G Huiskamp

University Medical Center Utrecht, Utrecht, Utrecht, Netherlands

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Publications (21)20.23 Total impact

  • G. Huiskamp
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    ABSTRACT: Interictal spikes as observed in epilepsy patients are assumed to be generated by relatively large patches of activated cortex. In order to check the validity of single dipole solutions to such spikes in EEG and MEG a simulation study is performed in a realistically shaped cortical model. It is shown that both in EEG and MEG the center of activated cortex can be misrepresented by single dipole solutions by more then 1 cm. The geometry of sulci and gyri determines where, for which modality, this effect is larger.
    Noninvasive Functional Source Imaging of the Brain and Heart & 2011 8th International Conference on Bioelectromagnetism (NFSI & ICBEM), 2011 8th International Symposium on; 01/2011
  • Source
    G Huiskamp, T Oostendorp, F. Leijten
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    ABSTRACT: this paper, we investigate the interpretability and validity of dipole source localization based on EEG, MEG and simultaneous EEG+MEG for TLE interictal spikes. Real validation can be attempted if epicorticograms (ECoG's) recorded prior to resection are available. It must be realized, however, that ECoG's are remote measurements as well, and that often they don't provide exact localization information either. Therefore we simulated ECoG's as well, in order to investigate whether they can be used to validate results obtained non-invasively. Finally we looked at a set of real simultaneous EEG+MEG data from a patient of which an ECoG, recorded later, prior to surgery, was available as well. 2 Methods We set up a double layer source configuration on two areas of a triangulation of the left temporal lobe (fig. 2). The areas were switched on in sequence and overlapped in time. From this a reference EEG (63 electrodes), MEG (151 axial gradiometers) and ECoG (20 electrodes) were computed by placing the source in a realistic volume conductor model. This model contained a skin- (1527 points, 0.33 ), a skull- ) and a brain- compartment ). Spatially correlated noise was generated by placing 2121 dipoles at random positions in the cortex in the same volume conductor model. The source strength functions of all of these dipoles were temporally uncorrelated. This noise was added to the simulated EEG and MEG and bandpass filtered between 0.7 and 70 Hz to mimic real measurements. This resulted in a SNR of 3.4 for the EEG and 1.5 for the MEG. When added to the ECoG the resulting SNR was 6.8. A two stationary dipoles model was fitted to the EEG, the MEG and to the simultaneous EEG+MEG. Dipole positions and strengths were compared to the original distributed source configuration, and...
    12/2001;
  • F Greensite, G Huiskamp
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    ABSTRACT: 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.
    Annals of Biomedical Engineering 02/2000; 28(10):1253-68. · 3.23 Impact Factor
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    ABSTRACT: 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.
    IEEE Transactions on Biomedical Engineering 12/1999; 46(11):1281-7. · 2.35 Impact Factor
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    ABSTRACT: 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.
    Scandinavian Audiology 02/1999; 28(4):249-55.
  • Geertjan Huiskamp, Fred Greensite
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    ABSTRACT: New methods of adding temporal constraints to the inverse EEG problem for spatially constrained distributed sources are presented and compared to standard minimum norm solutions. It is shown that under assumption of uncorrelated noise the new methods allow for a more accurate determination of location and timing of large scale cortical electrical activity
    01/1999; 1.
  • R. Hoekema, G. Huiskamp, G. Wieneke
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    ABSTRACT: In epilepsy surgery, electrode grids are often used to measure electrocorticograms (ECoGs). In these electrode grids, the distance between contacts is usually 10 mm. This may not be adequate to sample neocortical electrical activity. In this study, ECoGs were simulated, assuming the location of the electrical sources in the brain generating epileptic spikes. It is concluded that for adequately measuring the electric field generated by cortical sources, the contact distance should be decreased to 5 mm
    01/1999; 1.
  • G Huiskamp
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    ABSTRACT: 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.
    IEEE Transactions on Biomedical Engineering 08/1998; 45(7):847-55. · 2.35 Impact Factor
  • F Greensite, G Huiskamp
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    ABSTRACT: 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.
    IEEE Transactions on Biomedical Engineering 02/1998; 45(1):98-104. · 2.35 Impact Factor
  • G Huiskamp, F Greensite
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    ABSTRACT: 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.
    IEEE Transactions on Biomedical Engineering 07/1997; 44(6):433-46. · 2.35 Impact Factor
  • T. Oostendorp, J. Nenonen, G. Huiskamp
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    ABSTRACT: The ECG and MCG of a healthy subject were recorded simultaneously. The activation sequence at the surface of the heart was estimated independently from the ECG and the MCG data using individual torso geometry and the uniform double layer source model. The similarity between the two solutions indicates that ECG and MCG perform equally well in cardiac source estimation
    Engineering in Medicine and Biology Society, 1996. Bridging Disciplines for Biomedicine. Proceedings of the 18th Annual International Conference of the IEEE; 01/1996
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    ABSTRACT: Two formulations of the inverse problem of electrocardiography are presented which implicitly contain physiologically based spatio-temporal constraints. One formulation is based on the UDL model of ventricular activation, the other on a new, well posed formulation of the inverse problem. When applied to data of a WPW patient, results show agreement with invasive information regarding the site of pre-excitation, right ventricular epicardial breakthrough and termination of activation
    Engineering in Medicine and Biology Society, 1995., IEEE 17th Annual Conference; 10/1995
  • F. Greensite, Yao-Jin Qian, G. Huiskamp
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    ABSTRACT: We present a theorem which is the basis of a new approach to imaging ventricular surface activation. This is used to provide motivation for an algorithm which is presented and applied elsewhere in this conference
    Engineering in Medicine and Biology Society, 1995., IEEE 17th Annual Conference; 10/1995
  • F Greensite, Yao-Jin Qian, G Huiskamp
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    ABSTRACT: We reduce the task of noninvasive computation of the ventricular surface activation map to a well-posed problem. The resulting algorithm is shown to lead to stably computed solution generators (activation map extrema) in both a realistic numerical simulation, and with human data
    Engineering in Medicine and Biology Society, 1994. Engineering Advances: New Opportunities for Biomedical Engineers. Proceedings of the 16th Annual International Conference of the IEEE; 12/1994
  • G Huiskamp, Yao-Jin Qian, F Greensite
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    ABSTRACT: A method is evaluated for computing images of myocardial activation based on a well-posed formulation of the inverse problem of electrocardiography. The method may be interpreted as an adaptation of an existing method for solving an ill-posed formulation of the inverse problem that is based on Tikhonov regularization. It is shown that problems associated with conventional regularization techniques can be circumvented
    Engineering in Medicine and Biology Society, 1994. Engineering Advances: New Opportunities for Biomedical Engineers. Proceedings of the 16th Annual International Conference of the IEEE; 12/1994
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    ABSTRACT: In this simulation study the site of earliest activation is estimated using a single reference set of activation data and resulting electrocardiograms (ECGs) to which different noise realizations have been added. The estimated site of earliest ventricular depolarization found by using the single dipole model (SDM) and the one found by using the double layer model (DLM) are compared to the actual site of the reference activation sequence. The results show that the DLM is less sensitive to noise in the data than the SDM
    Computers in Cardiology 1991, Proceedings.; 10/1991
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    ABSTRACT: 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.
    Medical Physics 01/1990; 17(3):369-79. · 2.91 Impact Factor
  • G. Huiskamp, A. van Oosterom
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    ABSTRACT: A simulation study on the ECG is performed using the measured thorax geometry of a healthy subject and a measured ventricular activation sequence, known from the literature. Resulting QRS waveforms are compared with those actually measured on the subject. An adapted activation sequence is derived which, while differing only a small amount from the measured data, produces ECGs more closely resembling the ones measured on the subject. Measurements of the QRS complexes of the healthy subject have been performed using a 64-channel body-surface mapping system
    Engineering in Medicine and Biology Society, 1989. Images of the Twenty-First Century., Proceedings of the Annual International Conference of the IEEE Engineering in; 12/1989
  • G Huiskamp, A van Oosterom
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    ABSTRACT: 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.
    IEEE Transactions on Biomedical Engineering 09/1989; 36(8):827-35. · 2.35 Impact Factor
  • G Huiskamp, A Van Oosterom
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    ABSTRACT: A method for computing the activation sequence at the ventricular surface from body surface potentials, has been adapted to handle measured data. By using measured anatomical data together with a 64-channel ECG (electrocardiogram) recording of the same subject for three subjects, it is shown that the model is able to determine activation sequences on the heart surface which closely resemble similar data obtained through invasive measurement as reported in literature.
    IEEE Transactions on Biomedical Engineering 01/1989; 35(12):1047-58. · 2.35 Impact Factor

Publication Stats

517 Citations
20.23 Total Impact Points

Institutions

  • 1998–2011
    • University Medical Center Utrecht
      • Department of Neurology
      Utrecht, Utrecht, Netherlands
  • 1990–2000
    • University of California, Irvine
      • Department of Radiological Sciences
      Irvine, CA, United States
  • 1989–1997
    • Radboud University Nijmegen
      Nymegen, Gelderland, Netherlands