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.
I. M. Graf, G. Seemann, D. L. Weiss, and O. Dössel. Influence of electrophysiological heterogeneity on electrical stimulation in healthy and failing human hearts. In Medical & Biological Engineering & Computing, vol. 43(6) , pp. 783-792, 2005
The application of strong electrical stimuli is a common method used for terminating irregular cardiac behaviour. The study presents the influence of electrophysiological heterogeneity on the response of human hearts to electrical stimulation. The human electrophysiology was simulated using the ten Tusscher-Noble-Noble-Panfilov cell model. The anisotropic propagation of depolarisation in three-dimensional virtual myocardial preparations was calculated using bidomain equations. The research was carried out on different types of virtual cardiac wedge. The selection of the modelling parameters emphasises the influence of cellular electrophysiology on the response of the human myocardium to electrical stimulation. The simulations were initially performed on a virtual cardiac control model characterised by electrophysiological homogeneity. The second preparation incorporated the transmural electrophysiological heterogeneity characteristic of the healthy human heart. In the third model type, the normal electrophysiological heterogeneity was modified by the conditions of heart failure. The main currents responsible for repolarisation (Ito, IKs and IKI) were reduced by 25%. Successively, [Na+]i was increased by the regulation of the Na+-Ca2+ exchange function, and fibrosis was represented by decreasing electrical conductivity. Various electrical stimulation configurations were used to investigate the differences in the responses of the three different models. Monophasic and biphasic electrical stimuli were applied through rectangular paddles and needle electrodes. A whole systolic period was simulated. The distribution of the transmembrane voltage indicated that the modification of electrophysiological heterogeneity induced drastic changes during the repolarisation phase. The results illustrated that each of the heart failure conditions amplifies the modification of the response of the myocardium to electrical stimulation. Therefore a theoretical model of the failing human heart must incorporate all the characteristic features.
In this study the performance of a planar array for magnetic induction tomography (MIT) was investigated and the results of measurements to determine the precision and sensitivity of the sensor were undertaken. A planar-array MIT system utilizing flux-linkage minimization for the primary field has been constructed and evaluated. The system comprises 4 printed excitation coils of 4 turns which were shielded, 8 surface-mount inductors of inductance 10 microH as sensor, mounted such that in principle no primary-field flux threads them, and a calibration coil to produce a strong primary field. The excitation current was multiplexed via relays to drive the excitation and reference coils. The noise values were similar in real and imaginary components in the lower frequencies and the factor to which the primary field could be reduced was greatest in the nearest coil. Methods for determining the true real and imaginary components and for flux-linkage minimization for the primary field for variations in channel sensitivities are described and the results of measurements of the system's noise and drift are given. A SNR of 47 dB was observed at 4 MHz when a 0.3 Sm-1 saline filled tank of dimensions 20 cmx20 cmx10 cm was placed centrally over the array. Finally, images were reconstructed from measurements of saline samples in a free space background, with the samples moved past the array in 21 1 cm steps to emulate mechanical scanning of the array. The image reconstruction characteristics of the planar array in conjunction with the reconstruction technique employed are discussed.
C. Stehning, P. Bornert, K. Nehrke, and O. Dössel. Free breathing 3D balanced FFE coronary magnetic resonance angiography with prolonged cardiac acquisition windows and intra-RR motion correction. In Magnetic Resonance in Medicine : Official Journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine, vol. 53(3) , pp. 719-723, 2005
A shortcoming of today's coronary magnetic resonance angiography (MRA) is its low total scan efficiency (<5%), as only small well-defined fractions of the respiratory (50%) and cardiac (10%) cycle are used for data acquisition. These precautions are necessary to prevent blurring and artifacts related to respiratory and cardiac motion. Hence, scan times range from 4 to 9 min, which may not be tolerated by patients. To overcome this drawback, an ECG-triggered, navigator-gated free breathing radial 3D balanced FFE sequence with intra-RR motion correction is investigated in this study. Scan efficiency is increased by using a long cardiac acquisition window during the RR interval. This allows the acquisition of a number of independent k-space segments during each cardiac cycle. The intersegment motion is corrected using a self-guided epicardial fat tracking procedure in a postprocessing step. Finally, combining the motion-corrected segments forms a high-resolution image. Experiments on healthy volunteers are presented to show the basic feasibility of this approach.
In magnetic induction tomography reducing the influence of the primary excitation field on the sensors can provide a significant improvement in SNR and/or allow the operating frequency to be reduced. For the purposes of imaging, it would be valuable if all, or a useful subset, of the detection coils could be rendered insensitive to the primary field for any excitation coil activated. Suitable schemes which have been previously suggested include the use of axial gradiometers and coil-orientation methods (Bx sensors). This paper examines the relative performance of each method through computer simulation of the sensitivity profiles produced by a single sensor, and comparison of reconstructed images produced by sensor arrays. A finite-difference model was used to determine the sensitivity profiles obtained with each type of sensor arrangement. The modelled volume was a cuboid of dimensions 50 cmx50 cmx12 cm with a uniform conductivity of 1 S m-1. The excitation coils were of 5 cm diameter and the detection coils of 5 mm diameter. The Bx sensors provided greater sensitivity than the axial gradiometers at all depths, other than on the surface layer of the volume. Images produced using a single-planar array were found to contain distortion which was reduced by the addition of a second array.
R. Winkelmann, P. Bornert, and O. Dössel. Ghost artifact removal using a parallel imaging approach. In Magnetic Resonance in Medicine : Official Journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine, vol. 54(4) , pp. 1002-1009, 2005
Parallel imaging techniques, which use several receive coils simultaneously, have been shown to enable a significant scan time reduction by subsampling k-space. Nevertheless, the data acquired with multiple coils in parallel exhibit some redundancy if the number of receive coils exceeds the subsampling factor. This redundancy leads to an overdetermination of the reconstruction problem, which is generally used to optimize the signal-to-noise ratio (SNR). However, it can yield further information about the quality of the reconstructed image, and can thus be used to identify and correct image artifacts. While some known approaches try to solve this problem in k-space, this study addresses it in the spatial domain and uses a modified SENSE reconstruction to reduce or completely remove ghost-type artifacts arising from processes such as motion or flow during data acquisition. Phantom and in vivo studies show significant improvements in image quality after correction, and serve as a basis for the discussion of the performance and limitations of this new approach.
Parallel imaging techniques, which in principle represent procedures of unfolding a reduced dataset, are well known and well established in MR imaging. This paper presents a further application of one particular reconstruction method, the SENSE algorithm, considered from a different point of view to remove potential foldover in conventional images acquired with multiple receive coils. Based on the coil sensitivity information, a body coverage map in the excited plane is calculated. This is used together with the measured raw data in a SENSE-type reconstruction to optimize the signal-to-noise ratio (SNR) as well as to remove foldover reliably by unfolding the image to a larger field of view. The reconstruction is performed automatically, without any user interaction, and does not affect data acquisition. Based on phantom and in vivo studies, which retain high image quality after the removal, the potential and limits of this approach are discussed, also taking into account future scanner hardware that will support a large number of parallel receiver channels.
The steady-state free precessing (SSFP) sequences, widely used in MRI today, acquire data only during a short fraction of the repetition time (TR). Thus, they exhibit a poor scan efficiency. In this paper, a novel approach to extending the acquisition window for a given TR without considerably modifying the basic sequence is explored for radial SSFP sequences. The additional data are primarily employed to increase the signal-to-noise ratio, rather than to improve the temporal resolution of the imaging. The approach is analyzed regarding its effect on the image SNR (signal to noise ratio) and the reconstruction algorithm. Results are presented for phantom experiments and cardiac functions studies. The gain in SNR is most notable in rapid imaging, since SNR enhancement for a constant repetition time may be used to compensate for the increase in noise resulting from angular undersampling.
O. Dössel. Vorlesungsskript 05 - Lineare Elektrische Netze. Institut für Biomedizinische Technik, Universität Karlsruhe (TH), 2005.
O. Dössel, W. Bauer, D. Farina, C. Kaltwasser, and O. Skipa. Imaging of bioelectric sources in the heart using a cellular automaton model. In Conference Proceedings : ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference, vol. 2, pp. 1067-1070, 2005
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.
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.
A. Khawaja, and O. Dössel. A PCA-Based Technique for QRS Complex Estimation. In Proc. Computers in Cardiology, vol. 32, pp. 747-750, 2005
In this paper, a new method for QRS complex prediction is presented. It is based on Principal Components Analysis (PCA) and polynomial fitting techniques. QRS complexes were extracted from multi-lead ECG signals and were aligned very perfectly. The covariance matrix was calculated from the QRS complex data matrix of many heartbeats. Afterwards, the corresponding eigenvectors and eigenvalues were computed and the reconstruction parameters vectors were derived by expansion of every beat in terms of the first eigenvectors. Performing the first order poly-fit method on the elements of the reconstruction parameter vectors yielded certain linear functions. Thereafter, the following QRS complexes were estimated by calculating the corresponding reconstruction parameter vectors derived from these functions. The similarity, absolute error and RMS error between the original and predicted QRS complexes were measured
A. Khawaja, S. Sanyal, and O. Dössel. A wavelet-based technique for baseline wander correction in ECG and multi-channel ECG. In IFMBE Proceedings, vol. 9, pp. 291-292, 2005
In this paper, a new offline method for automatic baseline drift correction in Electrocardiogram is presented. It is based on Discrete Wavelet Transform (DWT) and analyzing high scale Approximation Coefficients (AC). A set of 650 noisy ECG signals was created by mixing different artificially generated noise-free ECGs and baseline wanders. By applying different mother Wavelets on each noisy signal, twelve stage DWT decomposition was carried out and twelve filtered ECGs were reconstructed by canceling the highest level AC at each stage. The similarities between initially generated baseline and canceled AC, as well as between the corresponding noise-free and reconstructed ECGs were examined every time by means of Correlation technique. The results from all 650 signals were considered in order to find the suitable Wavelet and AC level. The highest correlations, better than 99.9% for baseline and 99.99% for filtered ECG, were found with the ninth scale approximation coefficients when using Daubechies11 or Symlet12 as prototype wavelet. The algorithm was applied on various MIT-BIH and Multi-channel ECG signals. Furthermore, the baseline elimination results were considered to be very promising.
J. Qin, M. Reumann, S. H. Osswald, and O. Dössel. Developing Algorithms for The Optimization of Overpacing Strategies in Patients with Atrial Fibrillation. In Biomedizinische Technik, vol. 50(S1) , pp. 1426-1427, 2005
Atrial Fibrillation (AF) is the most common cardiac arrhythmia with overall prevalence of almost 1%. Treatment options range from pharmacological to surgical options. While ablation strategies are the most common thera- pies, it has been testified that overdrive stimulation can prevent AF successfully. The presented work suggests an approach to simulate the excitation propagation with various overdrive pacing frequencies based on a detailed cell model in a simplified anatomical structure. Through simulations, a relationship between the overdrive pac- ing position and its frequency with respect to the onset of AF was shown. The method applied in this work can be used to further develop optimization strategies for overdrive pacing. In the long run, the application trans- ferred to individual patient data might be used to increase the success rate of overdrive pacing in terminating atrial fibrillation.
G. Seemann, D. L. Weiß, F. B. Sachse, and O. Dössel. Electrophysiology and Tension Development in a Transmural Heterogeneous Model of the Visible Female Left Ventricle. In Lecture Notes in Computer Science, vol. 3504, pp. 172-182, 2005
D. L. Weiss, G. Seemann, and O. Dössel. Efficient Solving of Mathematical Models Describing the Behavior of Cardiac Myocytes. In Biomedizinische Technik, vol. 50(1) , pp. 566-567, 2005
D. L. Weiss, G. Seemann, and O. Dössel. Epicardial stimulation of a virtual left ventricular wall comprising heterogeneity and anisotropy. In Proc. IFMBE / EMBEC, vol. 11, 2005
D. L. Weiss, G. Seemann, F. B. Sachse, and O. Dössel. Epicardial Activation Increases Transmural Dispersion of Repolarization in a Heterogeneous Model of Wild-Type and Short QT Mutant Tissue. In Proc. Computers in Cardiology, vol. 32, pp. 117-120, 2005