H.-A. Wischmann

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

SummaryAt 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.

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

SummarySource 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.

A 31-channel squid system for biomagnetic imaging

A modular multichannel SQUID-system, in which every single channel can be optimized or replaced individually, is presented. The DC-SQUIDs based on the materials NbN/MgO are prepared by thin film technology and show noise values below 10μΦ0/√Hz. A simplified way of coupling the modulation and feedback current directly to the coupling coil is realized The complete SQUID module including the superconducting shield was miniaturized down to a diameter of 5mm. The gradiometers are wire wound and an as made balancing better than 10−3 is achieved. The cryogenic system was optimized with respect to low vibrations and low helium boil off rate. Simple conductive paint with precisely adjusted surface resistivity is used for RF-shielding. The complete SQUID-electronic of one channel has been realized on one single board and uses a new bias modulation scheme to completely suppress intrinsic 1/f noise. The noise level of the complete system is below 10fT/√Hz. Biomagnetic measurements of the human heart and brain are presented. Single current dipole reconstructions and current density imaging techniques can be used to find the underlying sources. Using a special coil positioning system an overlay of the functional current images with morphological MR-images can be carried out.

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. The authors 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.<<ETX>>

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.

Depth normalization in MEG/EEG current density imaging

We analyze the minimum norm solution of the biomagnetic inverse problem. It is shown that the bias of this technique, i.e. the tendency to reconstruct superficial currents, can be reduced by the use of an appropriate weighting, which depends the used norm. The weighting factors are calculated by a singular value decomposition of the lead field matrix for each location and a noise regularization technique. With this weighting, the ability to reconstruct deep sources is maintained over the whole volume of sensitivity. The extend of this volume depends on the measurement setup and the noise level.

Current Density Reconstructions Using the L1 Norm

A major goal in biomagnetism is the reconstruction of current distributions without making preliminary assumptions about number and temporal properties of the sources to be reconstructed. We propose a new current density reconstruction method that computes source images with high resolution and small blurring. A nonlinear functional, the L1-norm, is used as a source constraint. The L1-norm is the sum of absolute current densities. A simple Simulation shows, that the L1-norm does neither impose artificial smoothness nor sharpness upon the reconstructed sources. We have implemented three different minimization schemes for the L1-norm, which we compare against each other and against the MNLS (minimum norm least squares, L2-norm) method [1]. Unlike the L1-norm method described earlier [2], our approaches tolerate noisy data and arbitrary source orientations.

A modular 31-channel SQUID system for biomagnetic measurements

A modular multichannel superconducting quantum interference device (SQUID) system, in which every channel can be optimized or replaced individually, was further improved. The number of channels was increased to 31. The noise level is better than 10 fT/ square root Hz. A novel way of RF shielding using conductive paint avoids degradation of the SQUID characteristics due to RF interference without introducing significant extra noise, so that the system works without any Faraday cage. A simplified way of coupling the modulation and feedback signal directly to the SQUID was developed and tested successfully. The SQUID module with superconducting connections to the gradiometer and its superconducting shield was miniaturized to an outer diameter of 5 mm, so that it can be placed near the gradiometer without introducing significant unbalance. Tests have demonstrated that the accuracy of the system with respect to the localization of a single current dipole is better than 2 mm.<<ETX>>

Localization of current dipoles with multichannel SQUID systems

Current sources in the human body can be localized by measuring the biomagnetic fields with multichannel SQUID systems. Important system aspects are the noise level, the ambient field suppression, the dynamic range, the reliability, the number of channels, and the arrangement of gradiometers. From the user’s point of view the most important quality factor is the accuracy with which a current dipole can be localized. A test procedure is proposed to determine the localization power of the system. A 31‐channel‐SQUID system is presented together with the results of the test. The crucial parts of the system determining the accuracy are pointed out.

Imaging Properties of “Myocardial Current Imaging”

The imaging properties of the new technique “Myocardial Current Imaging” (MCI) [1] are investigated using Computer simulations. MCI has the potential to depict complex current distributions on the human heart from multichannel Magnetocardiography. This might have important applications in diagnosis of complex arrhythmias or myocardial infarction. MCI basically is a minimum norm estimate using the morphological constraint that the impressed currents must be on the heart muscle.