30 peaks, located close to the posterior bank of the central sulcus, in area 3b (mean deviation: 3 mm). The tangential sources in the electrical data were located 6 mm on average from the area 3b. MEG and EEG was able to locate the sources of finger stimulated SEFs in accordance with the somatotopic arrangement along the central fissure. A combined analysis demonstrated that MEG can provide constraints to the orientation and location of sources and helps to stabilize the inverse solution in a multiple-source model of the EEG.

Adult↗

PubMed1992

Effect of the signal-to-noise ratio on the quality of linear estimation reconstructions of distributed current sources.

Currently, linear estimation reconstruction is the only feasible method for extracting information about spatially distributed current sources from measurements of neural magnetic fields. We present the results of a systematic study of the effect of the signal-to-noise ratio on the imaging quality of one such algorithm in over-as well as undetermined circumstances. In particular, we will discuss the necessary trade-off between the contradictory goals of a minimum norm of the reconstructed current density distribution and of a minimal deviation of the reconstructed fields from the measured fields. As an example, we show the reconstruction of a simple arrangement of two nearly parallel dipoles in two different depths inside a spherical volume conductor, discussing the differences between the computer simulation without noise and simulation with a realistic noise level.

Algorithms↗