Progress in Biomagnetic Imaging of Heart Arrhythmias. In: Baert AL, Heuck FHW, editors. Frontiers in European Radiology. Berlin: Springer Berlin Heidelberg; 1991. pp. 1-19. [DOI: 10.1007/978-3-642-76099-0_1]

QRS amplitude and shape variability in magnetocardiograms

In magnetocardiography, averaging of QRS complexes is often used to improve the signal-to-noise ratio. However, averaging of QRS complexes ignores the variation in amplitude and shape of the signals caused, for example, by respiration. This may lead to suppression of signal portions within the QRS complexes. Furthermore, for inverse source, reconstructions of dipoles and of current density distributions errors in the spacial arrangement may occur. To overcome these problems we developed a method for separating and selective averaging QRS complexes with different shapes and amplitudes. The method is based on a spline interpolation of the QRS complex averaged by a standard procedure. This spline function then is fitted to each QRS complex in the raw data by means of nonlinear regression (Levenberg-Marquardt method). Five regression parameters are applied: a linear amplitude scaling, two parameters describing the baseline drift, a time scaling parameter, and a time shift parameter. We found that both amplitude and shape of the QRS complex are influenced by respiration. The baseline shows a weaker influence of the respiration. The regression parameters of two neighboring measurement channels correlate linearly. Thus, selective averaging of a larger number of sensors can be performed simultaneously.

Right ventricular volume unloading evaluated by tangential magnetocardiography

OBJECTIVE

The evaluation of right ventricular volume overload in the presence of a right bundle branch block based solely on the results of an electrocardiogram is difficult. The purpose of this study was to purify acute right ventricular volume unloading from the tangential magnetocardiography.

METHODS

We measured the tangential (x-y plane) magnetocardiogram and electrocardiogram simultaneously in nine patients with a secundum atrial septal defect. The magnetocardiograms were obtained before surgical closure and during the immediate postoperative period using the 32-channel superconducting quantum interference device system in a magnetically shielded room.

RESULTS

The QRS duration on the surface electrocardiogram decreased significantly (p < 0.05) in the immediate postoperative period. The right ventricular depolarization time as measured by the magnetocardiogram was shortened from 40.3 +/- 6.1 to 25.3 +/- 7.4 milliseconds (p < 0.0005). The maximum peak amplitude during right ventricular depolarization decreased from 17.9 +/- 4.8 to 11.1 +/- 4.9 pT (p < 0.01).

CONCLUSIONS

We conclude that acute volume unloading of the right ventricle was indicated quantitatively by shortening of the right ventricular depolarization time and a reduction in the amplitude of current vectors originating from the right ventricular depolarization on the tangential magnetocardiography.

Arrhythmia vulnerability assessment using magnetic field maps and body surface potential maps

Magnetic field maps and body surface potential maps can be used to measure cardiac activity. The ability of magnetic and potential body surface maps to identify patients' vulnerable to recurrent sustained ventricular arrhythmia (VA) were compared. Magnetic field maps (MFM) and body surface potential mapping (BSPM) were obtained from 76 normal (N) subjects, 15 myocardial infarct (MI) patients, and 15 VA patients. QRST integral maps were calculated for each subject and nondipolar content was determined using Karhunen-Loeve transform eigen-maps. Although differences in nondipolar content were significant between the normal and patient groups (P = 2.4 x 10(-5) for BSPM and P = 6.0 x 10(-8) for MFM), differences in nondipolar content between MI and VA patients using QRST integral BSPM and MFM maps were not significant. The trajectory of the location of the maxima and minima on the map area during the QRS and ST-T intervals were also constructed. Discrimination between MI and VA patients was based on intergroup differences in the amount of fragmentation of the trajectory plots. The ST-T trajectory plots were significantly more fragmented (P < 0.0001 and P < 0.05 for MFM and BSPM, respectively) for VA than for MI patients. The ST-T interval MFM and BSPM trajectory plots enabled separation of MI and VA patients with accuracies of 83% and 73%, respectively. These results suggest that repolarization MFM and BSPM extrema trajectory plots can be used effectively as a means of identifying patients at risk for VA.

[Magnetocardiographic diagnostic of late fields. Current state and future perspectives]

The occurence of ventricular late potentials in the signal averaged surface ECG is an indicator for slow electrical excitation in myocardial tissue prone to arrhythmias. Signal averaged surface ECG is performed for identification of patients with ventricular late potentials who have a high risk for life threatening arrhythmias. Since every electrical field is combined with a magnetic field, established methods of the signal averaged surface ECG were applied on recorded magnetocardiograms of patients. The incidences and details of late ventricular activity in the signal averaged electrocardiogram (ventricular late potentials) were compared with those in the magnetocardiogram (ventricular late fields). A close correlation of the two different methods was found. The magnetocardiogram has the option for the determination of the site of origin of magnetic signals three-dimensionally. In the future, this method may help to find areas of slow conduction for ablative procedures to cure patients with a high risk for malignant arrhythmias.

[Relevance of magnetocardiography in coronary artery disease and myocardial infarction]

Multichannel magnetocardiography (MCG) noninvasively registers the magnetic activity of the heart at different points above the thorax. This information can be used to determine the magnetic field produced by cardiac activity as well to reconstruct the current density distribution in the myocardium, which can then be examined during cardiac de- and repolarisation. First studies have shown that the detection of disease specific changes of the magnetic field and current density permit the diagnosis and localization of myocardial infaction (MI) and myocardial ischemia within the context of coronary artery disease (CAD). In these studies various approaches were used to quantify and condense the temporal and spatial changes in the magnetic signals. The integration of defined time intervals of cardiac de- and repolarisation in form of iso-integral magnetic field maps allowed a discrimination between myocardial infarct groups. Furthermore residual maps, calculated by subtracting the MCG map components of MI patients from those of normal subjects, were used to describe the infarcted region. On the basis of trajectory plots which represent the course of magnetic map extrema, patients with ventricular tachycardia after MI could be identified. Current density reconstruction during ST-segment permitted the visualization of biological injury currents during induced ischemia and infarction. Beyond the consideration of the overall magnetic activity, the signal in single channels may be examined and interpreted as is done in the body surface electrocardiogram. Morphological criteria such as the course of the ST-segment as well as the spatial distribution of cardiac time intervals may be considered. Risk stratification of patients after MI with regard to an increased risk of malignant arrhythmia is possible by making use of the spatial distribution of QT dispersion. The promising preliminary results suggest that the current methods must be developed and investigated further in studies with the appropriate number and kind of subjects in order to assess the clinical value of the MCG in patients with CAD and MI.

[Present state and future of magnetocardiographic localization]

The magnetic fields caused by the human heart's electrical excitation can be recorded without contact over the body surface to obtain the "magnetocardiogram" (MCG). As compared to the conventional electrocardiogram (ECG), the magnetic fields are influenced far less by the conductive properties of the body tissues, so that the MCG permits a more direct and accurate analysis of cardiac electrical excitation. Most important, the MCG allows an exact localization of the underlying electrical activity, based on the recorded magnetic field distribution. For localization, the MCG does not rely on pattern recognition algorithms such as the ECG, instead, a computational 3-D localization is performed using simplified source and volume conductor models. The spatial accuracy of this method, in combination with magnetic resonance imaging for anatomical assignment of the localization results, has been determined to be 10 to 15 mm for sources close to the body surface and 15 to 20 mm for sources in the posterior parts of the heart.Clinically, the magnetocardiogram can be applied for the non-invasive localization of accessory pathways in Wolff-Parkinson-White syndrome, and of ventricular ectopies (PVC and VT). Especially in combination with a subsequent interventional treatment by catheter ablation, the method may improve the clinical management of these conditions.While the registration techniques are standardized in a way that permits routine clinical application, the data evaluation has to be optimized and simplified before this method can be completely handed over for physicians to use.

Magnetocardiographically-guided catheter ablation

After more than 30 years since the first magnetocardiographic (MCG) recording was carried out with induction coils, MCG is now approaching the threshold of clinical use. During the last 5 years, in fact, there has been a growing interest of clinicians in this new method which provides an unrivalled accuracy for noninvasive, three-dimensional localization of intracardiac source. An increasing number of laboratories are reporting data validating the use of MCG as an effective method for preoperative localization of arrhythmogenic substrates and for planning the best catheter ablation approach for different arrhythmogenic substrates. In this article, available data from literature have been reviewed. We consider the clinical use of MCG to localize arrhythmogenic substrates in patients with Wolff-Parkinson-White syndrome and in patients with ventricular tachycardia in order to assess the state-of-the-art of the method on a large number of patients. This article also addresses some suggestions for industrial development of more compact, medically oriented MCG equipments at reasonable cost.

Localisation of myocardial ischaemia from the magnetocardiogram using current density reconstruction method: computer simulation study

A computer simulation study is performed to investigate the method of current density reconstruction to localise myocardial ischaemia. A computer model of the entire human heart is used to simulate the excitation and repolarisation process in eight topographically different cases of myocardial ischaemia. The associated magnetocardiogram is calculated at 37 positions of the KRENIKON biomagnetic measurement equipment. The method of current density reconstruction is applied at the S-point (the last discernible deviation from the ST-segment at the end of the QRS-complex) of the MCG to find characteristics of the myocardial ischaemia simulated by the model. The results show that it is possible to determine the location of the ischaemia. The current density distribution may be interpreted physiologically in terms of the so-called 'injury-current'. This indicates that magnetocardiography might be a suitable method for noninvasive ischaemia diagnosis, and further investigations of the current density reconstruction method for the injury current should be performed on patients with ischaemic heart disease.

Biomagnetic multi-channel systems. Principles and application in cardiology

The non-invasive measurement of the extremely weak magnetic fields generated by heart and brain is motivated by the possibility of obtaining quantitative diagnostic information about electric function. Magnetic signals (MCG, MEG) are significantly less influenced by body tissue than the corresponding electric signals (ECG, EEG). Measurement of biomagnetic signals is performed by superconducting sensors, consisting of pickup coils and SQUIDs (superconducting quantum interference device) operating in liquid Helium. For clinical investigations a biomagnetic multi-channel system (KRENIKONR) has been designed. It uses a flat array of 37 magnetic field sensors and is operated inside a shielded room. Evaluation of biomagnetic signals by use of simple source and body models and in combination with anatomical data from 3D MR- or CT-images yields sequences of locations of electrical function with a spatial resolution of some millimeters and a time resolution better than one millisecond. More than three years of clinical studies have demonstrated the value of the method primarily in cases with localized functional pathologies. In cardiology this is pathologies of the cardiac conductive pathway, ectopies, and arrhythmias. Validation has been performed by catheter stimulation in volunteers, and by catheter mapping and nuclear medical methods in patients. Extension of modelling and evaluation to cases with distributed activity, e.g. ventricular excitation, is under investigation.

Biomagnetically localizable multipurpose catheter and method for MCG guided intracardiac electrophysiology, biopsy and ablation of cardiac arrhythmias

A multipurpose catheter, specially designed to be biomagnetically localizable and the method for magnetocardiographic (MCG) guided intracardiac electrophysiological recordings, endomyocardial biopsy and ablation of cardiac arrhythmias are described. The catheter features two non-polarizable non-ferrous magnetic electrodes, arranged in such a way that, connected to an external current generator, an electromagnetic field of dipolar configuration can be generated in the heart. The connection is done with twisted pairs of non-ferrous magnetic conductors, to avoid the occurrence of spurious magnetic fields along the catheter during current injection to the electrodes. With this assembly the tip of the catheter can be localized (and driven close to an arrhythmogenic area) by MCG mapping. The same electrodes are feasible for monophasic action potential (MAP) recordings. One or more lumen allow fluid infusion, blood sampling, pressure measurements and introduction of steerable wires, pacing or ablation electrodes, bioptic devices, or optic fibers. On the basis of preoperative MCG three-dimensional localization of the arrhythmogenic substrate, the biomagnetically localizable catheter is driven, under fluoroscopic control, as close as possible to the suspected arrhythmogenic zone. MCG mapping is then performed under pacing, with adjustments of the catheter's tip, until the electrically induced magnetic field and catheter's localization parameters fit those generated by the spontaneous arrhythmia. MAP is recorded. The catheter position is accepted for ablation when electrophysiological abnormalities are identified in the MCG localized area.

Clinical magnetocardiography. 10 years experience at the Catholic University

Since the introduction, in 1982, of a Biomagnetic facility in the clinical environment, efforts were concentrated to investigate whether magnetocardiography could really provide new information of potential diagnostic use, even avoiding electromagnetic shielding to facilitate simultaneous biomagnetic and conventional cardiac investigations, including cardiac catheterization for invasive electrophysiological procedures. More than 350 patients have been magnetically investigated using a single-channel second-order gradiometer. Results of 281 MCG studies, whose data have been extensively analyzed with updated software programs, are reported. Magnetocardiographic (MCG) mapping during endocardial pacing was performed to quantify the accuracy of MCG localization of intracardiac dipolar sources. MCG classification of ventricular preexcitation has been attempted in 70 patients with overt preexcitation. MCG localization of the ventricular preexcited area was accurate and reproducible, provided that during mapping a sufficient degree of ventricular preexcitation was present. MCG mapping during orthodromic A-V re-entry tachycardia has been also employed to attempt the localization of retrograde atrial preexcitation as well as the site of origin of atrial and ventricular tachyarrhythmias. For validation, the results of catheter and epicardial mappings have been used. Other applications of clinical magnetocardiography are under evaluation. The use of the Relative smoothness index needs, in our opinion, a larger experience to define its reliability as a predictor of risk for sudden death. MCG follow-up study of patients with transplanted hearts seems to be a promising application, for early detection of acute graft rejection reaction. Our reported case strongly supports this potentiality. Present work is also addressed to develop an integrated system allowing easy MCG mapping during cardiac catheterization, as a new method to guide diagnostic and therapeutic procedures as close as possible to the arrhythmogenic substrate.

Clinical magnetocardiography: experience with a biomagnetic multichannel system

The magnetic fields caused by the human heart's electrical activity were coherently recorded with a biomagnetic multichannel system (KRENIKON) during 1 to 10 minutes in 49 patients. 31 to 37 magnetic channels were recorded simultaneously with the ECG and respiration. Comparison of a magnetic index and the Sokolow-Lyon index to echocardiographic findings in the quantification of left ventricular hypertrophy demonstrated the superiority of the magnetocardiogram (MCG) as compared to the ECG. The magnetocardiographic investigation of patients with WPW-Syndrome, ventricular extrasystoles, ventricular tachycardia, and paced ventricular beats demonstrated that multichannel magnetocardiography permits the non-invasive three dimensional localization of arrhythmogenic tissue with high spatial accuracy.

Experiences in data analysis and modelling with a multichannel biomagnetic system

Evaluation of MEG/MCG data, measured with the Siemens biomagnetic multichannel system KRENIKON, in patients with epilepsy, infarction, Wolff-Parkinson-White (WPW) syndrome or extra systoles are in good agreement with the results of different investigation techniques. The evaluations have been performed using an equivalent current dipole model within a sphere or a half-space with homogeneous conductivity. In cases where the current dipole model is not adequate, multiple dipoles or complete distributions of current sources have to be considered. Results from simulations and applications to in vivo data and the influence of geometries better adjusted to realistic geometries are discussed.

Design and performance of a biomagnetic multichannel system for MEG and MCG studies

Considerations which have lead to the design of the Siemens biomagnetic multichannel system are discussed. Algorithms developed for data evaluation include removal of periodic signals, averaging of sporadic events, and separation of background activity. Means are described to fuse biomagnetic locations with three-dimensional medical images.