Publications

Abstract:

Non-invasive localization of premature ventricular beat (PVB) foci is very important for medical treatment of numerous cardiac diseases. In this work a model-based method of reconstruction of ectopic center locations is investigated.Within the scope of this method patient's multichannel ECG is used as a reference for optimization of an electrophysiological cardiac model. This model is based on the cellular automaton principle and utilizes anatomical data of the patient. Optimized are coordinates of the ectopic focus as well as excitation conduction velocity of ventricular myocardium. Initial values for these parameters are obtained by solving the linearized problem of electrocardiography in terms of activation times. Optimization is performed by minimization of discrepancy between the simulated and reference ECGs.The aim of the current work is to estimate the quality of ectopic focus localization delivered by this method. Four sample ectopic beats have been simulated, with their foci located in different regions of the left ventricle. 1% Gaussian noise has been introduced into the resulting ECGs. In this way the "measured" ECG signals for this investigation have been obtained. Afterwards the origin of each ectopic beat has been reconstructed using the model-based approach. The method has demonstrated reliable localization of PVB foci, reconstruction errors have not exceeded 6.1 mm.

Abstract:

The distributions of transmembrane voltage (TMV) within the cardiac tissue are linearly connected with the patient's body surface potential maps (BSPMs) at every time instant. The matrix describing the relation between the respective distributions is referred to as the transfer matrix. This matrix can be employed to carry out forward calculations in order to find the BSPM for any given distribution of TMV inside the heart. Its inverse can be used to reconstruct the cardiac activity non-invasively, which can be an important diagnostic tool in the clinical practice.The computation of this matrix using the finite element method can be quite time-consuming. In this work, a method is proposed allowing to speed up this process by computing an approximate transfer matrix instead of the precise one. The method is tested on three realistic anatomical models of real-world patients. It is shown that the computation time can be reduced by 50% without loss of accuracy.

Abstract:

In this work an optimization-based method of modeling the cardiac activity is presented. The method employs a personalized anatomical 3D model of the patients thorax provided by the segmentation of MRI data as well as an electrophysiological model of the heart.Cellular automaton is used to model the propagation of depolarization and repolarization fronts through the myocardium. The form of action potential (AP) curves was previously derived from the coupled myocardium cell models developed by Noble, Priebe-Beuckelmann and ten Tusscher. The results provided by these three cell models are compared.A series of body surface potential maps (BSPMs) is calculated, the signals on the nodes representing the electrodes are recorded, providing thus a simulated multichannel ECG. A root-mean-square of the difference between simulated and measured ECGs is taken as a criterion for optimization of heart model parameters.The method provides a time-dependent distribution of transmembrane voltages within the heart muscle of a patient.

Abstract:

Over the last years, nonfluoroscopic in vivo cardiac mapping and navigation systems have been developed and successfully applied in clinical electrophysiology. Clearly, a trend can be observed to introduce more sensors into the measurement system so that physiological information can be gathered simultaneously and more efficiently and the duration of procedure can be shortened significantly. However, it would not be realistic to equip each catheter electrode with a localizer, e.g., by embedding a miniature magnetic location sensor. Therefore, in this paper, an alternate approach has been worked out to efficiently localize multiple catheter electrodes by considering the impedance between electrodes in the heart and electrode patches on the body surface. In application of the new technique, no additional expensive and sophisticated hardware is required other than the currently existing cardiac navigation system. A tank model and a computerized realistic human model are employed to support the development of the positioning system. In the simulation study, the new approach achieves an average localization error of less than 1 mm, which proves the feasibility of the impedance-based catheter positioning system. Consequently, the new positioning system can provide an inexpensive and accurate solution to improve the efficiency and efficacy of catheter ablation.

Abstract:

BACKGROUND: The efficacy of cardiac resynchronization therapy through biventricular pacing (BVP) has been demonstrated by numerous studies in patients suffering from congestive heart failure. In order to achieve a guideline for optimal treatment with BVP devices, an automated non-invasive strategy based on a computer model of the heart is presented. MATERIALS AND METHODS: The presented research investigates an off-line optimization algorithm regarding electrode positioning and timing delays. The efficacy of the algorithm is demonstrated in four patients suffering from left bundle branch block (LBBB) and myocardial infarction (MI). The computer model of the heart was used to simulate the LBBB in addition to several MI allocations according to the different left ventricular subdivisions introduced by the American Heart Association. Furthermore, simulations with reduced interventricular conduction velocity were performed in order to model interventricular excitation conduction delay. More than 800,000 simulations were carried out by adjusting a variety of 121 pairs of atrioventricular and interventricular delays and 36 different electrode positioning set-ups. Additionally, three different conduction velocities were examined. The optimization measures included the minimum root mean square error (E(RMS)) between physiological, pathological and therapeutic excitation, and also the difference of QRS-complex duration. Both of these measures were computed automatically. RESULTS: Depending on the patient's pathology and conduction velocity, a reduction of E(RMS) between physiological and therapeutic excitation could be reached. For each patient and pathology, an optimal pacing electrode pair was determined. The results demonstrated the importance of an individual adjustment of BVP parameters to the patient's anatomy and pathology. CONCLUSION: This work proposes a novel non-invasive optimization algorithm to find the best electrode positioning sites and timing delays for BVP in patients with LBBB and MI. This algorithm can be used to plan an optimal therapy for an individual patient.

Abstract:

Reduced cardiac output, dysfunction of the conduction system, atrio-ventricular block, bundle branch blocks and remodeling of the chambers are results of congestive heart failure (CHF). Biventricular pacing as Cardiac Resynchronization Therapy (CRT) is a recognized therapy for the treatment of heart failure. The present paper investigates an automated non-invasive strategy to optimize CRT with respect to electrode positioning and timing delays based on a complex threedimensional computer model of the human heart. The anatomical model chosen for this study was the segmented data set of the Visible Man and a set of patient data with dilated ventricles and left bundle branch block. The excitation propagation and intra-ventricular conduction were simulated with Ten Tusscher electrophysiological cell model and adaptive cellular automaton. The pathologies simulated were a total atrioventricular (AV) block and a left bundle branch block (LBBB) in conjunction with reduced interventricular conduction velocities. The simulated activation times of different myocytes in the healthy and diseased heart model are compared in terms of root mean square error. The outcomes of the investigation show that the positioning of the electrodes, with respect to proper timing delay influences the efficiency of the resynchronization therapy. The proposed method may assist the surgeon in therapy planning.

Abstract:

An optimal electrode position, atrio-ventricular (AV) and interventricular (VV) delay in cardiac resynchronization therapy (CRT) improves its success. An optimization strategy does not yet exist. A computer model of the Visible Man and a patient heart was used to simulate an atrio-ventricular and a left bundle branch block with 0%, 20% and 40% reduction in interventricular conduction velocity, respectively. The minimum error between physiological excitation and pathology/therapy was automatically computed for 12 different electrode positions. AV and VV delay timing was adjusted accordingly. The results show the importance of individually adjusting the electrode position as well as the timing delays to the patient's anatomy and pathology, which is in accordance with current clinical studies. The presented methods and strategy offer the opportunity to carry out non-invasive, automatic optimization of CRT preoperatively. The model is subject to validation in future clinical studies.

Abstract:

Es wird eine Methode beschrieben, wie medizinische Bilder des Herzens modellbasiert mit EKG-Daten verknüpft werden können, um damit zu einer spezifischen Diagnostik und zu einer besseren Therapieplanung in der Kardiologie zu gelangen. Zunächst wird aus MRT- oder CT-Bildern des Patienten die Geometrie seines Herzens ermittelt. Elektrokardiographische Messungen an der Körperoberfläche (EKG oder Body Surface Potential Mapping) und aus dem Inneren des Herzens (intracardial mapping) werden aufgenommen und die Orte der Messung in den Bilddatensatz eingetragen (registration). Ein elektrophysiologisches Computermodell vom Herzen des Patienten wird mit Hilfe der elektrophysiologischen Messdaten iterativ angepasst. Schließlich entsteht im Computer ein virtuelles Herz des Patienten, welches sowohl die Geometrie als auch die Elektrophysiologie wiedergibt. Ein Modell der Vorhöfe hat beispielsweise das Potenzial, die Ursachen von Vorhofflimmern zu erkennen und die Radiofrequenz-Ablationsstrategie zu optimieren. Ein Modell der Ventrikel des Herzens kann helfen, genetisch bedingte Rhythmusstörungen besser zu verstehen oder auch die Parameter bei der kardialen Resynchronisationstherapie zu optimieren. Die Modellierung des Herzens mit einem Infarktgebiet könnte die elektrophysiologischen Auswirkungen des Infarktes beschreiben und die Risikostratifizierung für gefährliche ventrikuläre Arrhythmien unterstützen oder die Erfolgsrate bei ventrikulären Ablationen erhöhen.

Abstract:

A model-based approach to noninvasively determine the location and size of the infarction scar is proposed, that in addition helps to estimate the risk of arrhythmias. The approach is based on the optimization of an electrophysiological heart model with an introduced infarction scar to fit the multichannel ECG measured on the surface of the patient's thorax. This model delivers the distributions of transmembrane voltages (TMV) within the ventricles during a single heart cycle. The forward problem of electrocardiography is solved in order to obtain the simulated ECG of the patient. This ECG is compared with the measured one, the difference is used as the criterion for optimization of model parameters, which include the site and size of infarction scar.

Abstract:

The forward problem of electrocardiography aims at obtaining a better understanding of cardiac electrophysiological activities, by means of computer modeling and simulation. Whereas, the inverse electrocardiographic problem provides a direct insight of electrical sources into the heart without interventional procedures. Nowadays, the forward and inverse problems are mostly solved in static models, which do not take into account heart motion and respiration. Besides heart motion, neglecting respiration may also lead to remarkable uncertainties in both forward and inverse solutions. In the present work a dynamic lung model is developed. With this model the effect of respiration on the forward and inverse solutions is studied.

Abstract:

After mathematical modeling of the healthy heart now modeling of diseases comes into the focus of research. Modeling of arrhythmias already shows a large degree of realism. This offers the chance of more detailed diagnosis and computer assisted therapy planning. Options for genetic diseases (channelopathies like Long-QT-syndrome), infarction and infarction-induced ventricular fibrillation, atrial fibrillation (AF) and cardiac resynchronization therapy are demonstrated.

Abstract:

Ventricular premature beats (VPB) occur when a cardiac depolarization is initiated from a focus in the ventricle instead of the sinoatrial node. Because the ventricular electrical excitation is not started from the intraventriclular conduction system, the excitation propagation in the ventricles behaves in an abnormal manner. This results in an extra asynchronous contraction of the ventricles. In addition VPBs can trigger life-threatening heart arrhythmias. Applying catheter ablation can cure VPB. Therefore it is of importance to localize the origin of VPB using a non-invasive approach before interventional treatment. In this work the inverse problem of electrocardiography is deployed to reconstruct electrical sources in the ventricles, from which the origin of VPB can be identified. By using a spatiotemporal maximum a posteriori (MAP) based regularization the quality of reconstructions is improved. In this work forward calculations with various VPBs are employed to construct a statistical a priori information.

Abstract:

Solving the forward problem of electrocardiography provides a better understanding of electrical activities in the heart. The inverse problem of electrocardiography enables a direct view of cardiac sources without catheter interventions. Today the forward and inverse computation is most often performed in a static model, which doesn't take into account the heart motion and may result in considerable errors in both forward and inverse solutions. In this work a dynamic heart model is developed. With this model the effect of the heart motion on the forward and inverse solutions is investigated.

Abstract:

Many studies conducted on patients suffering from congestive heart failure have shown the efficacy of cardiac resynchronization therapy (CRT). The presented research investigates an off-line optimization algorithm based on different electrode positioning and timing delays. A computer model of the heart was used to simulate left bundle branch block (LBBB), myocardial infarction (MI) and reduction of intraventricular conduction velocity in order to customize the patient symptom. The optimization method evaluates the error between the healthy heart and pathology with/without pacing in terms of activation time and QRS length. Additionally, a torso model of the patient is extracted to compute the body surface potential map (BSPM) and to simulate the ECG with Wilson leads to validate the results obtained by the electrophysiological heart model optimization.

Abstract:

The treatment of ventricular arrhythmia often requires detailed information about the location of ectopic beats. A noninvasive procedure is adopted to achieve this purpose. The aim of this work is the reconstruction of ectopic foci by using the critical point theory introduced by Greensite and Huiskamp [1]. The reliability and adaptability of the obtained simulation results are evaluated with regard to the reconstruction error.The reconstruction of bioelectrical sources from measured body surface potentials is an ill-posed problem and requires regularization. The advantage of the presented method is to deal with a well-posed formulation of the problem. Locations of ectopic beats can be detected by the critical point theorem. Four simulated ectopic centers have been localized to evaluate the method. The influence of Gaussian noise is considered.The reconstruction depends on the effective rank of a singular value decomposition (SVD) of the multi-channel ECG matrix. Regarding lower ranks, many critical points presenting ectopic foci can be observed. For higher ranks, the detection leads more and more to a stable estimation of ectopic locations. The detected critical points are shown to be reliable approximations of the simulated ectopic foci.

Abstract:

A computer model of the human heart is presented, that starts with the electrophysiology of single myocardial cells including all relevant ion channels, spans the de- and repolarization of the heart including the generation of the Electrocardiogram (ECG) and ends with the contraction of the heart that can be measured using 4D Magnetic Resonance Imaging (MRI). The model can be used to better understand physiology and pathophysiology of the heart, to improve diagnostics of infarction and arrhythmia and to enable quantitative therapy planning. It can also be used as a regularization tool to gain better solutions of the ill-posed inverse problem of ECG. Movies of the evolution of electrophysiology of the heart can be reconstructed from Body Surface Potential Maps (BSPM) and MRI, leading to a new non-invasive medical imaging technique.

Abstract:

The congenital long-QT syndrome is commonly associated with a high risk for polymorphic ventricular tachy-cardia and sudden cardiac death. This is probably due to an intensification of the intrinsic heterogeneities present in ventricular myocardium. Increasing the electrophysiological heterogeneities amplifies the dispersion of repolarization which directly affects the morphology of the T wave in the ECG. The aim of this work is to investigate the effects of LQT2, a specific subtype of the long-QT syndrome (LQTS), on the Body Surface Potential Maps (BSPM) and the ECG. In this context a three-dimensional, heterogeneous model of the human ventricles is used to simulate both physiological and pathological excitation propagation. The results are used as input for the forward calculation of the BSPM and ECG. Characteristic QT prolongation is simulated correctly. The main goal of this study is to prepare and evaluate a simulation environment that can be used prospectivley to find features in the ECG or the BSPM that are characteristic for the LQTS. Such features might be used to facilitate the identification of LQTS patients.

Abstract:

Congestive heart failure (CHF) is affecting more than 15 million people in the western population with an increasing number. Biventricular pacing as Cardiac Resynchronization Therapy (CRT) is a recognized therapy for the treatment of heart failure. The present paper investigates the optimal pacing sites and stimuli delays for stimulation, based on a complex three-dimensional computer model of the human heart. The anatomical features were derived from the Visible Man data set. The excitation propagation and intraventricular conduction were simulated with Ten Tusscher electrophysiological cell model and an adaptive cellular automaton. Biventricular pacing in AV block III and LBBB with different interventricular conduction delays were investigated. The simulated activation times of different myocytes in the healthy and diseased heart model are compared in terms of root mean square error (ERMS). The outcomes of the investigation underline that the positioning of the electrodes considering a proper atrioventricular and intraventricular delay influences the efficiency of the resynchronization therapy. The results of this optimization strategy may assist the surgeon in therapy planning.

Abstract:

Heterogeneity of ion channel properties within human ventricular tissue determines the sequence of repolarization under healthy conditions. In this computational study, the impact of different extend of electrophysiological heterogeneity in both human ventricles on the ECG was investigated by a forward calculation of the cardiac electrical signals on the body surface. The gradients ranged from solely transmural, interventricular and apico-basal up to full combination of these variations. As long interventricular heterogeneities were neglected, the transmural gradient generated a positive T wave that was increased when apico-basal variations were considered. Inclusion of interventricular changes necessitated the incorporation of both transmural and apico-basal heterogeneities to reproduce the positive T wave.

Abstract:

The approach to solve the inverse problem of electrocardiography presented here is using a computer model of the individual heart of a patient. It is based on a 3D-MRI dataset. Electrophysiologically important tissue classes are incorporated using rules. Source distributions inside the heart are simulated using a cellular automaton. Finite Element Method is used to calculate the corresponding body surface potential map. Characteristic parameters like duration and amplitude of transmembrane potential or velocity of propagation are optimized for selected tissue classes or regions in the heart so that simulated data fit to the measured data. This way the source distribution and its time course of an individual patient can be reconstructed.

Abstract:

After myocardial infarction, ischemic lesions within the myocardium can be the origin of malignant arrhythmias by the mechanism of re-entry. Surface-ECG and MR-imaging data can be used to detect and classify such re- gions in a non-invasive way. For this purpose a model of the electric conductivity of the tissues within the pa- tients chest and a model of cardiac sources must be constructed out of MR-imaging data. Employing finite- element algorithms the inverse problem of electrocardiology can then be solved, leading to the reconstructionof electrical sources within the myocardium during the process of depolarisation and repolarisation.