Prenatal diagnosis of QT prolongation by fetal magnetocardiogram--use of QRS and T-wave current-arrow maps

Genetic algorithms for dipole location of fetal magnetocardiography

In this paper, we explore the use of Maximum Likelihood (ML) method with Genetic Algorithms (GA) as global optimization procedure for source reconstruction in fetal magnetocardiography (fMCG) data. A multiple equivalent current dipole (ECD) model was used for sources active in different time samples. Inverse solutions across time were obtained for a single-dipole approximation to estimate the trajectory of the dipole position. We compared the GA and SIMPLEX methods in a simulation environment under noise conditions. Methods are applied on a real fMCG data. Results show robust estimators of the cardiac sources when GA is used as optimization technique.

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.

Surface current density mapping for identification of gastric slow wave propagation

The magnetogastrogram (MGG) records clinically relevant parameters of the electrical slow wave of the stomach noninvasively. Besides slow wave frequency, gastric slow wave propagation velocity is a potentially useful clinical indicator of the state of health of gastric tissue, but it is a difficult parameter to determine from noninvasive bioelectric or biomagnetic measurements. We present a method for computing the surface current density from multichannel MGG recordings that allows computation of the propagation velocity of the gastric slow wave. A moving dipole source model with hypothetical as well as realistic biomagnetometer parameters demonstrates that while a relatively sparse array of magnetometer sensors is sufficient to compute a single average propagation velocity, more detailed information about spatial variations in propagation velocity requires higher density magnetometer arrays. Finally, the method is validated with simultaneous MGG and serosal electromyography measurements in a porcine subject.

Repolarization spatial-time current abnormalities in patients with coronary heart disease

BACKGROUND

Magnetocardiography (MCG) is a new technique for visualizing a current distribution in the myocardium. In recent years, current distribution parameters (CDPs) have been developed based on the distribution. The CDPs reflect spatial-time current abnormalities in patients with coronary heart disease (CHD). However, the criteria and scoring method of the abnormalities using CDPs are still controversial.

METHOD

We measured MCG signals for 101 normal controls and 56 CHD patients (single-, double-, and triple-vessel diseases) using a MCG system. The CDPs (maximum current vector [MCV], total current vector [TCV], current integral map, and current rotation) during ventricular repolarization were analyzed. To evaluate the CDPs that are effective in distinguishing between normal controls and CHD patients, the areas under the receiver operating characteristic curve (A(z)) are calculated. Furthermore, the total scores ("0" to "4") of four CDPs with high A(z) values are also calculated.

RESULTS

MCV and TCV angles at the T-wave peak had the highest A(z) value. Furthermore, TCV angular differences between the ST-T segment also had high A(z) values. Using the four CDPs, the averaged total score for patients with triple-vessel disease was the highest ("2.67") compared to the other groups (normal controls: 0.53). Furthermore, based on the assumption that subjects with a total score over "1" were suspected of having CHD, sensitivity and specificity were 85.7% and 74.3%, respectively.

CONCLUSION

We concluded that the score and criteria using MCV and TCV during repolarization in CHD patients can reflect lesion areas and time changes of electrical activation dispersion due to ischemia.

Detection of non-ST-elevation myocardial infarction using magnetocardiogram: new information from spatiotemporal electrical activation map

BACKGROUND AND AIM

Non-ST-segment elevation myocardial infarction (NSTEMI) cannot be easily detected in the emergency room. We evaluate a method to detect NSTEMI using 64-channel magnetocardiography (MCG).

METHODS

MCG recordings were made in 20 NSTEMI patients (aged 59.7+/-12.4 years), 15 young (aged 26.8+/-3.4 years), and 13 age-matched control subjects (aged 57.3+/-3.6). We evaluated three approaches to analysis, including 1) determination when individual subjects' MCG results fell outside normal ranges for ten MCG parameters, 2) the magnetic field map at the T-wave peak (T-MFM), and 3) a pair of spatiotemporal activation graphs (STAGs) showing two projections of electrical excitation during repolarization.

RESULTS

Significant differences were found between normal controls and patients for all MCG parameters. None of the healthy controls had more than four MCG abnormal parameters, whereas 19 NSTEMI patients (95%) were abnormal in more than four parameters. STAGs and T-MFM also showed clear differences between healthy controls and NSTEMI patients.

CONCLUSIONS

These results suggest that the MCG is sensitive to changes in the cardiac electrical pathway after myocardial infarction as described by these graphs and parameters, and therefore MCG may be a useful tool to detect severe ischemic patients.

Space-time database for standardization of adult magnetocardiogram-making standard MCG parameters

BACKGROUND

The magnetocardiogram (MCG) is a promising medical tool for detecting and visualizing abnormal cardiac electrical activation in heart-disease patients. However, there is no large-scale MCG database of healthy subjects, and there is little knowledge of gender- and age-related influences on MCG data.

METHODS AND RESULTS

We obtained MCG data from 869 subjects (554 men, 315 women) using a conventional 64-channel MCG system, which covers the whole heart. Electrocardiogram (ECG) data were also obtained; 464 people (268 men, 196 women) were identified as a normal group using ECG data. Time intervals (PQ, QRS, QT, and QTc), current distributions (maximum current vector (MCV), and the total current vector (TCV)) of MCG data of the 464 normal subjects were analyzed to obtain basic MCG parameters. Although mean values of PQ and QRS intervals of the male subjects were slightly longer than those of the female subjects, no intervals were correlated with gender or age. The correlation between PQ intervals of ECG and those of MCG was better than the correlation between QRS and QT intervals of ECG and those of MCG. Both MCV and TCV angles were much smaller than the electrical-axis angle in ECG. Although TCVs of the QRS and T waves were stable, the women's mean T-wave-TCV angles significantly increased with age. The maximum amplitude of the P wave was about 1.7 pT, and the value of the QRS complex was about 20-25 pT. Moreover, the T-wave amplitude decreases with age.

CONCLUSION

The MCG standard space-time parameters determined here provide a normal range for MCG parameters.

Can magnetocardiography detect patients with non-ST-segment elevation myocardial infarction?

BACKGROUND AND AIM

Magnetocardiography (MCG) has been proposed as a noninvasive diagnostic tool to risk-stratify patients with myocardial infarction (MI) and ischemia. The purpose of this study is to find the MCG parameters that are sensitive enough to detect the non-ST-segment elevation myocardial infarction (NSTEMI) patients.

METHODS

MCG data were recorded and analyzed from 165 young controls (mean age = 27.2 +/- 9.0 years), 57 age-matched controls (mean age = 55.9 +/- 10.5 years) and 83 NSTEMI patients (mean age = 59.7 +/- 11.1 years). The MCG recordings were obtained using a 64-channel MCG system in a magnetically shielded room. Statistical analyses were performed for 24 parameters derived from QRS-, R-, T-wave, and ST-T period. Binary boundaries to detect NSTEMI patients out of control subjects were found using the receiver operating characteristic (ROC) curve for each parameter.

RESULTS

Fifteen parameters showed a significant difference (P < 0.05 and P < 0.01) between NSTEMI and both of the control groups. For detection of NSTEMI, the angle of the maximum current and the filed map angle on T-wave peak showed the highest diagnostic performance from 75% to 92% including accuracy, sensitivity, specificity, positive predictive value, and negative predictive value (area under ROC curve = 0.87 approximately 0.93).

CONCLUSIONS

Our study showed that MCG has potential clinical application for detection of NSTEMI and should be further investigated.

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.

Pseudo current density maps of electrophysiological heart, nerve or brain function and their physical basis

Background

In recent years the visualization of biomagnetic measurement data by so-called pseudo current density maps or Hosaka-Cohen (HC) transformations became popular.

Methods

The physical basis of these intuitive maps is clarified by means of analytically solvable problems.

Results

Examples in magnetocardiography, magnetoencephalography and magnetoneurography demonstrate the usefulness of this method.

Conclusion

Hardware realizations of the HC-transformation and some similar transformations are discussed which could advantageously support cross-platform comparability of biomagnetic measurements.

Vector projection of biomagnetic fields

Biomagnetic measurements are increasingly popular as functional imaging techniques for the non-invasive assessment of electrically active tissue. Although most currently available magnetometers utilise only one component of the vector magnetic field, some studies have suggested the possibility of obtaining additional information from recordings of the full magnetic field vector. Three projection techniques were applied to different biomagnetic signals for analysis of the three orthogonal components of the vector magnetic field. Vector magnetic fields obtained from fetal cardiac activity were projected into evenly spaced directions around a unit sphere. The vector magnetic field recorded from multiple intestinal current sources with independent temporal frequencies was then projected. Finally, an external reference signal from an invasive electrode was used to project the recorded vector magnetic fields due to gastric electrical activity. In each case, it was found that the information obtained by examination of the projected magnetic field vectors gave superior clinical insight to that obtained by analysis of any single magnetic field component.

Review of electromagnetic source investigations of the fetal heart

There is at present no reliable clinical technique for the assessment of cardiac electrophysiological activity in the fetus. There are two primary requirements of this type of monitoring: (i) sequential assessment of morphological and temporal parameters of cardiac electrical activity during advancing gestation, and (ii) description of the cardiac electrical activity in terms of an electrophysiologically realistic model. Fetal electrocardiography may be performed using maternal abdominal electrodes but this is only reliable prior to the 27th week of gestation. This is primarily because of the electrically insulating effects of the vernix caseosa and the existence of preferred conduction pathways between the fetal heart and maternal abdomen after this time. Fetal magnetocardiography is largely unaffected by these factors and so enables a reliable assessment of fetal electrocardiological activity throughout the second and third trimesters of pregnancy. This method can also be used to model fetal electrophysiological activity in terms of a current dipole or magnetic dipole. The vectorcardiogram is a plot of the dynamic change in dipole parameters during the cardiac cycle, allowing the study of growth-related or pathology-related electromagnetic changes in the heart. Fetal magnetocardiography and the fetal vectorcardiogram may thus provide important additions to current methods of antenatal monitoring.

Classifying cases of fetal Wolff-Parkinson-White syndrome by estimating the accessory pathway from fetal magnetocardiograms

The paper presents an evaluation of the possibility of using fetal magnetocardiogram (FMCG) signals to estimate and classify the accessory pathway in fetal Wolff-Parkinson-White (WPW) syndrome. The FMCG signals of two fetuses with WPW syndrome (type A) were detected using a 64-channel superconducting quantum-interference device system. An average across the cycles of these signals was taken to obtain clear WPW signals. To determine the direction and position of the accessory pathway in a fetal heart accurately, the accessory pathway and activated pathway at the peak of the QRS complex thus obtained were estimated for each fetus, using a single-dipole model. The phase angle (about 90 degrees) between the equivalent current dipoles (ECDs) was the same for both fetuses. This angle suggested that the accessory pathway is in the left side of the heart, i.e. that the pathway exists in the left ventricle, which indicates type A WPW syndrome. Identification of the position of the accessory pathway in a fetus with WPW syndrome from the angle between the ECD of the accessory pathway and the ECD of the peak in the QRS complex was thus demonstrated.

Detection of spatial repolarization abnormalities in patients with LQT1 and LQT2 forms of congenital long-QT syndrome

The aim of this study is to detect the spatial current dispersion that appears in the T-wave of patients with congenital long-QT syndrome (LQTS). To observe this dispersion, magnetocardiograms (MCGs)--which have a high spatial resolution--of LQT1 patients (n = 7), LQT2 patients (n = 9) and a control group (n = 33) were recorded. The dispersion was evaluated by plotting current-arrow maps (CAMs) calculated from the MCG signals. In the case of LQT1, abnormal current arrows in the CAMs appeared above the inferior part of the heart in two LQT1 patients with a long corrected QT interval (QTc) (>0.6), and the current direction was from the left (origin side) to the right ventricular muscle (110 degrees). In six out of nine LQT2 patients, abnormal current arrows with angles below 20 degrees were observed above the right inferior part or lower septum; the current direction was from the right (origin side) to the left ventricular muscle. However, in the case of the LQT2 patients, the QTc values did not correlate with the abnormal current. These findings suggest that the origin of abnormal repolarization in LQT1 is the left ventricular muscle and the origin of that in LQT2 is the right ventricular muscle or lower septum. The estimation of the origin in LQTS patients can provide important information such as the risk factor of sudden death.

Two-dimensional mapping of impedance magnetocardiograms

A new method for measuring two-dimensional (2-D) impedance magnetocardiograms (I-MCGs) and magnetocardiograms (MCGs) above the heart simultaneously, has been developed. The I-MCG's and MCG's signals are recorded by using a superconducting interference device (SQUID) system. To measure the I-MCG and MCG signals, four first-order gradiometers with an 18-mm diameter and a 50-mm baseline were used. The SQUIDs are driven by a flux-locked-loop circuit with a frequency range higher than that of an ac-current (40 kHz) with constant amplitude passed through a subject. The output of the circuit is filtered through two circuits: one for measuring the I-MCG signals and one for measuring the MCG signals. The I-MCG signals are obtained by demodulating the magnetic field, which is detected by the gradiometers, at the frequency of the ac current. As a result, the I-MCG signal reflects the change in spatial distribution of conductivity caused by the movement of the heart muscle and blood volume. A contour map of the 2-D I-MCG signals showed the largest signals occur above the right ventricle and right atrium. In a corresponding current-arrow map, it was found that the large current arrows occurred above the right side of the right ventricle. Furthermore, it was found that the systole and diastole timings obtained from the first-derivative I-MCG signal and the phonocardiogram were different. These results show that primitive 2-D I-MCG signal can provide much physiological information on the circulatory movement of the heart.

Detection of atrial-flutter and atrial-fibrillation waveforms by fetal magnetocardiogram

Two cases of fetal tachycardia are reported: atrial flutter and fibrillation. The waveforms from each case were detected by fetal magnetocardiograms (FMCGs) using a 64-channel superconducting quantum interference device (SQUID) system. Because the magnitude of supraventricular arrhythmia signals is very weak, two subtraction methods were used to detect the fetal MCG waveforms: subtraction of the maternal MCG signal, and subtraction of the fetal ORS complex signal. It was found that atrial-flutter waveforms showed a cyclic pattern and that atrial-fibrillation waveforms showed f-waves with a random atrial rhythm. Fast Fourier transform analysis determined the main frequency of the atrial flutter to be about 7Hz, and the frequency distribution of atrial fibrillation consisted of small, broad peaks. To visualise the current pattern, current-arrow maps, which simplify the observation of pseudo-current patterns in fetal hearts, of the averaged atrial flutter and fibrillation waveforms were produced. The map of the atrial flutter had a circular pattern, indicating a re-entry circuit, and the map of the atrial fibrillation indicated one wavelet, which was produced by a micro-re-entry circuit. It is thus concluded that an FMCG can detect supraventricular arrhythmia, which can be characterised by re-entry circuits, in fetuses.