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The assessment of myocardial viability is a major diagnostic challenge in patients with coronary artery disease (CAD) after myocardial infarction. Novel threedimensional current density (CD) imaging algorithms use high-resolution magnetic field mapping to determine the electrical activity of myocardial segments at rest. We, for the first time, compared CD activity obtained with several algorithms to 18-F-fluoro-deoxyglucose positron emission tomography (FDG-PET) in evaluation of myocardial viability. Magnetic field maps were obtained in nine adult patients (pt) with CAD and a history of infarction. The criterion for non-viable myocardium was an FDG-PET uptake with less than 45% of the maximum in the respective segments. CD imaging was applied to the left ventricle by using six different methods to solve the inverse problem. Mean CD activity was calculated for a close meshed grid of 90 locations of the left ventricle. A cardiologist compared bull's eye plots of CD and FDG-PET activity by eye. Spearman's correlation coefficients and specificity at a given level of sensitivity (70%) were calculated. Bull's eye plots revealed a significant correlation of CD/PET in 5 pt and no correlation in 3 pt. One pt had a negative correlation. The six different CD reconstruction methods performed similar. While CD reconstruction has the principal potential to image viable myocardium, we found that the reconstructed CD magnitude was low in scar segments but also reduced in some segments with preserved metabolic activity under resting conditions. New vector measurement techniques, the use of additional stress testing and advances in mathematical methodology are expected to improve CD imaging in future.

The heart rate variability (HRV) is related to several mechanisms of the complex autonomic functioning such as respiratory heart rate modulation and phase dependencies between heart beat cycles and breathing cycles. The underlying processes are basically nonlinear. In order to understand and quantitatively assess those physiological interactions an adequate coupling analysis is necessary. We hypothesized that nonlinear measures of HRV and cardiorespiratory interdependencies are superior to the standard HRV measures in classifying patients after acute myocardial infarction. We introduced mutual information measures which provide access to nonlinear interdependencies as counterpart to the classically linear correlation analysis. The nonlinear statistical autodependencies of HRV were quantified by auto mutual information, the respiratory heart rate modulation by cardiorespiratory cross mutual information, respectively. The phase interdependencies between heart beat cycles and breathing cycles were assessed basing on the histograms of the frequency ratios of the instantaneous heart beat and respiratory cycles. Furthermore, the relative duration of phase synchronized intervals was acquired. We investigated 39 patients after acute myocardial infarction versus 24 controls. The discrimination of these groups was improved by cardiorespiratory cross mutual information measures and phase interdependencies measures in comparison to the linear standard HRV measures. This result was statistically confirmed by means of logistic regression models of particular variable subsets and their receiver operating characteristics.

The noninvasive measurement of magnetic heart activity based on high- T C DC SQUIDs, could be a tool for investigating cardiac electrophysiological properties, if such a system is able to work in unshielded environments. For the realization of such a system we use thin film planar SQUID gradiometers with bicrystal or step-edge Josephson junctions. Even for a small baseline of approximately 4 mm (limited by substrate dimensions), the field gradent resolution is in the range required for clinical analysis. We use a portable system which consists of a glass fiber cryostat with a measuring unit where we can place up to four sensors, different read-out electronics, and signal filtering methods. Firstly, measurements inside a magnetically shielded room were used to show the capability of our magnetocardiography (MCG) system in medical applcations. The system works in an unshielded environment also, without additional field compensation. We demonstrate a measurement of a magnetocardiogram in a real clinical environment, and discuss the possibilties as well as the limitations of this system in magnetocardiography.

Objective: In this paper we present an attempt at noninvasive imaging of distributed myocardial electrical activity in patients suffering from myocardial infarction and in healthy subjects. Although advances have been made, noninvasive three-dimensional imaging of cardiac electrophysiological activity is still in its infancy and extending our knowledge of cardiac electrophysiological properties may be a valuable guide in the treatment of patients with coronary artery disease. Methods: Magnetic field mapping data formed the input for an inverse solution that is based on a multiple dipole model. The lead field normalized minimum norm least square criterion was applied to predefined myocardial source geometry. Current density distributions were calculated for the left ventricle during ventricular depolarization. Images from two patients with previous myocardial infarction were compared to images from two healthy subjects. Results: Low regional and global current density was found in the infarction patients. Regions of low current density corresponded to infarcted segments. The images of the healthy subjects displayed less marked areas of low current density. Conclusion: The proposed multiple dipole model may be able to distinguish viable from scarred myocardium. A prospective clinical study should be undertaken to investigate the spatial resolution and the diagnostic performance of this method.

A wide range of cardiac imaging procedures is available to assess left-ventricular function, geometry, perfusion, and altered metabolism. There is no non-invasive imaging procedure to detect electrically unstable myocardium. We assessed the electrical function of the myocardium by biomagnetic imaging of high-frequency depolarisation signals in a patient aged 70 years who had non-sustained ventricular tachycardia that developed after anterior left-ventricular myocardial infarction and apical aneurysm. Imaging was done on the basis of magnetic-field measurement and inverse computation of the magnitudes of 1200 left-ventricular endocardial dipoles regularly spaced (mean distance 3 mm). Such endocardial activity images have been computed for the unfiltered depolarisation signal (QRS) and for the last 40 ms of the bidirectional 30 Hz highpass filtered depolarisation signal (LP). In theory, three types of QRS/LP image pairs could appear. If areas of high dipole magnitude of LP and QRS images match, LP generators can be taken to lie in the intact myocardium. If areas of high dipole magnitude in LP images match areas of low dipole magnitude in QRS images, LP generators may belong to the area infarcted and, therefore, display properties of the surviving subendocardial layer. If no correlation of LP and QRS is seen, it will not fit into the concepts of arrhythmogenesis after infarction. By comparison of LP and QRS images (figure), we observed regional mismatch of dipole magnitudes. The QRS image shows low dipole magnitudes in the apical segment of the left ventricle (the apical aneurysm) with a loss of electrically active myocardium. The opposite has appeared in the LP image with high dipole magnitudes in the segments infarcted. This finding could be an indication of fragmented delayed intraventricular conduction in this region. We conclude that this method supports the imaging of delayed high-frequency depolarisation signals, which are known to correlate with conduction disturbances and, therefore, risk of arrhythmia. Our findings may help in assessing patients with malignant tachyarrhythmias after myocardial infarction.