Current-density estimation of exercise-induced ischemia in patients with multivessel coronary artery disease

Clinical magnetocardiography: the unshielded bet-past, present, and future

Magnetocardiography (MCG), which is nowadays 60 years old, has not yet been fully accepted as a clinical tool. Nevertheless, a large body of research and several clinical trials have demonstrated its reliability in providing additional diagnostic electrophysiological information if compared with conventional non-invasive electrocardiographic methods. Since the beginning, one major objective difficulty has been the need to clean the weak cardiac magnetic signals from the much higher environmental noise, especially that of urban and hospital environments. The obvious solution to record the magnetocardiogram in highly performant magnetically shielded rooms has provided the ideal setup for decades of research demonstrating the diagnostic potential of this technology. However, only a few clinical institutions have had the resources to install and run routinely such highly expensive and technically demanding systems. Therefore, increasing attempts have been made to develop cheaper alternatives to improve the magnetic signal-to-noise ratio allowing MCG in unshielded hospital environments. In this article, the most relevant milestones in the MCG's journey are reviewed, addressing the possible reasons beyond the currently long-lasting difficulty to reach a clinical breakthrough and leveraging the authors' personal experience since the early 1980s attempting to finally bring MCG to the patient's bedside for many years thus far. Their nearly four decades of foundational experimental and clinical research between shielded and unshielded solutions are summarized and referenced, following the original vision that MCG had to be intended as an unrivaled method for contactless assessment of the cardiac electrophysiology and as an advanced method for non-invasive electroanatomical imaging, through multimodal integration with other non-fluoroscopic imaging techniques. Whereas all the above accounts for the past, with the available innovative sensors and more affordable active shielding technologies, the present demonstrates that several novel systems have been developed and tested in multicenter clinical trials adopting both shielded and unshielded MCG built-in hospital environments. The future of MCG will mostly be dependent on the results from the ongoing progress in novel sensor technology, which is relatively soon foreseen to provide multiple alternatives for the construction of more compact, affordable, portable, and even wearable devices for unshielded MCG inside hospital environments and perhaps also for ambulatory patients.

Investigations of sensitivity and resolution of ECG and MCG in a realistically shaped thorax model

Solving the inverse problem of electrocardiography (ECG) and magnetocardiography (MCG) is often referred to as cardiac source imaging. Spatial properties of ECG and MCG as imaging systems are, however, not well known. In this modelling study, we investigate the sensitivity and point-spread function (PSF) of ECG, MCG, and combined ECG+MCG as a function of source position and orientation, globally around the ventricles: signal topographies are modelled using a realistically-shaped volume conductor model, and the inverse problem is solved using a distributed source model and linear source estimation with minimal use of prior information. The results show that the sensitivity depends not only on the modality but also on the location and orientation of the source and that the sensitivity distribution is clearly reflected in the PSF. MCG can better characterize tangential anterior sources (with respect to the heart surface), while ECG excels with normally-oriented and posterior sources. Compared to either modality used alone, the sensitivity of combined ECG+MCG is less dependent on source orientation per source location, leading to better source estimates. Thus, for maximal sensitivity and optimal source estimation, the electric and magnetic measurements should be combined.

Clinical application of magnetocardiography

Magnetocardiography is a noninvasive contactless method to measure the magnetic field generated by the same ionic currents that create the electrocardiogram. The time course of magnetocardiographic and electrocardiographic signals are similar. However, compared with surface potential recordings, multichannel magnetocardiographic mapping (MMCG) is a faster and contactless method for 3D imaging and localization of cardiac electrophysiologic phenomena with higher spatial and temporal resolution. For more than a decade, MMCG has been mostly confined to magnetically shielded rooms and considered to be at most an interesting matter for research activity. Nevertheless, an increasing number of papers have documented that magnetocardiography can also be useful to improve diagnostic accuracy. Most recently, the development of standardized instrumentations for unshielded MMCG, and its ease of use and reliability even in emergency rooms has triggered a new interest from clinicians for magnetocardiography, leading to several new installations of unshielded systems worldwide. In this review, clinical applications of magnetocardiography are summarized, focusing on major milestones, recent results of multicenter clinical trials and indicators of future developments.

Usefulness of ventricular endocardial electric reconstruction from body surface potential maps to noninvasively localize ventricular ectopic activity in patients

As radio frequency (RF) catheter ablation becomes increasingly prevalent in the management of ventricular arrhythmia in patients, an accurate and rapid determination of the arrhythmogenic site is of important clinical interest. The aim of this study was to test the hypothesis that the inversely reconstructed ventricular endocardial current density distribution from body surface potential maps (BSPMs) can localize the regions critical for maintenance of a ventricular ectopic activity. Patients with isolated and monomorphic premature ventricular contractions (PVCs) were investigated by noninvasive BSPMs and subsequent invasive catheter mapping and ablation. Equivalent current density (CD) reconstruction (CDR) during symptomatic PVCs was obtained on the endocardial ventricular surface in six patients (four men, two women, years 23-77), and the origin of the spontaneous ectopic activity was localized at the location of the maximum CD value. Compared with the last (successful) ablation site (LAS), the mean and standard deviation of localization error of the CDR approach were 13.8 and 1.3 mm, respectively. In comparison, the distance between the LASs and the estimated locations of an equivalent single moving dipole in the heart was 25.5 ± 5.5 mm. The obtained CD distribution of activated sources extending from the catheter ablation site also showed a high consistency with the invasively recorded electroanatomical maps. The noninvasively reconstructed endocardial CD distribution is suitable to predict a region of interest containing or close to arrhythmia source, which may have the potential to guide RF catheter ablation.

Localization of endocardial ectopic activity by means of noninvasive endocardial surface current density reconstruction

Localization of the source of cardiac ectopic activity has direct clinical benefits for determining the location of the corresponding ectopic focus. In this study, a recently developed current-density (CD)-based localization approach was experimentally evaluated in noninvasively localizing the origin of the cardiac ectopic activity from body-surface potential maps (BSPMs) in a well-controlled experimental setting. The cardiac ectopic activities were induced in four well-controlled intact pigs by single-site pacing at various sites within the left ventricle (LV). In each pacing study, the origin of the induced ectopic activity was localized by reconstructing the CD distribution on the endocardial surface of the LV from the measured BSPMs and compared with the estimated single moving dipole (SMD) solution and precise pacing site (PS). Over the 60 analyzed beats corresponding to ten pacing sites (six for each), the mean and standard deviation of the distance between the locations of maximum CD value and the corresponding PSs were 16.9 mm and 4.6 mm, respectively. In comparison, the averaged distance between the SMD locations and the corresponding PSs was slightly larger (18.4 ± 3.4 mm). The obtained CD distribution of activated sources extending from the stimulus site also showed high consistency with the endocardial potential maps estimated by a minimally invasive endocardial mapping system. The present experimental results suggest that the CD method is able to locate the approximate site of the origin of a cardiac ectopic activity, and that the distribution of the CD can portray the propagation of early activation of an ectopic beat.

A comparison of two-dimensional techniques for converting magnetocardiogram maps into effective current source distributions

The aim of this study was to develop a method for converting the pseudo two-dimensional current given by a current-arrow map (CAM) into the physical current. The physical current distribution is obtained by the optimal solution in a least mean square sense with Tikhonov regularization (LMSTR). In the current dipole simulation, the current pattern differences (ΔJ) between the results of the CAM and the LMSTR with several regularization parameters (α = 10(-1)-10(-15)) are calculated. In magnetocardiographic (MCG) analysis, the depth (z(d)) of a reconstruction plane is chosen by using the coordinates of the sinus node, which is estimated from MCG signals at the early p-wave. The ΔJs at p-wave peaks, QRS-complex peaks, and T-wave peaks of MCG signals for healthy subjects are calculated. Furthermore, correlation coefficients and regression lines are also calculated from the current values of the CAM and the LMSTR during p-waves, QRS-complex, and T-waves of MCG signals. In the simulation, the ΔJs (α ≈ 10(-10)) had a minimal value. The ΔJs (α = 10(-10)) at p-wave peaks, QRS-complex peaks, and T-wave peaks of MCG signals for healthy subjects also had minimal value. The correlation coefficients of the current values given by the CAM and the LMSTR (α = 10(-10)) were greater than 0.9. Furthermore, slopes (y) of the regression lines are correlated with the depth (z(d)) (r = -0.93). Consequently, the CAM value can be transformed into the LMSTR current value by multiplying it by the slope (y) obtained from the depth (z(d)). In conclusion, the result given by the CAM can be converted into an effective physical current distribution by using the depth (z(d)).

From 3D to 4D imaging: is that useful for interventional cardiac electrophysiology?

Three-dimensional electroanatomical imaging is increasingly used in interventional cardiac electrophysiology, to guide catheter ablation of cardiac arrhythmias. At the same time, there is a growing interest for non-invasive methods, such as magnetocardiographic mapping (MCG), to localize the arrhythmogenic substrates, to test their reproducibility and to plan the most appropriate interventional approach. So far electroanatomical imaging has relayed on static mathematical modeling of the heart and more recently on direct merging with three-dimensional rendering of cardiac anatomy from multidetector computer tomography or magnetic resonance imaging. Merging electrophysiological information with static anatomical structures, can surely be a source of uncertainty for MCG-based pre-interventional localization of the arrhythmogenic substrate and causes mismatch between the real-time imaging of moving catheters and the static geometry of the cardiac chambers reconstructed with invasive electroanatomical imaging. The implementation of recent realistic numerical models of the beating heart in a breathing thorax can improve accuracy and fill the gap between non-invasive and interventional electroanatomical imaging.

Contactless magnetocardiographic mapping in anesthetized Wistar rats: evidence of age-related changes of cardiac electrical activity

Magnetocardiography (MCG) is the recording of the magnetic field (MF) generated by cardiac electrophysiological activity. Because it is a contactless method, MCG is ideal for noninvasive cardiac mapping of small experimental animals. The aim of this study was to assess age-related changes of cardiac intervals and ventricular repolarization (VR) maps in intact rats by means of MCG mapping. Twenty-four adult Wistar rats (12 male and 12 female) were studied, under anesthesia, with the same unshielded 36-channel MCG instrumentation used for clinical recordings. Two sets of measurements were obtained from each animal: 1) at 5 mo of age (297.5 ± 21 g body wt) and 2) at 14 mo of age (516.8 ± 180 g body wt). RR and PR intervals, QRS segment, and QTpeak, QTend, JTpeak, JTend, and Tpeak-end were measured from MCG waveforms. MCG imaging was automatically obtained as MF maps and as inverse localization of cardiac sources with equivalent current dipole and effective magnetic dipole models. After 300 s of continuous recording were averaged, the signal-to-noise ratio was adequate for study of atrial and ventricular MF maps and for three-dimensional localization of the underlying cardiac sources. Clear-cut age-related differences in VR duration were demonstrated by significantly longer QTend, JTend, and Tpeak-end in older Wistar rats. Reproducible multisite noninvasive cardiac mapping of anesthetized rats is simpler with MCG methodology than with ECG recording. In addition, MCG mapping provides new information based on quantitative analysis of MF and equivalent sources. In this study, statistically significant age-dependent variations in VR intervals were found.

Three-dimensional recovery time dispersion map by 64-channel magnetocardiography may demonstrate the location of a myocardial injury and heterogeneity of repolarization

BACKGROUND

QT dispersion reveals heterogeneities in the repolarization time in the three-dimensional (3D) structure of the ventricular myocardium. In this study, we report on a 3D function map of recovery time (RT) dispersions as measured by 64-channel magnetocardiography (MCG).

METHODS

MCG were simultaneously recorded in 29 controls and 21 patients with previous myocardial infarction (MI). The 3D current density was calculated from 64-channel MCG data in the Bz component using a space filter. The heart outline, reconstructed from the integrated the current density, revealed both the atrium and ventricle. The RT for the intervals between QRS onset and the time of the maximum dT/dt of T wave, and the peak to the end of the T wave (T(peak)-negative dT/dt) were automatically measured by means of a computer from 3D MCG data. The corrected RT (RTc) and corrected T(peak)-negative dT/dt were then calculated using Bazett's formula. The 3D RTc and the corrected T(peak)-negative dT/dt dispersion map were superimposed on the heart outline generated by MCG.

RESULTS

The RTc was significantly longer for the MI group than in the control group (67+/-25 ms1/2 vs. 16+/-6 ms1/2) (p<0.0001). The corrected T(peak)-negative dT/dt dispersions in each patient was also significantly longer for the MI group than in the control group (35+/-27 ms1/2 vs. 10+/-5 ms1/2) (p<0.0001). Furthermore, the 3D RTc and T(peak)-negative dT/dt dispersion maps corresponded with the space location of MI, as defined by Tc-99m tetrofosmin myocardial imaging

CONCLUSIONS

3D RTc and T(peak)-negative dT/dt dispersion maps in the ST segment, obtained by 64-channel MCG may be used demonstrate the location of a myocardial injury and heterogeneities of repolarization.

The Value of Magnetocardiography in Patients with and Without Relevant Stenoses of the Coronary Arteries Using an Unshielded System

BACKGROUND

The diagnostic management of patients with chest pain remains a clinical challenge. Magnetocardiography (MCG) is a noninvasive method for the recording of cardiac electromagnetic signals at multiple sites above the chest cage. Contrary to electrocardiogram (ECG) the magnetic field is unaltered by surrounding tissues. The present study aimed to analyze the diagnostic value of an unshielded four-channel MCG for the detection of coronary artery disease (CAD) in patients with chest pain.

METHODS

The study included 417 subjects: 177 patients with angiographically documented CAD (stenoses > or =50%), 123 symptomatic patients without hemodynamically relevant stenosis (nCAD) and 117 healthy subjects. Twelve-lead ECG was obtained in all subjects. The magnetocardiography recordings were taken from 36 positions at rest. From these current density vector maps were generated during the ST-T interval. Each map was classified using a classification system with a scale from 0 (normal) to 4 (grossly abnormal).

RESULTS

While the ECG was normal in all subjects the MCG revealed typical differences. In normals most maps were classified as category 0, 1 or 2, in nCAD and more so in CAD patients the categories 3 and 4 prevailed. Using a cut-off value of 39.2% for the discrimination between normals and CAD patients sensitivity was 73.3%, specificity 70.1%.

CONCLUSION

Contrary to ECG, unshielded MCG reveals significant differences between normals and symptomatic patients with and without relevant stenoses using current density reconstruction during repolarization at rest. This method might be a suitable noninvasive tool for the management of patients with chest pain.

A 3-D model-based registration approach for the PET, MR and MCG cardiac data fusion

In this paper, a new approach is presented for the assessment of a 3-D anatomical and functional model of the heart including structural information from magnetic resonance imaging (MRI) and functional information from positron emission tomography (PET) and magnetocardiography (MCG). The method uses model-based co-registration of MR and PET images and marker-based registration for MRI and MCG. Model-based segmentation of MR anatomical images results in an individualized 3-D biventricular model of the heart including functional parameters from PET and MCG in an easily interpretable 3-D form.

Baseline reconstruction for localization of rapid ventricular tachycardia from body surface potential maps

Determination of an accurate electrocardiographic (ECG) baseline is generally needed for localization of ventricular arrhythmias with body surface potential mapping (BSPM). We suggest a novel signal processing method for ECG baseline reconstruction during monomorphic ventricular tachycardias (VT). The method is based on an assumption that VT consists of similar ventricular extrasystolic beats with overlapping depolarization and repolarization. The sequential reconstruction algorithm utilizes information of small variations in the heart rate and yields a non-overlapping QRST-signal, provided that the measurement set-up has a high enough temporal resolution to avoid distortions due to sampling differences and misalignment of individual beats. The reconstructed QRST-signal is utilized to subtract overlapping T-waves from the QRS complexes during VT. The use of the method is demonstrated with clinically measured BSPM data.

Postprocessing of 3-D current density reconstruction results with equivalent ellipsoids

A method of postprocessing and visualizing three-dimensional vector fields, such as current density reconstruction results, is presented. This method is based on equivalent ellipsoids fitted to the vector fields. The technique has been tested with simulated data and current density reconstructions based on bioelectromagnetic data obtained from a physical thorax phantom. Three different approaches based on: 1) longest distance; 2) dominant direction; and 3) principal component analysis, for fitting the equivalent ellipsoids are proposed. Multiple foci in vector fields are extracted by multiple ellipsoids which are fitted iteratively. The method enables statistical postprocessing for the sake of comparisons of different source reconstructions algorithms or comparisons of groups of patients or volunteers.

Noninvasive myocardial activation time imaging: a novel inverse algorithm applied to clinical ECG mapping data

Linear approaches like the minimum-norm least-square algorithm show insufficient performance when it comes to estimating the activation time map on the surface of the heart from electrocardiographic (ECG) mapping data. Additional regularization has to be considered leading to a nonlinear problem formulation. The Gauss-Newton approach is one of the standard mathematical tools capable of solving this kind of problem. To our experience, this algorithm has specific drawbacks which are caused by the applied regularization procedure. In particular, under clinical conditions the amount of regularization cannot be determined clearly. For this reason, we have developed an iterative algorithm solving this nonlinear problem by a sequence of regularized linear problems. At each step of iteration, an individual L-curve is computed. Subsequent iteration steps are performed with the individual optimal regularization parameter. This novel approach is compared with the standard Gauss-Newton approach. Both methods are applied to simulated ECG mapping data as well as to single beat sinus rhythm data from two patients recorded in the catheter laboratory. The proposed approach shows excellent numerical and computational performance, even under clinical conditions at which the Gauss-Newton approach begins to break down.