Jukka Nenonen

Magnetocardiographic functional localization using a current dipole in a realistic torso

A fast and numerically effective biomagnetic inverse solution using a moving current dipole in a realistic homogeneous torso is described. The localization model and high-resolution magnetocardiographic (HR-MCG) mapping were applied to localize noninvasively the ventricular preexcitation site in ten patients suffering from Wolff-Parkinson-White syndrome. In all cases, the computed localization results were compared to the results obtained by the invasive catheter technique. Using a standard-size torso model in all cases, the average 3-D distance between the computed noninvasive locations and the invasively obtained results was 2.8+or-1.4 cm. When the torso was rescaled to better match the true shape of the subject, the 3-D average was improved to 2.2+or-1.0 cm. This accuracy is very satisfactory, suggesting that the method would be clinically useful.<<ETX>>

A Matlab library for solving quasi-static volume conduction problems using the boundary element method

The boundary element method (BEM) is commonly used in the modeling of bioelectromagnetic phenomena. The Matlab language is increasingly popular among students and researchers, but there is no free, easy-to-use Matlab library for boundary element computations. We present a hands-on, freely available Matlab BEM source code for solving bioelectromagnetic volume conduction problems and any (quasi-)static potential problems that obey the Laplace equation. The basic principle of the BEM is presented and discretization of the surface integral equation for electric potential is worked through in detail. Contents and design of the library are described, and results of example computations in spherical volume conductors are validated against analytical solutions. Three application examples are also presented. Further information, source code for application examples, and information on obtaining the library are available in the WWW-page of the library: (http://biomed.tkk.fi/BEM).

Detection of Exercise‐Induced Myocardial Ischemia by Multichannel Magnetocardiography in Single Vessel Coronary Artery Disease

Background: Detection of myocardial ischemia was studied with multichannel exercise magneto‐cardiography (MCG). A surface gradient method was applied to analyze cardiac magnetic fields.

Activation Dynamics in Anisotropic Cardiac Tissue via Decoupling

Bidomain theory for cardiac tissue assumes two interpenetrating anisotropic media—intracellular (i) and extracellular (e)—connected everywhere via a cell membrane; four local parameters σi,eℓ,t specify conductivities in the longitudinal (ℓ) and transverse (t) directions with respect to cardiac muscle fibers. The full bidomain model for the propagation of electrical activation consists of coupled elliptic–parabolic partial differential equations for the transmembrane potential vm and extracellular potential φe, together with quasistatic equations for the flow of current in the extracardiac regions. In this work we develop a preliminary assessment of the consequences of neglecting the effect of the passive extracardiac tissue and intracardiac blood masses on wave propagation in isolated whole heart models and describe a decoupling procedure, which requires no assumptions on the anisotropic conductivities and which yields a single reaction–diffusion equation for simulating the propagation of activation. This reduction to a decoupled model is justified in terms of the dimensionless parameter ∈ = (σℓiσte − σtiσℓe)/(σℓi+σℓe)(σti+σte). Numerical simulations are generated which compare propagation in a sheet H of cardiac tissue using the full bidomain model, an isolated bidomain model, and the decoupled model. Preliminary results suggest that the decoupled model may be adequate for studying general properties of cardiac dynamics in isolated whole heart models.

Recording locations in multichannel magnetocardiography and body surface potential mapping sensitive for regional exercise-induced myocardial ischemia.

Introduction This study aimed to identify the optimal locations in multichannel magnetocardiography (MCG) and body surface potential mapping (BSPM) to detect exercise-induced myocardial ischemia. Methods We studied 17 healthy controls and 24 coronary artery disease (CAD) patients with stenosis in one of the main coronary artery branches: left anterior descending (LAD) in 11 patients, right (RCA) in 7 patients, and left circumflex (LCX) in 6 patients. MCG and BSPM signals were recorded during a supine bicycle stress test. The capability of a recording location to separate the groups was quantified by subtracting the mean signal amplitude of the normal group from that of the patient group during the ST segment and at the T-wave apex, and dividing the resulting amplitude difference by the corresponding standard deviation within all subjects. Results In MCG the optimal location for ST depression was at the right inferior grid for the RCA, at the mid-inferior grid for the LCX, and in the middle of these locations for the LAD subgroup (mean ST amplitudes: CAD −80 ± 360fT, controls 610 ± 660fT; p < 0.001). In BSPM it was on the left upper anterior thorax for the LAD, left lower anterior thorax for the RCA, and on the lower back for the LCX subgroup (mean ST amplitudes: CAD −39 ± 61 μV and controls 38 ± 38 μV; p < 0.001). In MCG the optimal site for T-wave amplitude decrease was the same as the one for the ST depression. In BSPM it was on the middle front for the LAD, on the back for the LCX and on the left abdominal area for the RCA group. In accordance with electromagnetic theory, the largest ST segment and T-wave amplitude changes took place in MCG in locations orthogonal to those in BSPM. Conclusion This study identified magnetocardiographic and BSPM recording locations which are sensitive for detecting transient myocardial ischemia by evaluation of the ST segment as well as the T-wave. These locations strongly depend on ischemic regions and are outside the conventional 12-lead ECG recording sites.

A 3-D model-based registration approach for the PET, MR and MCG cardiac data fusion

In this paper, a new approach is presented for the assessment of a 3-D anatomical and functional model of the heart including structural information from magnetic resonance imaging (MRI) and functional information from positron emission tomography (PET) and magnetocardiography (MCG). The method uses model-based co-registration of MR and PET images and marker-based registration for MRI and MCG. Model-based segmentation of MR anatomical images results in an individualized 3-D biventricular model of the heart including functional parameters from PET and MCG in an easily interpretable 3-D form.

Minimum-norm estimation in a boundary-element torso model

The paper deals with the bioelectric and biomagnetic inverse problems. The authors present a method to estimate primary-current distributions in a homogeneous, realistically shaped boundary-element torso model. The reconstruction surface is triangulated to keep the procedure computationally feasible. The minimum-norm estimate is computed on the basis of separate electric and magnetic signals, as well as from combined data. The method can be used both for heart and brain studies. Simulation results for current-dipole sources in a homogeneous realistic torso are discussed.

A triangulation method of an arbitrary point set for biomagnetic problems

A new triangulation method has been developed for extracting isosurface from volume data. The nodes for triangulation can be selected arbitrarily from the surface of the object of interest. The Voronoi polygons for nodes are searched on the surface and triangulation is accomplished by connecting the neighboring Voronoi areas. The method is basically Delaunay triangulation using geodesic distances instead of Euclidean ones. In areas where the curvature of the surface is low, the Delaunay criteria are fulfilled. When the curvature is high, the geometry of the object is described more accurately than in Euclidean Delaunay methods. Since geodesic distances are utilized, i.e., the surface information is used in triangulation, the topology of the object can be preserved more easily than in the Euclidean cases. Our fully automatic method has been developed for boundary element modeling and it has been successfully applied in magnetocardiographic and electrocardiographic forward and inverse studies. However, the method can be utilized in any triangulation problem if the surface description is provided.

Conversion of magnetocardiographic recordings between two different multichannel SQUID devices

Comparison of biomagnetic measurements performed with different multichannel magnetometers is difficult, because differing sensor types and locations do not allow measurements from the same locations in respect to the body. In this study, two transformation procedures were utilized to compare magnetocardiograms (MCG) recorded with two different multisensor systems. Signals from one sensor array were used to compute parameters of a multipole expansion or minimum-norm estimates at 1-ms steps over the cardiac cycle. The signals of the second sensor array were then simulated from the computed estimates and compared against measured data. Both the multipole- and the minimum-norm-based transformation method yielded good results; the average correlation between simulated and measured signals was 93%. Thus, the methods are useful to compare MCG recordings performed using differing sensor configurations, e.g., for multicenter patient studies. This study provides the first empirical basis for assessing the transformation of MCG data of differing devices by general model-based field reconstructions.

Thermal noise in biomagnetic measurements

Studies of weak magnetic fields are generally influenced by magnetic noise emanating from thermal agitation of electric charge (Johnson noise) in electrically conducting materials surrounding the magnetic‐field sensor. In this article, the thermal magnetic noise fields generated by slabs with high electric conductivity (copper, aluminum) or high magnetic permeability (mu metal) are studied. The analysis is based both on a previously published phenomenological model and on measurements with an ultrasensitive superconducting magnetometer. Both the spectral densities and spatial correlations of the magnetic field fluctuations are evaluated. The computed correlation coefficients are utilized to develop a practical method for estimating the thermal noise due to thin conducting foils, such as thermal radiation shields in a cryogenic measurement Dewar. Also experiments to reduce the Dewar noise are reported. Finally, estimations are presented for the thermal noise fields arising in the walls of a magnetically sh...