Reproducibility of HTS-SQUID magnetocardiography in an unshielded clinical environment

The magnetocardiogram

The magnetic field produced by the heart's electrical activity is called the magnetocardiogram (MCG). The first 20 years of MCG research established most of the concepts, instrumentation, and computational algorithms in the field. Additional insights into fundamental mechanisms of biomagnetism were gained by studying isolated hearts or even isolated pieces of cardiac tissue. Much effort has gone into calculating the MCG using computer models, including solving the inverse problem of deducing the bioelectric sources from biomagnetic measurements. Recently, most magnetocardiographic research has focused on clinical applications, driven in part by new technologies to measure weak biomagnetic fields.

Electromagnetic field and cardiovascular diseases: A state-of-the-art review of diagnostic, therapeutic, and predictive values

Despite encouraging advances in early diagnosis and treatment, cardiovascular diseases (CVDs) remained a leading cause of morbidity and mortality worldwide. Increasing evidence has shown that the electromagnetic field (EMF) influences many biological processes, which has attracted much attention for its potential therapeutic and diagnostic modalities in multiple diseases, such as musculoskeletal disorders and neurodegenerative diseases. Nonionizing EMF has been studied as a therapeutic or diagnostic tool in CVDs. In this review, we summarize the current literature ranging from in vitro to clinical studies focusing on the therapeutic potential (external EMF) and diagnostic potential (internal EMF generated from the heart) of EMF in CVDs. First, we provided an overview of the therapeutic potential of EMF and associated mechanisms in the context of CVDs, including cardiac arrhythmia, myocardial ischemia, atherosclerosis, and hypertension. Furthermore, we investigated the diagnostic and predictive value of magnetocardiography in CVDs. Finally, we discussed the critical steps necessary to translate this promising approach into clinical practice.

Unshielded magnetocardiography: Repeatability and reproducibility of automatically estimated ventricular repolarization parameters in 204 healthy subjects

BACKGROUND

Magnetocardiographic mapping (MCG) provides quantitative assessment of the magnetic field (MF) induced by cardiac ionic currents, is more sensitive to tangential currents, and measures vortex currents undetectable by ECG, with higher reported sensitivity of MCG ventricular repolarization (VR) parameters for earlier detection of acute myocardial ischemia. Aims of this study were to validate the feasibility of in-hospital unshielded MCG and to assess repeatability and reproducibility of quantitative VR parameters, considering also possible gender- and age-related variability.

METHODS

MCG of 204 healthy subjects [114 males-mean age 43.4 ± 17.3 and 90 females-mean age 40.2 ± 15.7] was retrospectively analyzed, with a patented proprietary software automatically estimating twelve VR parameters derived from the analysis of the dynamics of the T-wave MF extrema (five parameters) and from the inverse solution with the effective magnetic dipole model giving the effective magnetic vector components (seven parameters). MCG repeatability was calculated as coefficient of variation (CV) ±standard error of the mean (SEM). Reproducibility was assessed as intraclass correlation coefficient (ICC).

RESULTS

The repeatability of all MCG parameters was 16 ± 1.2 (%) (average CV ± SEM). Optimal (ICC > 0.7) reproducibility was found for 11/12 parameters (mean values) and in 8/12 parameters (single values). No significant gender-related difference was observed; six parameters showed a strong/moderate correlation with age.

CONCLUSION

Reliable MCG can be performed into an unshielded hospital ambulatory, with repeatability and reproducibility of quantitative assessment of VR adequate for clinical purposes. Wider clinical use is foreseen with the development of multichannel optical magnetometry.

Optimal Magnetic Sensor Vests for Cardiac Source Imaging

Magnetocardiography (MCG) non-invasively provides functional information about the heart. New room-temperature magnetic field sensors, specifically magnetoresistive and optically pumped magnetometers, have reached sensitivities in the ultra-low range of cardiac fields while allowing for free placement around the human torso. Our aim is to optimize positions and orientations of such magnetic sensors in a vest-like arrangement for robust reconstruction of the electric current distributions in the heart. We optimized a set of 32 sensors on the surface of a torso model with respect to a 13-dipole cardiac source model under noise-free conditions. The reconstruction robustness was estimated by the condition of the lead field matrix. Optimization improved the condition of the lead field matrix by approximately two orders of magnitude compared to a regular array at the front of the torso. Optimized setups exhibited distributions of sensors over the whole torso with denser sampling above the heart at the front and back of the torso. Sensors close to the heart were arranged predominantly tangential to the body surface. The optimized sensor setup could facilitate the definition of a standard for sensor placement in MCG and the development of a wearable MCG vest for clinical diagnostics.

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.

Diagnostic value of magnetocardiography in coronary artery disease and cardiac arrhythmias: a review of clinical data

Despite the availability of several advanced non-invasive diagnostic tests such as echocardiography and magnetic resonance imaging, electrocardiography (ECG) remains as the most widely used diagnostic technique in clinical cardiology. ECG detects electrical potentials that are generated by cardiac electrical activity. In addition to electrical potentials, the same electrical activity of the heart also induces magnetic fields. These extremely weak cardiac magnetic signals are detected by a non-invasive, contactless technique called magnetocardiography (MCG), which has been evaluated in a number of clinical studies for its usefulness in diagnosing heart diseases. We reviewed the basic principles, history and clinical data on the diagnostic role of MCG in coronary artery disease and cardiac arrhythmias.

Noninvasive assessment of the effects of glucagon on the gastric slow wave

Hyperglycemic effects on the gastric slow wave are not well understood, and no studies have examined the effects that hyperglycemia has on gastric slow wave magnetic fields. We recorded multichannel magnetogastrograms (MGGs) before and after intravenous administration of glucagon and subsequent modest hyperglycemia in 20 normal volunteers. Normal slow waves were evident in baseline MGG recordings from all 20 subjects, but within 15 min after glucagon had been given, we noted significant effects on MGG signals. In addition to an overall decrease in the slow wave frequency from 2.9 +/- 0.5 cycles per min (cpm) to 2.2 +/- 0.1 cpm (P < 0.05), we observed significant changes in the number and range of spectral peaks recorded. Furthermore, the propagation velocity determined from surface current density maps computed from the multichannel MGG decreased significantly (7.1 +/- 0.8 mm/s to 5.0 +/- 0.3 mm/s, P < 0.05). This is the first study of biomagnetic effects of hyperglycemia in normal subjects. Our results suggest that the analysis of the MGG provides parameter quantification for gastric electrical activity specific to and characteristic of slow wave abnormalities associated with increased serum glucose by injection of glucagon.

Reproducibility of Quantitative Estimate of Magnetocardiographic Ventricular Depolarization and Repolarization Parameters in Healthy Subjects and Patients with Coronary Artery Disease

Magnetocardiography (MCG) has been introduced as an innovative non-invasive diagnostic tool to identify various heart diseases. However, there have been little data on the reliability of MCG parameters. The purpose of this study is to examine the test-retest reliability of different diagnostic parameters derived from MCG. We investigated short-, intermediate-, and long-term reliability of nine parameters from T (max/3)-T (max) interval, and five parameters from each time point such as QRS-wave, the peak of R-, and T-wave were evaluated. Short-term reliability was tested in the youngest 20 subjects (mean age = 26.3 +/- 4.9 years) in three sessions separated by 5 min. Intermediate-term reliability was tested in the 35 subjects with coronary artery disease (CAD) (65.1 +/- 7.1 years) with two recording sessions each in the morning and afternoon, separated by more than four hours. Long-term reliability was tested in seven subjects (37.1 +/- 8.8 years) using seven daily sessions. Interclass correlation coefficients (ICC) showed that test-retest reliability was good to excellent (0.99 > or = ICC > or = 0.80) for six out of nine parameters within T (max/3)-T (max). In addition, all parameters on the peak of R-wave, T-wave, and QRS-wave integrated were good to excellent (0.99 > or = ICC > or = 0.80) except for one parameter of CAD patients showing lower ICC values under 0.7. In conclusion, our study showed that the test-retest characteristics of the studied MCG parameters are generally stable and reliable over periods of minutes to days in subjects with different age spectrums.

Magnetocardiogram recordings in a nonshielded environment--reproducibility and ischemia detection

BACKGROUND

Magnetocardiography (MCG) is a noninvasive technology that measures the magnetic field of the heart by superconducting quantum interference devices (SQUID) sensors. The novelty of the present system is that the sensors can be operated without electromagnetic shielding of the examination room, thus allowing the system to be easily installed in the emergency department or chest pain unit. Studies in shielded rooms, found that this imaging modality may have better sensitivity as compared to ECG in detecting ischemia. We aimed (1) to assess the reproducibility, intra-observer, and interobserver interpretation variability and (2) to assess the MCG maps in the presence of coronary narrowings.

METHODS AND RESULTS

All measurements were performed in a nonshielded room. For the first part of the study, two MCG maps were recorded in 24 otherwise healthy volunteers (age 20-44 years, median 24, 16 male) in an interval ranging from 2 to 48 hours. The maps were interpreted using the CardioMag software for contour maps, averaged MCG time traces, and waveform morphology of repolarization by two observers blinded to each other. The parameters tested had low disagreement between repeated measurements. The correlations of the intra-observer and interobserver interpretation were excellent. Secondly, MCG maps were obtained in 29 patients referred for angiography due to suspected coronary artery disease. Nineteen of them had coronary narrowings defined as more than 50%. In this group, 16 (84.2%) had abnormal MCG maps as compared to only 5 (26.3%) who had abnormal ECGs (P < 0.01).

CONCLUSIONS

MCG maps can be successfully obtained in a nonshielded room and allow feasible, accurate, and reproducible measurements with little intra-observer and interobserver variability. Ischemic changes in the heart's magnetic field may occur before electrical changes. Our pilot data suggests that this imaging modality may potentially offer better sensitivity as compared to rest ECG in detecting ischemia in a cohort of patients who had coronary narrowings identified by angiography.

Recent advances in magnetocardiography

New developments in instrumentation, in clinical application, as well as in data analysis and visualization have provided new momentum to magnetocardiography (MCG). On one hand robust, easy to use and budget-priced MCG-systems entered the market and are applied to a multi-centred clinical study. On the other hand highly sophisticated vectormagnetometer systems with >300 SQUID sensors are opening new perspectives in electrocardiology research. Several parameters have recently been introduced to evaluate MCG-signals in order to support diagnosis, therapy follow-up and risk stratification. Particularly interesting is the renaissance of the Hosaka-Cohen-transformation which allows to visualize so-called pseudo current density (PCD) maps. A few examples are given to emphasise the value of these maps.

Myocardial viability evaluation using magnetocardiography in patients with coronary artery disease

OBJECTIVE

Magnetocardiography (MCG) has been used to risk stratify patients in terms of sudden death or to detect ischemia. We evaluated the potential of this technique to assess myocardial viability in coronary artery disease.

METHODS

Fifteen patients aged 36-75 (median, 59) years with stable single-vessel disease (> or =70% diameter stenosis) and corresponding regional wall-motion abnormality underwent (1) echocardiography to evaluate wall motion, (2) Tl dipyridamole single-photon emission computed tomography to document perfusion and (3) quantitative F-fluorodeoxyglucose positron emission tomography to assess viability in 16 left-ventricular wall segments. MCG was performed in each patient using a shielded prototype 49-channel low-temperature superconducting quantum interference device (SQUID) system. Multiple time and area parameters were extracted automatically from each baseline-corrected data set.

RESULTS

Eleven patients had prior myocardial infarction. In each patient, four to 12 (median, seven) segments were lesion dependent, totalling up to 117 out of 240 segments. A total of 88 segments (75%) were viable and 29 segments (25%) represented scar. Patients were divided into three categories: (a) no scar segments (five patients), (b) scar in one to three segments (six patients) and (c) scar in > or = four segments (four patients). The three MCG parameters with the best selectivity were identified using linear discriminant analysis with forward inclusion (P<0.10). The corresponding Fisher's discriminant functions classified all patients correctly (Wilks' lambda=0.079).

CONCLUSION

Selected MCG parameters yielded accurate patient classification with regard to the extension of myocardial scar within the viable tissue in retrospect. These findings indicate that MCG may contribute to the assessment of myocardial viability. Further evaluation in a comprehensive multicenter study is warranted.

Active magnetic shielding for biomagnetic measurement using spatial gradient fields

Biomagnetic measurement performed outside a magnetically shielded room is subject to distortion by strong magnetic fields. Reducing such disturbances can enhance and stabilize biomagnetic measurement conditions in the absence of passive shielding. We have developed an active magnetic shielding system that produces both homogeneous and spatial gradient magnetic fields. The system is composed of anisotropic magnetoresistive sensors, a digital signal processor controller and two different coil systems. In order to improve the measurement environment for a first-order gradient coil SQUID system, the disturbing vertical magnetic fields and vertical field gradients are reduced, thus achieving a shielding factor of approximately 6 at 100 Hz. Our system provides a more flexible and less costly alternative to magnetically shielded rooms.

Pre- and postsurgical biomagnetic activity in malt-type gastric lesions. A case report

PURPOSE

To evaluate the biomagnetic activity of a low-grade mucosa-associated lymphoid tissue (MALT-) type gastric lymphoma, and to determine if this procedure could be an adjunct to endoscopic ultrasonography (EUS) in the assessment of the disease pre- and postsurgically.

MATERIAL AND METHODS

A 47-year-old female with a 2-year clinical history of a low-grade MALT-type gastric lymphoma associated with Helicobacter pylori was examined. The disease showed no histologic remission after eradication of the H. pylori infection and subsequent treatment with chemotherapy and the patient was classified as stage II EA. A decision for surgical resection was made. Biomagnetic waveform recordings were made in the target area before and after surgery and the Fourier analysis of these recordings was performed.

RESULTS

The gastric lymphoma biomagnetic waveforms showed high amplitudes (1.8 pT) before and low amplitudes (0.6 pT) after resection. The corresponding Fourier analysis demonstrated that the maximum spectral power of the presurgical measurement was elevated (mean 235 +/- 847 pT2/Hz) compared with that obtained postsurgically (mean 725 +/- 89 pT2/Hz). The difference was of statistical significance ( p < 0.0001, t-test).

CONCLUSION

Biomagnetic monitoring of gastric lymphoma, which is an entirely new diagnostic modality, could be a supplement to EUS for assessing remission or persistence of disease with medical treatment during follow up.