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Biomedical subjects

T F Oostendorp

Publications and source records attributed to T F Oostendorp.

15 recordsLinked to original sources
PubMed2004

ECGSIM: an interactive tool for studying the genesis of QRST waveforms.

BACKGROUND: Discussion about the selection of diagnostic features of the ECG and their possible interpretation would benefit from a model of the genesis of these signals that has a sound basis in electrophysiology as well as in physics. Recent advances in computer technology have made it possible to build a simulation package whereby the genesis of ECG signals can be studied interactively. DESIGN: A numerical method was developed for computing ECG signals on the thorax, as well as electrograms on both endocardium and epicardium. The source representation of the myocardial electric activity is the equivalent double layer. The transfer factors between the electric sources and the resulting potentials on the heart surface as well as on the body surface were computed using a realistic thorax model. RESULTS AND CONCLUSION: The resulting transfer factors were implemented in a simulation program. The program allows the user to make interactive changes in the timing of depolarisation and repolarisation on the ventricular surface, as well as changing the local source strength, and to inspect or document the effect of such changes instantaneously on electrograms and body surface potentials, visualised by waveforms as well as by potential maps and movies. The entire simulation package can be installed free of charge from www.ecgsim.org.

Computer Simulation↗
PubMed2000

The conductivity of the human skull: results of in vivo and in vitro measurements.

The conductivity of the human skull was measured both in vitro and in vivo. The in vitro measurement was performed on a sample of fresh skull placed within a saline environment. For the in vivo measurement a small current was passed through the head by means of two electrodes placed on the scalp. The potential distribution thus generated on the scalp was measured in two subjects for two locations of the current injecting electrodes. Both methods revealed a skull conductivity of about 0.015 [symbol: see text]/m. For the conductivities of the brain, the skull and the scalp a ratio of 1:1/15:1 was found. This is consistent with some of the reports on conductivities found in the literature, but differs considerably from the ratio 1:1/80:1 commonly used in neural source localization. An explanation is provided for this discrepancy, indicating that the correct ratio is 1:1/15:1.

Biomedical Engineering↗
PubMed2000

Accuracy of a single equivalent moving dipole model in a realistic anatomic geometry torso model.

In this study, we investigated the accuracy of an algorithm to identify the spatial single equivalent moving dipole parameters in a realistic anatomic geometry torso model from potentials at the body surface. Specifically we investigated the effect of measurement noise, and dipole position and orientation in the accuracy of the algorithm. The boundary element method was used to calculate the forward potential distribution at 64 electrode positions on the body surface due to a point dipole. The mean and standard deviation of the distance of the true (obtained in the forward potential calculation) minus the estimated dipole location (obtained from the inverse algorithm) was estimated for each of the above three cases. Our results indicate that the dipole position has the most significant influence on the accuracy of our inverse algorithm.

Algorithms↗
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↗
PubMed1997

Electrical impedance of the cochlear implant lubricants hyaluronic acid, oxycellulose, and glycerin.

Hyaluronic acid (Healon), oxycellulose (hydroxypropyl methylcellulose), and glycerin are lubricants used in cochlear implant surgery for atraumatic deep insertion of the electrode array into the scala tympani. The electrical impedances of these three lubricants were measured to assess possible effects on intraoperative evoked response measurements, such as the electrically evoked stapedius reflex and auditory brain stem response. The impedances of hyaluronic acid, oxycellulose, and saline were very similar and independent of frequency (20 Hz to 1 MHz). Glycerin had an excessively high impedance at low frequencies. A film of hyaluronic acid or oxycellulose around the electrode array immersed in saline did not have any measurable effect on the impedance; a film of glycerin resulted in a strongly reactive polarized layer. However, neither the far-field current spread nor the impedance between stimulated electrodes was affected by any of the lubricants applied as a thin film. This suggests that none of these lubricants affect intraoperative responses, when applied as a thin film.

Cochlear Implants↗
PubMed1996

The surface Laplacian of the potential: theory and application.

The use of the surface Laplacian of the potential (Ls) in bioelectricity is discussed. Different estimates of Ls, in particular the field measured by coaxial electrodes, are compared to that of the true Laplacian. A method to compute Ls on the surface of an inhomogeneous volume conductor of arbitrary shape resulting from assumed electrical sources in introduced. In two applications the sensitivity of the body surface Laplacian is carried to that of body surface potentials. This comparison is carried out for dipolar sources within the human brain as well as for distributed sources within the heart.

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↗
PubMed1991

Modelling the fetal magnetocardiogram.

During gestation the abdominally recorded fetal ECG (FECG) changes as a result of changes in the conductive medium. In particular, a minimum occurs in the FECG amplitude around 30 weeks of gestation. Based on the results of an extensive FECG study, the effect of these changes on the fetal MCG are simulated. It is shown that the FMCG amplitude is not greatly affected by the changes in the conductive medium. The FMCG waveform, however, is.

Fetal Heart↗
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 fetal ECG throughout the second half of gestation.

The results of a study in which multi-lead simultaneous recordings of the abdominal fetal electrocardiogram (FECG) have been made are presented. A homogeneous volume conduction model based on the actual recorded geometry was made. Using this model, fetal vectorcardiograms (FVCGS) were computed. Before 28 weeks of gestation the inter- and intra-individual variability of the observed signals could be removed by computing FVCGs in a fetal reference frame. This opens up the possibility of using the shape of the QRS complex for diagnostic purposes.

Abdomen↗
PubMed1989

The effect of changes in the conductive medium on the fetal ECG throughout gestation.

The electrical conduction of the ECG from the fetal heart to the maternal abdomen has been modelled by using volume conductor models based on the measured actual geometry. The models have been verified by means of multi-lead recordings of the fetal ECG. The results show that early in pregnancy (less than 28 weeks) the conduction can successfully be described by an electrically homogeneous model. Based on this model, a description of the fetal ECG that is independent of the position of the fetus is derived. In late pregnancies, the conduction is dominated by the isolating effect of the vernix caseosa. As the distribution of the vernix over the fetal body is unknown, the shape of the fetal ECG is disturbed in an unintelligible way. As a consequence, caution has to be applied when using the shape of the fetal ECG for diagnostic means.

Electric Conductivity↗
PubMed1989

Source parameter estimation in inhomogeneous volume conductors of arbitrary shape.

In this paper it is demonstrated that the use of a direct matrix inverse in the solution of the forward problem in volume conduction problems greatly facilitates the application of standard, nonlinear parameter estimation procedures for finding the strength as well as the location of current sources inside an inhomogeneous volume conductor of arbitrary shape from potential measurements at the outer surface (inverse procedure). This, in turn, facilitates the inclusion of a priori constraints. Where possible, the performance of the method is compared to that of the Gabor-Nelson method. Applications are in the fields of bioelectricity (e.g., electrocardiography and electroencephalography).

Electric Conductivity↗
PubMed1986

The potential distribution generated by the fetal heart at the maternal abdomen.

The pathways along which the electrical currents generated by the fetal heart are conducted to the surface of the maternal abdomen are not known. As a consequence, in recording the fetal electrocardiogram (FECG) it is hard to predict where electrodes should be placed in order to obtain an optimal signal. The amplitude of the FECG varies with gestation, and there is a large interindividual variability in the amplitude of the FECG and in the optimal recording site among subjects within the same gestational age. Attempts have been made to explain these phenomena in terms of volume conduction. In this research the complete potential distribution on the maternal abdomen is studied in connection with the geometrical configuration of the electrical source (fetal heart) and the volume conductor (surrounding tissues). For a small group of pregnant women the abdominal FECG is recorded simultaneously in 32 leads during a period of about one minute, once every two weeks from 20 weeks of gestation onwards. A spatial filtering technique which combines information of all 32 leads is used to provide a trigger of the fetal QRS complexes. Using this trigger, an average fetal complex is constructed for each lead by time coherent averaging, after subtraction of the maternal contribution. These average fetal complexes are combined to plot the complete potential distribution generated by the fetal heart at the maternal abdomen (fetal body surface map, FBSM) at any given time instant during the fetal cardiac cycle. At these recording sessions the geometry is carefully quantified by making transverse scans every 2 cm with a compound echo scanner. The contours of fetal head and body, the placenta and the uterus are manually drawn on hardcopies of the video display images. Real time echoscopy is used to support the identification of the geometry. The contours are fed into a computer using a graphics tablet. The three dimensional surfaces of fetus, placenta and uterus are separately represented by a triangulation of the respective contour lines. Figures 5 and 6 show an example of the triangulated representation of the recorded geometry. Figure 7 shows the average fetal complexes of an individual at 26 weeks of gestation, plotted at the site where they have been recorded.(ABSTRACT TRUNCATED AT 400 WORDS)

Abdomen↗