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Biomedical subjects

G Huiskamp

Publications and source records attributed to G Huiskamp.

9 recordsLinked to original sources
PubMed2000

Temporally unconstrained space-time treatment of linear formulations of the inverse problem of electroencephalography.

This paper provides an optimal mechanism for the introduction of temporal constraints into linear imaging formulations of the inverse electroencephalography problem. The method is based on derivation of a "virtual-SVD," an extension of generalized singular value decomposition to the setting of random matrices. Surprisingly, the formalism is superior, in principle, to standard regularization methods even in the absence of known temporal constraints. Investigation of this basic temporally unconstrained setting was undertaken to illustrate the application of the method, and as a necessary first step in its systematic evaluation. Although abstract simulations demonstrate superior accuracy for the virtual-SVD method as compared with standard methods, investigation of a particular realistic simulation involving spatiotemporally distributed temporal lobe interictal spikes indicates that significant improvement in solution estimate quality under temporally unconstrained conditions may be limited to a very narrow range of the signal-to-noise ratio (particularly in the context of a markedly row-deficient transfer matrix). These results underline the prospective importance of investigation of the efficacy and feasibility of application of temporal constraints (such as those resulting from knowledge of the general time series format of epilepsy associated wave forms, evoked potentials, etc.) within the derived formalism.

Biomedical Engineering↗
PubMed1999

Modelling surface potentials from intracochlear electrical stimulation.

Volume conduction models were used qualitatively to model surface potentials from cochlear implant patients recorded earlier by the authors. These recorded potentials reflected the equivalent dipole orientation in the head in patients who are deaf due to otosclerosis, but increased uniformly with the distance between the stimulating electrodes along the basilar membrane in other patients, which suggested a low and high resistivity of the cochlear bone, respectively. Several models of the head were constructed, with compartments representing the skin, skull, brain, cochlea, internal and external ear canal. In the "petrous bone" model, the cochlea was modelled as a cavity in a bony layer surrounded by the brain compartment. Of all models, the petrous bone model using a high resistivity ratio (1:100) between the bony and the other compartments was the only one that produced outcomes similar to the potentials observed in non-otosclerosis patients. In conclusion, the results suggested that the surface potentials observed in non-otosclerosis patients are sufficiently explained by a high impedance between cochlear turns and a non-specific return of current via the wall of the petrous bone into the larger brain compartment.

Cochlea↗
PubMed1999

The need for correct realistic geometry in the inverse EEG problem.

For accurate electroencephalogram-based localization of mesial temporal and frontal sources correct modeling of skull shape and thickness is required. In a simulation study in which results for matched sets of computed tomography and magnetic resonance (MR) images are compared, it is found that errors arising from skull models based on smooth and inflated segmented MR images of the cortex are of the order of 1 cm. These errors are comparable to those found when overestimating or underestimating skull conductivity by a factor of two.

Brain↗
PubMed1998

An improved method for estimating epicardial potentials from the body surface.

We present a new method for regularizing the illposed problem of computing epicardial potentials from body surface potentials. The method simultaneously regularizes the equations associated with all time points, and relies on a new theorem which states that a solution based on optimal regularization of each integral equation associated with each principal component of the data will be more accurate than a solution based on optimal regularization of each integral equation associated with each time point. The theorem is illustrated with simulations mimicking the complexity of the inverse electrocardiography problem. As must be expected from a method which imposes no additional a priori constraints, the new approach addresses uncorrelated noise only, and in the presence of dominating correlated noise it is only successful in producing a "cleaner" version of a necessarily compromised solution. Nevertheless, in principle, the new method is always preferred to the standard approach, since it (without penalty) eliminates pure noise that would otherwise be present in the solution estimate.

Computer Simulation↗
PubMed1998

Simulation of depolarization in a membrane-equations-based model of the anisotropic ventricle.

The results of a simulation study of the propagation of depolarization in inhomogeneous anisotropic (monodomain) myocardial tissue are presented. Simulations are based on modified Beeler-Reuter membrane equations, and performed on a block of anisotropic myocardium with rotating fiber geometry, measuring 1 cm x 1 cm x 0.3 cm, at various levels of spatial discretization (0.15 mm, 0.30 mm, 0.60 mm). At a discretization level of 0.6 mm the algorithm allowed the simulation in a realistically shaped model of the ventricle, including rotational anisotropy, as well. For this simulation results are justified by comparing results for the block at various levels of discretization, for which the surface to volume ratio has been adjusted. By placing the model ventricle in a realistically shaped (human) volume conductor model, realistic body surface potentials (QRST waveforms) are simulated.

Anisotropy↗
PubMed1997

A new method for myocardial activation imaging.

Noninvasive images of the myocardial activation sequence are acquired, based on a new formulation of the inverse problem of electrocardiography in terms of the critical points of the ventricular surface activation map. It is shown that the method is stable with respect to substantial amounts of correlated noise common in the measurements and modeling of electrocardiography and that problems associated with conventional regularization techniques can be circumvented. Examples of application of the method to measured human data are presented. This first invasive validation of results compares well to previously published results obtained by using a standard approach. The method can provide additional constraints on, and thus improve, traditional methods aimed at solving the inverse problem of electrocardiography.

Algorithms↗
PubMed1989

Tailored versus realistic geometry in the inverse problem of electrocardiography.

The stability and applicability of a previously developed inverse procedure for the noninvasive determination of the activation sequence of the human heart has been evaluated. In particular, the possibility of using a standard geometrical configuration representing the heart and the inhomogeneous volume conductor in this procedure has been tested. Results show that in order to obtain reliable inverse solutions, true "tailored" geometry should be used.

Electric Conductivity↗
PubMedDate not supplied

New quantitative and qualitative approaches to the inverse problem of electrocardiology: their theoretical relationship and experimental consistency.

In addition to formidable theoretical obstacles that a proposed solution to the inverse electrocardiology problem must overcome, there are great practical difficulties in establishing its accuracy in actual clinical application. However, the recent appearance of two fundamentally independent treatments of the inverse problem raises the possibility that they may be used in tandem to help establish their individual accuracy. Thus, if the two methods give incompatible results in application then one of the methods must be inaccurate. Conversely, if the two methods give compatible results then the accuracy of both methods is supported (for the particular quantities measured) subject only to the validity of the assumptions common to both methods. We have compared results from the application of a quantitative "integral equations based" method with that of a qualitative "differential topology inspired" approach in three healthy volunteers. The output examined consists of measurements of the times of appearance of epicardial sources (depolarization wavefront breakthroughs) and sinks of the ventricular surface activation map. The extent of agreement on source/sink times between the methods was consistent with the resolution limits imposed by noise and discrete sampling on derivatives of the electrocardiogram. When events defined by the integral method occurring within 2 ms of each other are grouped together (and their times averaged), the two methods agreed on source/sink times to within 3 ms except in two instances where they differed by 5 ms. The measurements made by the two methods were found to be highly correlated (R = 0.95). While the quantitative method alone rests on a variety of modeling and procedural assumptions, the only assumption common to both methods is the uniform dipole layer hypothesis. Thus, subject to this single assumption, one may infer the accuracy of the quantitative method in healthy individuals for epicardial source/sink times. On the other hand, coupling with the far more detailed quantitative method allows further useful characterization of the output of the qualitative method. In particular, this study provides convincing evidence that the major deflections of the spatial velocity electrocardiogram are coupled to particular epicardial sources and sinks, as has been previously conjectured on theoretical grounds. This raises the possibility of bedside evaluation of these epicardial events.

Electrocardiography↗