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J W Meijs

Publications and source records attributed to J W Meijs.

7 recordsLinked to original sources
PubMed1989

On the numerical accuracy of the boundary element method.

The numerical accuracy of the boundary element (BE) method used to solve the volume conduction problem of nested compartments, each having a homogeneous conductivity, is studied. The following techniques for improving this accuracy are discussed: the handling of the auto solid angle element omega ii, the overall refinement of the level of discreteness, the use of a locally refined discrete grid, the isolated problem approach, and an adaptive refined computation of the discrete surface integrals involved in the BE method. The effects of these techniques on the numerical accuracy of the computed electrical potentials are illustrated by taking a volume conductor consisting of four concentric spheres representing the head since for this model an analytical (exact) solution is available. The techniques are of importance for numerically computed electroencephalograms (EEG's) since the numerically computed surface EEG's are severely affected by the relatively low conductivity of the compartment representing the skull.

Brain↗
PubMed1987

On the magnetic field distribution generated by a dipolar current source situated in a realistically shaped compartment model of the head.

The magnetic field distribution around the head is simulated using a realistically shaped compartment model of the head. The model is based on magnetic resonance images. The 3 compartments describe the brain, the skull and the scalp. The source is represented by a current dipole situated in the visual cortex. The magnetic field distribution due to the source and that due to the volume currents are calculated separately. The simulations are carried out in order to ascertain which matrix of grid points is suitable as a measuring grid. The possibilities studied are grid points situated in a plane, in a surface which follows the contours of the head and in a sphere. This sphere is taken concentric to the sphere which is the best possible fit for the head. Taking into account the relative contribution of the volume currents and the possible accuracy in the positioning of the magnetic field detector, it can be concluded that the best choice is to measure the normal component of the magnetic field at points which are situated in the spherical surface. The results of this study also show that the magnetic field distribution based on a realistically shaped compartment model differs from that based on a compartment model consisting of concentric spheres. In the spherical model of the head no contribution of the volume currents to the component of the field normal to the sphere can be expected. The difference between the results obtained with these two volume conductor models increases with source depth.

Brain↗
PubMed1987

Inverse solutions based on MEG and EEG applied to volume conductor analysis.

An inverse solution computer program, using a single current dipole in a selected volume conductor, calculates an equivalent dipole from a magneto- or electroencephalographic distribution. The program is used to evaluate several volume conductor models of the head by using one model when generating the distribution and another when calculating the equivalent dipole. Sources of errors in the equivalent dipole, namely uncertainties in the model parameters (e.g. conductivities) and noise in the MEG or EEG distribution, are investigated in the same way. A realistically shaped model of the head is introduced to investigate the extent to which sphere-shaped models can be used.

Adult↗
PubMed1986

Model evaluation using electroencephalography and magnetoencephalography.

The source of both the measured visual evoked potentials and the measured visual evoked magnetic fields was estimated by means of an inverse procedure. The model used consisted of a single current dipole positioned in a volume conductor consisting of four concentric spheres. Comparison of the results showed that the estimations did not always match. In order to reveal a possible cause of this mismatch a realistically shaped multicompartment model of the head was constructed. From forward simulations it followed that the influence of the realistic shape was apparent, especially when the dipole was positioned deep within the brains.

Electroencephalography↗