Multichannel Magnetocardiography Research Articles

Enhancing the efficiency and cost-effectiveness of magnetocardiography by optimal channel selection for cardiac diagnosis

Background. Magnetocardiography (MCG) is a non-invasive and non-contact technique that measures weak magnetic fields generated by the heart. It is highly effective in the diagnosis of heart abnormalities. Multichannel MCG provides detailed spatio-temporal information of the measured magnetic fields. While multichannel MCG systems are costly, usage of the optimal number of measurement channels to characterize cardiac magnetic fields without any appreciable loss of signal information would be economically beneficial and promote the widespread use of MCG technology. Methods. An optimization method based on the sequential selection approach is used to choose channels containing the maximum signal information while avoiding redundancy. The study comprised 40 healthy individuals, along with two subjects having ischemic heart disease and one subject with premature ventricular contraction. MCG measured using a 37 channel MCG system. After revisiting the existing methods of optimization, the mean error and correlation of the optimal set of measurement channels with those of all 37 channels are evaluated for different sets, and it has been found that 18 channels are adequate. Results. The chosen 18 optimal channels exhibited a strong correlation (0.99 ± 0.006) between the original and reconstructed magnetic field maps for a cardiac cycle in healthy subjects. The root mean square error is 0.295 pT, indicating minimal deviation. Conclusion. This selection method provides an efficient approach for choosing MCG, which could be used for minimizing the number of channels as well as in practical unforeseen measurement conditions where few channels are noisy during the measurement.

A movable unshielded magnetocardiography system.

Magnetocardiography (MCG), which uses high-sensitivity magnetometers to record magnetic field signals generated by electrical activity in the heart, is a noninvasive method for evaluating heart diseases such as arrhythmia and ischemia. The MCG measurements usually require the participant keeping still in a magnetically shielded room due to the immovable sensor and noisy external environments. These requirements limit MCG applications, such as exercise MCG tests and long-term MCG observations, which are useful for early detections of heart diseases. Here, we introduce a movable MCG system that can clearly record MCG signals of freely behaving participants in an unshielded environment. On the basis of optically pumped magnetometers with a sensitivity of 140 fT/Hz1/2, we successfully demonstrated the resting MCG and the exercise MCG tests. Our method is promising to realize a practical movable multichannel unshielded MCG system that nearly sets no limits to participants and brings another kind of insight into the medical diagnosis of heart disease.

Calibration of SQUID Magnetometers in Multichannel MCG System Based on Bi-Planar Coil

In a practical multichannel magnetocardiography (MCG) system based on superconducting quantum interference device (SQUID) magnetometers, the Tesla/Volt coefficient must be precisely calibrated for accurate analysis of heart diseases. In this article, we utilize the biplanar coil to calibrate the SQUID magnetometers in an MCG system for the first time. The coil design theory is discussed in detail, including the target-field theory and the Tikhonov regularization method. Also, the coil winding patterns can be obtained after discretization of the stream function, which is related to the current density. By defining the parameter of calibration uncertainty, the performance of the biplanar coil is analyzed and compared with a rectangular Helmholtz coil based on the COMSOL simulation method and experimental measurement. The measurement results are well in coincidence with the simulation ones, indicating that the biplanar coil has remarkable advantages over the Helmholtz coil. Also, Tesla/Volt coefficient calibrations of four SQUID magnetometers in an MCG system are conducted inside a magnetically shielded room (MSR) depending on these two coils. The final results show the superiority of the biplanar coil method in MCG system calibration.

Weakly Damped SQUID Gradiometer With Low Crosstalk for Magnetocardiography Measurement

In a multichannel magnetocardiography system, the problem of crosstalk is serious due to mutual inductances between the pickup coils of adjacent channels. In this paper, a superconducting quantum interference device gradiometer module based on weakly damped Josephson junctions with large $\beta _{c}\;(\approx 5)$ is presented. An external feedback design is also adopted to suppress the crosstalk between different channels. With a special superconducting connection between input coil and pickup coil, an axial first-order gradiometer with baseline of 70 mm and diameter of 18 mm is fabricated. Using a direct readout circuit with equivalent flux noise of $1.8\;\mu \Phi _{{0}}{\rm{/ \surd Hz}}$ , the flux noise of this gradiometer is measured to be $6.5\;\mu \Phi _{{0}}{\rm{/ \surd Hz}}$ in a white noise range, corresponding to a field sensitivity of ${\rm{0.5\;fT/ (cm\bullet \surd Hz)}}$ . The crosstalk rates in external feedback mode and internal feedback mode are both successfully simulated with MAXWELL software. The results by experimental measurements using these gradiometers are in coincidence with the simulated ones; both simulation and experimental results show a substantial suppression of crosstalk in the external feedback design.

Early Myocardial Repolarization Heterogeneity Is Detected by Magnetocardiography in Diabetic Patients with Cardiovascular Risk Factors.

Multi-channel magnetocardiography (MCG) is a sensitive technique to map spatial ventricular repolarization with high resolution and reproducibility. Spatial ventricular repolarization heterogeneity measured by MCG has been shown to accurately detect and localize myocardial ischemia. Here, we explored whether these measurements correlated with cardiovascular risk factors in patients with type 2 diabetes. Two hundreds and seventy-seven type 2 diabetic patients without known coronary artery disease (CAD) and arrhythmia were recruited consecutively from the outpatient clinic of National Taiwan University Hospital. The spatially distributed QTc contour maps were constructed with 64-channel MCG using the superconducting quantum interference device (SQUID) system. Indices of myocardial repolarization heterogeneity including the smoothness index of QTc (SI-QTc) and QTc dispersion were derived and analyzed for association with conventional cardiovascular risk factors. SI-QTc correlated strongly with the QTc dispersion (r = 0.70, p <0.0001). SI-QTc was significantly higher in patients with presence of metabolic syndrome in comparison to those without metabolic syndrome (8.56 vs. 7.96 ms, p = 0.02). In univariate correlation analyses, QTc dispersion was associated with smoking status (average 79.90, 83.83, 86.51, and 86.00 ms for never smokers, ex-smokers, current smokers reporting less than 10 cigarettes daily, and current smoker reporting more than 10 cigarettes daily, respectively, p = 0.03), body weight (r = 0.15, p = 0.01), and hemoglobin A1c (r = 0.12, p = 0.04). In stepwise multivariate regression analyses, QTc dispersion was associated with smoking (p = 0.02), body weight (p = 0.04), total cholesterol levels (p = 0.05), and possibly estimated glomerular filtration rate (p = 0.07). In summary, spatial heterogeneity of myocardial repolarization measured by MCG is positively associated cardiovascular risk factors including adiposity, smoking, and total cholesterol levels.

Denoising of multichannel MCG data by the combination of EEMD and ICA and its effect on the pseudo current density maps

The signal preprocessing is prerequisite for reduction of noise and for better estimation of sources from the measured field distribution of multichannel data, since different measurement channels may be contaminated by different types of artifacts and noise. Toward this, we use a combination of independent component analysis (ICA) and ensemble empirical mode decomposition (EEMD) to denoise the multichannel magnetocardiography (MCG) data. In this technique, MCG time series data is first subjected to ICA to obtain the statistically independent components (ICs) and subsequently the EEMD-interval threshold based denoising is applied to the ICs prior to the reconstruction of the signal. We compare the results obtained from EEMD-ICA with those obtained using the conventional ICA alone and also using the wavelet enhanced ICA (wICA). We illustrate the effect of these denoising techniques on the pseudo current density (PCD) maps, which aid in visualizing the source location. The results obtained from the EEMD-ICA are seen to be decidedly superior compared to those obtained by ICA alone and wICA methods.

Abstract 13892: Fragmented QRS Activity Representing Inhomogeneous Left Ventricular Conduction on Magnetocardiography Predicts Adverse Outcomes in Patients With LBBB and Left Ventricular Dysfunction

Background: Left bundle branch block (LBBB) is known to be a risk factor in patients with heart failure or structural heart diseases. We have already reported that the fragmented QRS (fQRS) activity on QRS vector magnitude waveform (VMW) derived from high-resolution multi-channel magnetocardiography (MCG) represents heterogeneous left ventricular (LV) activation sequences due to myocardial scar and can predict adverse outcomes in patients with dilated and hypertrophic cardiomyopathy. Therefore, we investigated whether MCG also can predict the outcome of patients with LBBB noninvasively using fQRS. Methods: 64-ch MCG was recorded in 58 patients with LV dysfunction (LVEF&lt;45%) and LBBB (QRS interval ≥120ms). Clinical characteristics, electrocardiographic parameters, and clinical outcomes were compared between patients with and without fQRS on the QRS VMW, which was constructed from 64 superimposed signals during sinus rhythm. The fQRS on MCG was defined as the presence of ≥1 discrete component larger than 20% of the maximum amplitude within QRS complex. Results: The fQRS was identified on MCG in 29 patients (Group-A), but not in the rest of 29 patients (Group-B). Age, Sex, presence of structural heart disease, QRS or QTc interval, fQRS (≥8 spikes in leads I, aVL, V5 and V6) on 12-lead ECG, history of arrhythmias, LVEF, and medications were not different between 2 groups. During the follow-up of 30 ± 23 months, cardiac events (sudden cardiac death, heart failure, hospitalization, and life-threatening arrhythmia) occurred more frequently in the Group-A (17/29, 58.6%) than in the Group-B (3/29, 10.3%; p &lt; 0.0002), although patients with fQRS on ECG showed the similar event rate to those without. Multivariate analysis revealed that the presence of fQRS on MCG was only an independent predictor of poor outcome (p=0.0137). Conclusions: The prognosis of LBBB patients with fQRS on MCG was poor. The fQRS on MCG, which indicates the presence of inhomogeneous LV conduction, can be a reliable marker for predicting adverse outcomes in patients not only with cardiomyopathy but also with LBBB.

Developing fetal motor-cardiovascular coordination analyzed from multi-channel magnetocardiography

Fetal movements (FM) related heart rate accelerations (AC) are an important maturation criterion. Since Doppler-based time resolution is not sufficient for accompanying heart rate variability analysis, the work is aimed at a comprehensive FM-AC analysis using magnetocardiographic recordings from fetuses during sleep.We identify FM and AC by independent component analysis and automatic recognition algorithms. We investigate associations between FM and AC of different magnitude by means of event coincidence and time series cross-correlation over the maturation period of 21–40 weeks of gestation (WGA).FM related AC appear with increasing AC magnitude and WGA. Vice versa, AC related FM appear independent of WGA, but more frequently with increasing AC amplitude. The FM-AC correlation exists already at 21 WGA and further increases with WGA while the variability of its time delay decreases. Hence, FM and AC are clearly associated over the whole investigated maturation period. The increase of FM related AC runs parallel to the increasing AC magnitude.The MCG methodology was confirmed and results from previous Doppler-based analyses reproduced. Hence, MCG recordings allow the collective analysis of heart rate variability based maturation indices and FM related AC. This synergism may improve the diagnosis of fetal developmental disorders.

Spatial Repolarization Heterogeneity Detected by Magnetocardiography Correlates with Cardiac Iron Overload and Adverse Cardiac Events in Beta-Thalassemia Major

BackgroundPatients with transfusion-dependent beta-thalassemia major (TM) are at risk for myocardial iron overload and cardiac complications. Spatial repolarization heterogeneity is known to be elevated in patients with certain cardiac diseases, but little is known in TM patients. The purpose of this study was to evaluate spatial repolarization heterogeneity in patients with TM, and to investigate the relationships between spatial repolarization heterogeneity, cardiac iron load, and adverse cardiac events.Methods and ResultsFifty patients with TM and 55 control subjects received 64-channel magnetocardiography (MCG) to determine spatial repolarization heterogeneity, which was evaluated by a smoothness index of QTc (SI-QTc), a standard deviation of QTc (SD-QTc), and a QTc dispersion. Left ventricular function and myocardial T2* values were assessed by cardiac magnetic resonance. Patients with TM had significantly greater SI-QTc, SD-QTc, and QTc dispersion compared to the control subjects (all p values<0.001). Spatial repolarization heterogeneity was even more pronounced in patients with significant iron overload (T2*<20 ms, n = 20) compared to those with normal T2* (all p values<0.001). Loge cardiac T2* correlated with SI-QTc (r = −0.609, p<0.001), SD-QTc (r = −0.572, p<0.001), and QTc dispersion (r = −0.622, p<0.001), while all these indices had no relationship with measurements of the left ventricular geometry or function. At the time of study, 10 patients had either heart failure or arrhythmia. All 3 indices of repolarization heterogeneity were related to the presence of adverse cardiac events, with areas under the receiver operating characteristic curves (ranged between 0.79 and 0.86), similar to that of cardiac T2*.ConclusionsMultichannel MCG demonstrated that patients with TM had increased spatial repolarization heterogeneity, which is related to myocardial iron load and adverse cardiac events.

Magnetocardiac signal analysis based on multiple time windows beamformer method

A beamformer method based on multiple time windows is proposed to estimate the equivalent current sources of different strength and duration from multi-channel magnetocardiography data. This technique of visualizing cardiac electric activities will be a great help to the diagnosis of relevant heart diseases. Feasibility of this beamformer method has been proved by simulation experiments of two known sources. Source estimation results and two-dimensional excitation propagation diagrams of a complete left bundle branch block and a complete right bundle branch block patients during QRS complex are taken to be compared. The revealed characteristics of their electrophysiological phenomena are analyzed.

Multichannel fetal magnetocardiography using SQUID bootstrap circuit

Fetal magnetocardiography (MCG) is a sophisticated non-invasive technique for the fetal heart diagnosis. We constructed a multichannel fetal MCG system based on a novel superconducting quantum interference device (SQUID) direct readout scheme called SQUID bootstrap circuit (SBC). The system incorporates four SBC gradiometers for the signal detection and three SBC magnetometers as the references. The fetal MCG signal at a 28-weeks' gestation was measured. By the fetal MCG signal separation and average, the P-wave and QRS complex can be clearly identified. These results indicate that the SBC is one of the most promising techniques for the fetal MCG recordings.

Thirty years of clinical magnetocardiography at the Catholic University of Rome: Diagnostic value and new perspectives for the treatment of cardiac arrhythmias

We read with much interest the excellent review article of Kwong et al. [1] which summarizes in detail most of the literature on the diagnostic value of magnetocardiography (MCG) in coronary artery disease (CAD). However, as concerns cardiac arrhythmias, we regrettably noticed that the research work carried out, since 1981, at the Catholic University in Rome, documented by multiple publications and summarized in previous review articles [2–4], has not been quoted in that review, although significantly contributing to the development of innovative instrumentation for unshielded multichannel MCG and to the clinical validation and routine application of such novel imaging method, especially for non-invasive three-dimensional (3D) electro-anatomical imaging (EAI) and localization of cardiac electrogenesis.

Magnetocardiographic Assessment of Healed Myocardial Infarction

We evaluated the capability of multichannel magnetocardiography (MCG) to detect healed myocardial infarction (MI). Multichannel MCG over frontal chest was recorded at rest in 21 patients with healed MI, detected by cine- and contrast-enhanced magnetic resonance imaging, and in 26 healthy controls. Of the 21 MI patients, 11 had non-Q wave and 10 Q wave MIs. QRS, ST-segment, T wave and ST-T wave integrals, ST-segment and T wave amplitudes, and QRS and ST-T wave magnetic field map orientations were measured. The MCG repolarization indexes, such as ST segment and ST-T wave integrals, separated the MI group from the controls (ST-T wave integral -1.4 +/- 5.3 vs 1.5 +/- 4.7 pTs, P = 0.034). The abnormalities were more distinct in the Q wave-MI than in the non-Q wave MI subgroup. In the latter, however, a trend similar to the Q wave MI group was found. The relation of QRS area to ST segment and T wave integral improved the detection of healed MIs compared to the ST-T wave indexes alone (QRS-ST-T discordance 14 +/- 10 vs 5.0 +/- 7.1 pTs, P = 0.003). When comparing the MI group to the controls, the orientation of the magnetic field maps differed in the ST-T wave maps (163 +/- 119 degrees vs 58 +/- 17 degrees, P < 0.001) but not in the QRS maps (111 +/- 95 degrees vs 106 +/-93 degrees, P = 0.646). The MCG repolarization variables can detect healed MI. These ST-T wave abnormalities are more pronounced in patients with Q wave MI than in patients with non-Q wave MIs. Relating the signals of depolarization and repolarization phases improves the detection of healed MI. Repolarization abnormalities are common in healed MI and thus should not always be interpreted as present ongoing ischemia.

Control System for Readout Electronics of Multi-Channel Magnetocardiographs Using High-Temperature DC Superconducting Quantum Interference Devices

We aimed to develop a control system for multichannel magnetocardiography (MCG) based on a high-temperature DC superconducting quantum interference device (high-Tc SQUID). To create this system, we used one oscillator as an AC bias controller to operate a multichannel high-Tc SQUID. To optimize the SQUID parameters (such as the AC bias, offset voltage), two new control sequences based on a cross-correlation method and a fast Fourier transform method were developed. Using the AC bias controller and the sequences, the typical white noise level of the SQUID was about 50–60 fT Hz-1/2 around 100 Hz. Multichannel MCG signals were detected clearly in the system with the SQUIDs. We conclude that our control system with one oscillator and new protocols can reliably operate a multichannel SQUID.

Measurement and reproducibility of magnetocardiographic filtered atrial signal in patients with paroxysmal lone atrial fibrillation and in healthy subjects

Magnetocardiography (MCG) is a method complementary to electrocardiography (ECG). We examined recording and reproducibility of atrial depolarization signal by MCG. Multichannel MCG over anterior chest and orthogonal 3-lead ECG were recorded in 9 patients who had paroxysmal lone atrial fibrillation and in 10 healthy subjects in duplicate at least 1 week apart. Data were averaged using atrial wave template and high-pass filtered at 25, 40, and 60 Hz. Atrial signal duration with automatic detection of onset and offset and root mean square amplitudes of the last portion of atrial signal were determined. Coefficient of variation of atrial signal duration by MCG at 40 Hz was 3.3% and difference between the measurements was 3.5 milliseconds on average. The corresponding figures obtained by signal-averaged ECG (SAECG) were 6.1% and 6.9 milliseconds. Coefficient of variation for root mean square of the last 40 milliseconds of atrial signal were 16% in MCG and 17% in SAECG. Reproducibility was best at 40-Hz filter and similar in patients and healthy subjects. In conclusion, the reproducibility of atrial signal variables in MCG is adequate and somewhat better than in SAECG and equal in patients with lone atrial fibrillation and healthy subjects. Magnetocardiography seems to be a potentially valuable method to evaluate features of atrial depolarization in patient studies.

COMPARISON OF CARDIAC MAGNETIC FIELD DISTRIBUTIONS DURING DEPOLARIZATION AND REPOLARIZATION

Multichannel magnetocardiography measures the magnetic field distribution of the human heart noninvasively from many sites over the body surface. Multichannel magnetocardiogram (MCG) analysis enables regional temporal differences in the distribution of cardiac magnetic field strength during depolarization and repolarization to be identified, allowing estimation of the global and local inhomogeneity of the cardiac activation process. The aim of this study was to compare the spatial distribution of cardiac magnetic field strength during ventricular depolarization and repolarization in both normal subjects and patients with cardiac abnormalities, obtaining amplitude measurements by magnetocardiography. MCGs were recorded at 49 sites over the heart from three normal subjects and two patients with inverted T-wave conditions. The magnetic field intensity during depolarization and repolarization was measured automatically for each channel and displayed spatially as contour maps. A Pearson correlation was used to determine the spatial relationship between the variables. For normal subjects, magnetic field strength maps during depolarization (R-wave) showed two asymmetric regions of magnetic field strength with a high positive value in the lower half of the chest and a high negative value above this. The regions of high R-wave amplitude corresponded spatially to concentrated asymmetric regions of high magnetic field strength during repolarization (T-wave). Pearson-r correlation coefficients of 0.7 (p&lt;0.01), 0.8 (p&lt;0.01) and 0.9 (p&lt;0.01) were obtained from this analysis for the three normal subjects. A negative correlation coefficient of -0.7 (p&lt;0.01) was obtained for one of the subjects with inverted T-wave abnormalities, suggesting similar but inverted magnetic field and current distributions to normal subjects. Even with the high correlation values in these four subjects, the MCG was able to identify differences in the distribution of magnetic field strength, with a shift in the T-wave relative to the R-wave. The measurement of cardiac magnetic field distribution during depolarization and repolarization of normal subjects and patients with clinical abnormalities should enable the improvement of theoretical models for the explanation of the cardiac depolarization and repolarization processes.

A new screening method to diagnose coronary artery disease using multichannel magnetocardiogram and simple exercise.

Magnetocardiography (MCG) is a non-contact mapping technique to record cardiac action currents. The Master's two-step electrocardiogram (ECG) test is a simple exercise method for screening coronary artery disease (CAD), but it is inadequate concerning the sensitivity. Our aim was to develop a new screening method using multichannel MCG instead of ECG. Thirty subjects (aged 54 +/- 16 years, 27 males), 17 of whom had CAD confirmed by coronary angiography, underwent the Master's exercise ECG test. After the exercise, MCG signals were acquired every minute during recovery with a 64-channel MCG system (MC-6400, Hitachi Ltd). We integrated tangential components of the MCG signals within QRS (during 20, 40, 80, and 120 ms centering on R-wave peak) immediately after exercise (Iex) and 5 minutes after exercise (Irec). The exercise-induced change of currents [(Iex-Irec)/Irec] was determined and normalized for each channel, and the maximal change among 64 channels, maximal QRS integral change, was used as a diagnostic index for myocardial ischemia. The maximal QRS integral change during 40 ms was significantly higher in the CAD group than in the control group (0.81 +/- 0.51 vs. 0.36 +/- 0.19, p < 0.01). A sensitivity and specificity for predicting CAD by the change > 0.44 were 82 % and 85 %, respectively, yielding a diagnostic accuracy of 83 %. The conventional Master's ECG test identified the CAD patients with a diagnostic accuracy of 63 % (sensitivity 47 %, specificity 85 %). The Master's two-step exercise test with a 64-channel MCG system showed the high diagnostic accuracy, despite of non-contact recording and simple exercise. The magnetic field in the depolarization process has the potential to detect the subtle myocardial ischemia induced by exercise.

Noninvasive study of ventricular preexcitation using multichannel magnetocardiography.

In clinical practice, noninvasive classification of ventricular preexcitation (VPX) is usually done with ECG algorithms, which provide only a qualitative localization of accessory pathways. Since 1984, single or multichannel magnetocardiography (MMCG) has been used for three-dimensional localization of VPX sites, but a systematic study comparing the results of ECG and MMCG methods was lacking. This study evaluated the reliability of MMCG in an unshielded electrophysiological catheterization laboratory, and compared VPX classification as achieved with the five most recent ECG algorithms with that obtained by MMCG mapping and imaging techniques. A nine-channel direct current superconducting quantum interference device (DC-SQUID) MMCG system (sensitivity is 20 fT/Hz0.5) was used for sequential MMCG from 36 points on the anterior chest wall, within an area 20 x 20 cm. Twenty-eight patients with Wolff-Parkinson-White syndrome were examined at least twice, on the same day or after several months to test the reproducibility of the measurements. In eight patients, the reproducibility of MMCG was also evaluated using different MCG instrumentation during maximal VPX and/or atrioventricular reentrant tachycardia induced by transesophageal atrial pacing via a nonmagnetic catheter. The results of VPX localization with ECG algorithms and MMCG were compared. Equivalent current dipole, effective magnetic dipole, and distributed currents imaging models were used for the inverse solution. MMCG classification of VPX was found to be more accurate than ECG methods, and also provided additional information for the identification of paraseptal pathways. Furthermore, in patients with complex activation patterns during the delta wave, distributed currents imaging revealed two different activation patterns, suggesting the existence of multiple accessory pathways.

Phantom validation of multichannel magnetocardiography source localization.

Multichannel magnetocardiography (MMCG) is used clinically for noninvasive localization of the site of origin of cardiac arrhythmias. However, its accuracy in unshielded environments is still unknown. The aim of this study was to test the accuracy of three-dimensional localization of intracardiac sources by means of MMCG in an unshielded catheterization laboratory using a saline-filled phantom, together with a nonmagnetic catheter designed for multiple monophasic action potential recordings in a clinical setting. A nine-channel direct current superconducting quantum interference device (DC-SQUID) system (sensitivity fT/Hz0.5) was used for MMCG from 36 points in a measuring area of 20 x 20 cm. The artificial sources to be localized were dipoles embedded in the distal end of the catheter, placed 12 cm below the sensor's plane. Equivalent current dipoles, effective magnetic dipoles, and distributed currents models were used for the inverse solution. The localization error was estimated as the three-dimensional difference between the physical position of the tip of the catheter and the three-dimensional localization of the dipoles derived by means of the inverse solution calculated from MMCG data. The reproducibility was tested by repeating the MMCG after repositioning the phantom and the measurement system. The average location error of the catheter dipole was 9 +/- 4 mm and was due primarily to imprecise depth estimation. Localization was reproducible within 0.73 mm. The distributed currents model provided an accurate image of current distribution centered over the catheter tip. The authors conclude that MMCG estimation is accurate enough to guarantee proper localization of cardiac dipolar sources even in an unshielded clinical electrophysiological laboratory.

Relation of magnetocardiographic arrhythmia risk parameters to delayed ventricular conduction in postinfarction ventricular tachycardia.

Time-domain late field and intra-QRS fragmentation parameters in magnetocardiography (MCG) identify patients prone to VT after myocardial infarction. This study investigated if they are related to slow ventricular conduction and affected by arrhythmia surgery. Twenty-two patients with old myocardial infarction undergoing map-guided subendocardial resection to treat sustained VT were included. Bipolar electrograms were recorded during operation using an epicardial jacket and endocardial balloon electrode array. The time from the QRS onset to the end of local ventricular excitation in each electrogram was measured during sinus rhythm. Multi-channel MCG was recorded before and after operation and filtered QRS duration (QRSd), root mean square amplitude of the magnetic field strength during the last 40 ms of the QRS complex (RMS40), duration of the low amplitude signal < 300 fT (LAS300), fragmentation index M (M), and fragmentation score S (S) were determined. All patients had one or two VT foci localized and resected. MCG parameters correlated with time to the latest end of ventricular excitation; r = 0.45 for QRSd (P = 0.035), r = 0.64 for M (P = 0.001), and r = 0.73 for S (P < 0.001). The correlations were even better in patients with anterior infarction (e.g., r = 0.87 for QRSd, P < 0.001; r = 0.91 for M, P < 0.001). The operation reduced the abnormalities in MCG parameters and 20 of the 21 patients tested postoperatively became noninducible. MCG parameters indicating postinfarction arrhythmia propensity are related to delayed ventricular conduction. Abolition of the arrhythmia substrate reverses the abnormality of these parameters.