The design of a system of adjustable superconducting plates for balancing a gradiometer

The gradiometer of a SQUID-system for measuring very small biomagnetic signals should be made as insensitive as possible to the much larger uniform background fields. The balance of a gradiometer can be improved considerably by modifying the effective area of the gradiometer loops by a system of small adjustable superconducting plates.\ud This paper is intended as an aid for designing such a balancing system. We calculated the effective area of a gradiometer loop perpendicular to a rectangular or parallel to a circular plate. The paper contains graphs showing how large the plates have to be and where they must be placed.\ud The calculations show that for accurate balancing the distance of a plate to the wire of a loop must be large and that the plates must be as small as possible

Inward continuation of the scalp potential distribution by means of the (vector) BEM

The vector Boundary Element Method (vBEM) is used for the calculation of a matrix that links the tangential components of the current density on the cortical and scalp surface. This so-called transfer matrix is compared to the transfer matrix that links the potential distribution on both surfaces. Forward and inverse calculations are performed to evaluate both types of transfer matrice

Optimization of a Third-Order Gradiometer for Operation in Unshielded Environments

The optimum geometry of a third-order gradiometer for operation in unshielded environments is discussed. The optimization result depends on the specific signal and noise conditions. The fetal heart is considered as an example of the signal source. We optimized the gradiometer such that the signal-to-noise ratio is maximized in an averaged sense for all relevant environmental noise conditions and distances to the signal source. The resulting design consists of two second-order gradiometers that can be combined to form a third-order gradiometer in noisy environments, whereas a single second-order gradiometer can be used in low-noise environments. The gradiometer can provide the signal-to-noise ratio that allows detection of fetal heart signals in all relevant environmental noise conditions

Information criteria determine the number of active sources

With the neuroelectromagnetic inverse problem, the optimal choice of the number of sources is a difficult problem, especially in the presence of correlated noise. In this paper we present a number of information criteria that help to solve this problem. They are based on the probability density function of the measurements or their eigenvalues. Make use of the Akaike or MDL (minimum description length) correction term and all employ some sort of noise information. By extensive simulations we investigated the conditions under which these criteria yield reliable estimations. We were able to quantify two major factors of influence: (1) the precision of the noise information and (2) the signal-to-noise ratio (SNR). Here defined as the ratio of the smallest signal eigenvalues and the average of the noise eigenvalues. Furthermore, we found that the Akaike correction term tends to overestimate, due to its greater sensibility to the precision of the noise informatio

CD40 is constitutively expressed on platelets and provides a novel mechanism for platelet activation

CD40 is a 48-kDa phosphorylated transmembrane glycoprotein belonging to the TNF receptor superfamily. CD40 has been demonstrated on a range of cell types, and it has an important role in adaptive immunity and inflammation. CD40 has recently been described on platelets but platelet activation by CD40 has not been described. In the present study, we use flow cytometry and immunoblotting to confirm that platelets constitutively express surface CD40. CD40 mRNA was undetectable, suggesting that the protein is synthesized early in platelet differentiation by megakaryocytes. Ligation of platelet CD40 with recombinant soluble CD40L trimer (sCD40LT) caused increased platelet CD62P expression, -granule and dense granule release, and the classical morphological changes associated with platelet activation. CD40 ligation also caused ß3 integrin activation, although this was not accompanied by platelet aggregation. These actions were abrogated by the CD40L blocking antibody TRAP-1 and the CD40 blocking antibodies M2 and M3, showing that activation was mediated by CD40L binding to platelet CD40. ß3 integrin blockade with eptifibatide had no effect, indicating that outside-in signaling via IIbß3 was not contributing to these CD40-mediated effects. CD40 ligation led to enhanced platelet-leukocyte adhesion, which is important in the recruitment of leukocytes to sites of thrombosis or inflammation. Our results support a role for CD40-mediated platelet activation in thrombosis, inflammation, and atherosclerosis

Measuring miniature eye movements by means of a SQUID magnetometer

A new technique to measure small eye movements is reported. The precise recording of human eye movements is necessary for research on visual fatigue induced by visual display units.1 So far all methods used have disadvantages: especially those which are sensitive or are rather painful.2,3 Our method is based on a transformation of mechanical vibrations into magnetic flux variations. In order to do this a small magnet is embedded in a close-fitting soft contact lens. The magnetic flux variations caused by eyeball movements during fixation are measured by means of a SQUID magnetometer. The recordings show the typical fixation pattern of a human eye. This pattern is composed of three kinds of movements: saccades, drift and microtremor. The last-mentioned type of movements are displacements in the order of 2 μm. It is possible to distinguish between movements which are perpendicular to each other

The influence of skull parameters in a realastically shaped head model on the accuracy of EEG dipole localization

Magnetic resonance imaging (MRI) provides the means for the generation of head models with a high geometrical detail. Errors in the generation of realistically shaped models are likely to be made, due to the identification of the different anatomical structures. The poorly conducting skull layer plays a special role, since it is known to have a large effect on the scalp potentials and is difficult to distinguish in MRI. If source localisation is applied to EEG, then the systematic errors in the parameters of the reconstructed sources are partly due to the misspecifications of the head model. - In this paper, the influence of certain head model parameters on the systematic errors of reconstructed dipole sources is investigated. Variations in the skull conductivity and thickness, as well as local variations in the skull thickness, are considered. In order to do so, the sensitivity measure is introduced, which specifies the amount of change of a dipole parameter due to a specific model variation. Sensitivity maps are constructed for layers of dipoles underneath the brain surface. The maps of dipole sensitivities show the local distribution of the systematic errors to be expected. The computations are performed by means of a derivative method, which utilizes a linearization of the forward problem

Post-movement Beta synchronization studied with linear estimation

Event-related desynchronization (ERS) describes a short-lasting and localized amplitude enhancement of specific frequency components. The spatial distribution of a post-movement beta ERS can be visualized by computing the local average reference (LAR). The Linear estimation (LE) method can also be applied to study the spatiotemporal ERS patterns. As source space an hemisphere was used with equally distributed radially oriented current dipoles. The lead field matrix is normalized to make sure that all dipoles have the same average impact on the sensors. A distributed source solution is found for each timestep and for each trial. Event-related Desynchronization calculations are carried out for every dipole (squaring of amplitude, averaging over all trials and time averaging over 16 time points). Both methods were conducted for the study of voluntary hand movement. The results are similar but in contrast to the LAR maps, the LE maps show a better spatial resolution. This is not surprising since the LAR method is limited to the electrode sites whereas with LE the EEG activity is projected onto the source space. Furthermore, the LE method counteracts the deblurring caused by the poorly conducting skull. Linear Estimation depends on several assumptions about the source space, volume conductor and the regularization parameter. Further investigation is needed to evaluate the application of LE for the study of Event-Related EEG phenomena