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Biomedical subjects

A van Oosterom

Publications and source records attributed to A van Oosterom.

At least 55 records · Page 3Linked to original sources
PubMed1993

The number of signals in multilead ECGs in individuals.

The number of independent signals in multilead electrocardiograms (ECGs) (body surface maps) of individuals have been determined by using two different methods. Both methods are based on an analysis of the eigenvalues of the spatial covariance matrix corresponding to the individual multilead ECG. One method takes only the noise level into account (threshold method). The other method is based on an information theoretic criterion (minimum description length) and uses explicitly the statistical properties of the noise (noise covariance). The analysis was performed on the QRS complexes and ST-T complexes of the multilead ECG of 61 subjects. The noise was estimated from three isoelectric intervals (PQ interval). For the different subjects, 0-14 signals (with a mean of 7 signals) were detected in the QRS interval and 0-9 signals (with a mean of 3 signals) were detected in the ST-T interval.

Electrocardiography↗
PubMed1993

Coping with toxicities of docetaxel (Taxotere).

BACKGROUND: In phase II studies of the EORTC-ECTG, acute hypersensitivity reaction, edema and skin toxicity were important side effects of docetaxel (TaxotereTM). PATIENTS AND METHODS: Fifty patients with solid tumors were treated with docetaxel at the University Hospital of Antwerp. Of these, 28% developed an acute hypersensitivity reaction and 20% edema. Twelve and three hours before the next docetaxel administrations those with the acute hypersensitivity reaction were subsequently pretreated orally with 32 mg of methylprednisolone, 10 mg of cetirizine and 1 mg of ketotifen. CONCLUSIONS: The acute hypersensitivity reaction could be completely blocked in all but in 1 of 14 patients, and this pretreatment schedule had an important prophylactic effect on edema formation. Skin toxicity was successfully treated with an ointment of glycerin and chlorhexidine.

Antineoplastic Agents, Phytogenic↗
PubMed1992

Heart position and orientation in forward and inverse electrocardiography.

A study has been made of the influence of the position and orientation of the heart within the thorax on computed ECG waveforms (forward model) and on computed activation sequences (inverse model) in three normal cases. Results show that differences in heart position and orientation, associated with shifts relative to the precordium of the order of 0.5 cm, may result in amplitude differences or QRS waveforms of typically tenths or millivolts, which constitute part of the observed interindividual variability of the ECG. The inverse study shows that, in spite of similar errors in estimated heart position and orientation, stable solutions of the ventricular activation sequence can still be found. However, in the case where the heart is very close to precordial leads, the stability of the inverse solution is found to be intrinsically poor.

Electrocardiography↗
PubMed1992

Implicit and explicit constraints in inverse electrocardiography.

This paper reviews the major distributed source models that have been postulated over the years to support the interpretation of observed body surface potentials: double-layer models, the source description in terms of epicardial (ie, pericardial) potentials, and its equivalent: the distributed monolayer. This includes a presentation and discussion of a source model that has been developed over the past decade: the uniform double-layer model. The properties of this model are contrasted to those of other distributed source models from the perspective of their inherent capacity for imposing the constraints that are essential for regularizing the involved inverse problem.

Electrocardiography↗
PubMed1992

Electrical impedance tomography. Solving anisotropic mixed boundary value problems by means of the boundary element method.

In a previous study we have shown that an anisotropic volume conduction problem involving the Neumann condition at the outer surface of a volume conductor can be solved by means of the boundary element method by applying an appropriate coordinate transformation (Zhou and van Oosterom 1991). In this paper we will show that this approach can also be applied to solve the anisotropic mixed boundary value condition problems studied in electrical impedance tomography (EIT). Results are presented of numerical computations, which were carried out for a volume conductor of irregular (torso) shape.

Electric Conductivity↗
PubMed1992

Application of electrical impedance tomography to the determination of lung volume.

Trans-thoracical electrical impedance measurements on a human subject have been performed using the Sheffield equipment. These measured transfer impedances are compared with computed ones, based on an inhomogeneous, torso-shaped volume conductor, for different values of the overall electrical conductivities sigma l and sigma r of left and right lung. Each measured set of impedances is compared with all calculated sets. The remaining (minimum) residual difference epsilon is used as a measure of the goodness of fit, and the values of sigma l and sigma r for which epsilon is minimal are taken as representing the overall conductivities of the lungs. For data measured at various stages within the respiratory cycle, sigma l and sigma r have been determined. The results are found to relate well to physiology: for a measurement taken at maximum expiration sigma l and sigma r indicate almost homogeneity, whereas for end inspiratory level sigma l and sigma r tend towards values of 0.2, which is a realistic value for the relative conductivity of lung tissue.

Electric Conductivity↗
PubMed1992

Computation of the potential distribution in a four-layer anisotropic concentric spherical volume conductor.

A method for solving the potential distribution in a multilayer anisotropic concentric spherical volume conductor, which has recently been described in the literature, has been tested and found to be numerically unstable. In this paper it is demonstrated how these numerical difficulties can be avoided. Moreover, the method is extended by lifting the previously imposed restriction on the innermost region to be isotropic. A convergence criterion for determining the required number of terms in the final series expansion is proposed. The influences of radial and tangential conductivity values of the skull and brain tissue on the dipole-induced potential are investigated.

Algorithms↗
PubMed1992

Experience with ifosfamide in the EORTC Soft Tissue and Bone Sarcoma Group.

The Soft Tissue and Bone Sarcoma Group of the European Organization on Research and Treatment of Cancer has conducted a number of studies of chemotherapy in advanced disease over the past 15 years. Following the discovery that the CYVADIC regimen (cyclophosphamide/vincristine/doxorubicin/dimethyl imidazole carboxamide) was less active than had been reported originally, the individual components of CYVADIC were studied. This showed that doxorubicin had considerable activity and that ifosfamide 5 g/m2 given over 24 hours was at least as active as cyclophosphamide. Subsequent studies have therefore centered on a combination of doxorubicin and ifosfamide. A large randomized trial showed no significant benefit of doxorubicin/ifosfamide over single-agent doxorubicin, but the doses of doxorubicin were different. More recently, using bone marrow growth factors, it has been possible to increase the doxorubicin dose to that given as a single agent and also maintain the ifosfamide dose. This is now being tested in a randomized trial to determine whether improved response rates and other indicators of outcome will be seen.

Antineoplastic Combined Chemotherapy Protocols↗
PubMed1992

On the detection of the number of signals in multi-lead ECGs.

New methods which have been developed in the field of signal processing to detect the number of intrinsic source signals in a set of multiple signals have been studied in their application to the analysis of multilead ECGs (body surface maps). All these methods are based on the analysis of the eigenvalues of the covariance matrix related to the observed data. By using artificial multilead ECGs in which the properties of the signal and noise parts were known, the effect of taking into account these properties was investigated. It was found that the method based on information-theoretic criteria was superior in detecting the number of signal components, in the case that the noise in the data is non-white, as compared to a simple threshold method.

Data Interpretation, Statistical↗
PubMed1992

The performance of information-theoretic criteria in detecting the number of independent signals in multi-lead ECGs.

Three different methods to detect the number of independent signals in multilead ECGs were evaluated by using different ECG realizations with known specifications for signal and noise: a threshold method (TM), the minimum description length (MDL) and Akaike's information criterion (AIC). The fundamental assumption in this kind of signal processing is that both the signal and the noise stem from independent stochastic generators. The consequence is that the detection of the number of signals is only possible if the noise is white, or if the noise properties have been specified. The evaluation was performed with respect to the QRS complex of individual multilead ECG simulations and to the entire ensemble. In the simulated ECGs the number of independent signals was fixed (eight). It was found that, out of the three methods studied, the performance of MDL was the best, especially when the number of available observations in the noise (used to estimate the noise specifications) was moderate.

Data Collection↗
PubMed1991

A realistic torso model for magnetocardiography.

This paper contains the description of an inhomogeneous, multi-compartmental volume conductor model which is in use in our group. Although initially developed for the study of the ECG, it has been found to serve equally well for simulating the magnetocardiogram (MCG), the forward problem, and as a basis for source analysis of the MCG, the inverse problem. For both problems some illustrative examples are included demonstrating the necessity of using an inhomogeneous volume conductor model of the torso, having realistic--preferably tailored--geometry. A simple inverse procedure, based on correlation techniques, is included for the solution of the problem of source localization when an accurate description of the inhomogeneous volume conductor is available.

Adult↗
PubMed1991

Implications of macroscopic source strength on cardiac cellular activation models.

This paper compares several cellular-level models of cardiac activation according to the sources that are generated and their macroscopic fields. Since macroscopic field patterns and strengths are well documented, by comparing the models of cardiac activation, the more controversial microscopic processes can be evaluated. The results show that it is most likely that both macroscopic and microscopic activation of cardiac tissue are uniform processes, and in a direction that is transverse to the fiber axis.

Animals↗
PubMed1991

The Brody effect revisited.

This paper reexamines the Brody effect, both in the far-field and in the near-field approximation. It stresses the fact that near an inhomogeneity the Brody factor is not a constant but a function of space. A full documentation of this function for realistic values of the inhomogeneity as relevant to electrocardiography is included. The existence of a zone having "anomalous" Brody factors is demonstrated. Moreover, the importance for this problem of the zero reference point is stressed.

Blood Volume↗
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↗
PubMed1991

History and evolution of methods for solving the inverse problem.

This article serves as an introduction to the other articles in this issue devoted to the problem of the localization of neural generators. Elements of the theory of electric volume conduction are briefly introduced, as far as these apply to the interpretation of observed scalp potentials. First, some basic methods for display of the different aspects of the spatiotemporal information are described. Next, the most prominent source and volume conductor models that have been postulated for the involved forward problem are summarized. The problems of source identification and source localization, known as the inverse problem, are then formulated in terms of a parameter estimation procedure. The importance of introducing a priori information in the inverse problem, aimed at stabilizing (regularizing) the obtained solution, is emphasized. Methods for imposing such constraints are briefly outlined.

Biophysics↗
PubMed1991

The potential distribution generated by surface electrodes in inhomogeneous volume conductors of arbitrary shape.

The use of the boundary element technique in the computation of the potential distribution within isotropic inhomogeneous volume conductors of arbitrary shape set up by current injected through surface electrodes is presented. The derived algorithm is validated by comparing its solution to analytical solutions in the case of a concentric bipolar electrode configuration on a homogeneous, spherical volume conductor. This problem is essentially a mixed boundary value problem. It is shown that approximations by treating this problem as a Neumann problem, which have recently appeared in the literature, are valid for remote field points only. Applications to the modeling of the field of cardiac defibrillation electrodes are presented.

Algorithms↗
PubMed1991

Mathematical aspects of source modeling.

This paper reviews the properties of the major source models which are used in the analysis of observed bioelectric signals in general, and of evoked potentials in particular. The interpretation of such models demands the solution of an associated inverse problem. Emphasis is laid on the possibility of imposing such constraints on the model which render stable inverse solutions and yet allow a physiological interpretation. The nature of this paper is tutorial.

Bioelectric Energy Sources↗