Clinical magnetocardiography: experience with a biomagnetic multichannel system

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.

Application of Magnetocardiography to Screen for Inflammatory Cardiomyopathy and Monitor Treatment Response

Background Inflammatory cardiomyopathy is one of the most common causes of sudden cardiac death in young adults. Diagnosis of inflammatory cardiomyopathy remains challenging, and better monitoring tools are needed. We present magnetocardiography as a method to diagnose myocardial inflammation and monitor treatment response. Methods and Results A total of 233 patients were enrolled, with a mean age of 45 (±18) years, and 105 (45%) were women. The primary analysis included 209 adult subjects, of whom 66 (32%) were diagnosed with inflammatory cardiomyopathy, 17 (8%) were diagnosed with cardiac amyloidosis, and 35 (17%) were diagnosed with other types of nonischemic cardiomyopathy; 91 (44%) did not have cardiomyopathy. The second analysis included 13 patients with inflammatory cardiomyopathy who underwent immunosuppressive therapy after baseline magnetocardiography measurement. Finally, diagnostic accuracy of magnetocardiography was tested in 3 independent cohorts (total n=23) and 1 patient, who developed vaccine-related myocarditis. First, we identified a magnetocardiography vector to differentiate between patients with cardiomyopathy versus patients without cardiomyopathy (vector of ≥0.051; sensitivity, 0.59; specificity, 0.95; positive predictive value, 93%; and negative predictive value, 64%). All patients with inflammatory cardiomyopathy, including a patient with mRNA vaccine-related myocarditis, had a magnetocardiography vector ≥0.051. Second, we evaluated the ability of the magnetocardiography vector to reflect treatment response. We observed a decrease of the pathologic magnetocardiography vector toward normal in all 13 patients who were clinically improving under immunosuppressive therapy. Magnetocardiography detected treatment response as early as day 7, whereas echocardiographic detection of treatment response occurred after 1 month. The magnetocardiography vector decreased from 0.10 at baseline to 0.07 within 7 days (P=0.010) and to 0.03 within 30 days (P<0.001). After 30 days, left ventricular ejection fraction improved from 42.2% at baseline to 53.8% (P<0.001). Conclusions Magnetocardiography has the potential to be used for diagnostic screening and to monitor early treatment response. The method is valuable in inflammatory cardiomyopathy, where there is a major unmet need for early diagnosis and monitoring response to immunosuppressive therapy.

Magnetocardiography for the diagnosis of coronary artery disease: a systematic review and meta-analysis

BACKGROUND

Coronary artery disease (CAD) has a significant disease burden making early diagnosis and management imperative. Magnetocardiography (MCG) is a relatively new noninvasive technique that allows diagnosis of CAD by recording the magnetic fields generated by the electrical activity of the heart.

METHODS

We searched MEDLINE and the Cochrane Central Register of Controlled Trials for prospective studies that evaluated the test characteristics (e.g., sensitivity, specificity, likelihood ratios) of MCG for detection of CAD. Studies were included if they evaluated either patients with stable CAD documented by angiogram or patients presenting initially with acute coronary syndrome and subsequently diagnosed with CAD. The quality of included studies was assessed using an adaptation of the Quality Assessment of Diagnostic Accuracy Studies (QUADAS) tool. We performed meta-analyses of sensitivity, specificity and positive and negative likelihood ratios using Meta-DiSc software.

RESULTS

Screening of titles and abstracts followed by full-text review yielded seven studies that met our inclusion criteria. Meta-analyses yielded a pooled sensitivity of 83% (95% confidence interval [CI] 80% to 86%) and a specificity of 77% (95% CI 73% to 81%). The pooled positive likelihood ratio was 3.92 (95% CI 2.30 to 6.66) and negative likelihood ratio was 0.20 (95% CI 0.12 to 0.35). Significant heterogeneity was present in all meta-analyses.

CONCLUSIONS

The pooled test characteristics for MCG are similar to those of existing noninvasive modalities for diagnosing CAD. Our results suggest that MCG is a potential complementary or alternative tool for noninvasive detection of CAD.

Magnetocardiography in coronary artery disease with a new system in an unshielded setting

BACKGROUND

The noninvasive detection of coronary artery disease (CAD) remains a clinical challenge. Magnetocardiography is a completely noninvasive method that permits the registration of cardiac electrical activity at multiple sites in a plane above the chest cage without the need for electrodes. In contrast to the electrocardiogram (ECG) which suffers from boundary effects and a variety of potential artifacts (electrode placement, etc.) the MCG is unaffected by such impediments as the magnetic field is unaltered by surrounding tissues.

HYPOTHESIS

Magnetocardiography with a newly developed single-channel system in an unshielded setting should be a better qualitative diagnostic tool than the standard ECG for the detection and assessment of CAD.

METHODS

In all, 52 patients with angiographically documented CAD and unimpaired ventricular function as well as 55 controls were included in this study. A standard 12-lead ECG was obtained in all subjects. The MCG recordings were taken from 36 positions under resting conditions. From these, current density vector maps were generated during the ST-T interval. Each map was then 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 in the controls was classified as category 0, 1, or 2. However, in patients with abnormal coronary angiograms, mainly maps in categories 3 and 4 were seen (p < 0.05).

CONCLUSION

A single-channel magnetometer in an unshielded setting reveals significant differences between normals and patients with CAD with normal ECG on the basis of current density reconstruction during the ST segment when measured under resting conditions. This method might be suitable for the noninvasive detection of CAD.

Visualization of regional myocardial depolarization by tangential component mapping on magnetocardiogram in children

BACKGROUND

Tangential components to the body surface on magnetocardiography theoretically reflect regional myocardial current sources just below the gradiometer. The usefulness of tangential component mapping on magnetocardiography in determination of regional myocardial abnormalities has not been investigated in children.

METHODS

Twenty-six children with ventricular hypertrophy, including a child with a left ventricular diverticulum (aged 7 to 15), and age matched 22 healthy children (aged 7 to 15) were studied. Tangential components on magnetocardiography were measured using a newly-developed super-conducting quantum interference device system housed in a magnetically shielded room. Isomagnetic maps and current vector maps were constructed from the data obtained.

RESULTS

The peak magnetic fields and current dipoles were demonstrated to be located at the interventricular septum initially, and then were shifted to the anterior and inferior walls of the left ventricle and to the right ventricular outflow tract, successively. In patients with right ventricular hypertrophy whose systolic right ventricular pressure was over 60 mmHg, the peak magnetic fields were located in the right half with rightward directed current vectors throughout ventricular depolarization. In patients with left ventricular hypertrophy, the maximal magnetic fields during depolarization were shifted to the hypertrophic site, showing significantly stronger forces than those in healthy children (35.5+/-11.7 pT vs 26.5+/-11.9 pT, p < 0.01). In a patient with left ventricular diverticulum, two discrete depolarizing current dipoles were visualized. The mean time required in measuring MCGs among all subjects was 10 minutes.

CONCLUSION

The time course as well as the location of the regional electrical activities of the myocardium in children can be visualized, in a short time, as a two-dimensional projection to the frontal plane by tangential component mapping on magnetocardiography.

[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.

Evaluation of the non-invasive localization accuracy of cardiac arrhythmias attainable by multichannel magnetocardiography (MCG)

The accuracy of multichannel magnetocardiography (MCG) for the non-invasive localization of cardiac arrhythmias was investigated. A non-magnetic catheter was used in phantom studies and for cardiac pacing of 6 patients. In a clinical setting, 32 patients with WPW-syndrome, 37 patients with premature ventricular complexes and 12 patients with ventricular tachycardia were studied and the MCG results compared to reference methods, including invasive electrophysiological mapping. Phantom and pacing studies demonstrated the spatial localization accuracy to be better than 15 mm for a dipole-to-dewar distance below 15 cm. In all patients with structural cardiac disease, the ectopic focus was localized at the margin of the damaged area, serving as a proof of MCG localization. Invasive mapping confirmed the MCG result whenever performed (42 patients). In 11 patients (9 WPW, 2 VT) the MCG localization result was verified by successful HF catheter ablation as a gold standard. MCG permits the non-invasive localization of cardiac arrhythmias with high spatial accuracy. MCG guided HF catheter ablation constitutes a new concept of non-invasive localization and minimally invasive causal therapy.