K Pesola

Bioelectromagnetic localization of a pacing catheter in the heart

The accuracy of localizing source currents within the human heart by non-invasive magneto- and electrocardiographic methods was investigated in 10 patients. A non-magnetic stimulation catheter inside the heart served as a reference current source. Biplane fluoroscopic imaging with lead ball markers was used to record the catheter position. Simultaneous multichannel magnetocardiographic (MCG) and body surface potential mapping (BSPM) recordings were performed during catheter pacing. Equivalent current dipole localizations were computed from MCG and BSPM data, employing standard and patient-specific boundary element torso models. Using individual models with the lungs included, the average MCG localization error was 7+/-3 mm, whereas the average BSPM localization error was 25+/-4 mm. In the simplified case of a single homogeneous standard torso model, an average error of 9+/-3 mm was obtained from MCG recordings. The MCG localization accuracies obtained in this study imply that the capability of multichannel MCG to locate dipolar sources is sufficient for clinical purposes, even without constructing individual torso models from x-ray or from magnetic resonance images.

Nonfluoroscopic localization of an amagnetic stimulation catheter by multichannel magnetocardiography.

This study was performed to: (1) evaluate the accuracy of noninvasive magnetocardiographic (MCG) localization of an amagnetic stimulation catheter; (2) validate the feasibility of this multipurpose catheter; and (3) study the characteristics of cardiac evoked fields. A stimulation catheter specially designed to produce no magnetic disturbances was inserted into the heart of five patients after routine electrophysiological studies. The catheter position was documented on biplane cine x‐ray images. MCG signals were then recorded in a magnetically shielded room during cardiac pacing. Noninvasive localization of the catheters tip and stimulated depolarization was computed from measured MCG data using a moving equivalent current‐dipole source in patient‐specific boundary element torso models. In all five patients, the MCG localizations were anatomically in good agreement with the catheter positions defined from the x‐ray images. The mean distance between the position of the tip of the catheter defined from x‐ray fluoroscopy and the MCG localization was 11 ± 4 mm. The mean three‐dimensional difference between the MCG localization at the peak stimulus and the MCG localization, during the ventricular evoked response about 3 ms later, was 4 ± 1 mm calculated from signal‐averaged data. The 95% confidence interval of beat‐to‐beat localization of the tip of the stimulation catheter from ten consecutive beats in the patients was 4 ± 2 mm. The propagation velocity of the equivalent current dipole between 5 and 10 ms after the peak stimulus was 0.9 ± 0.2 m/s. The results show that the use of the amagnetic catheter is technically feasible and reliable in clinical studies. The accurate three‐dimensional localization of this multipurpose catheter by multichannel MCG suggests that the method could be developed toward a useful clinical tool during electrophysiological studies.

Multichannel magnetocardiographic measurements with a physical thorax phantom

Artificial dipolar sources were applied inside a physical thorax phantom to experimentally investigate the accuracy obtainable for non-invasive magnetocardiographic equivalent current dipole localisation. For the measurements, the phantom was filled with saline solution of electrical conductivity 0.21 Sm−1. A multichannel cardiomagnetometer was employed to record the magnetic fields generated by seven dipolar sources at distances from 25 mm to 145 mm below the surface of the phantom. The inverse problem was solved using an equivalent current dipole in a homogeneous boundary element torso model. The dipole parameters were determined with a non-linear least squares fitting algorithm. The signal-to-noise ratio (SNR) and the goodness of fit of the calculated localisations were used in assessing the quality of the results. The dependence between the SNR and the goodness of fit was derived, and the results were found to correspond to the model. With SNR between 5 and 10, the average localisation error was found to be 9±8 mm, while for SNR between 30 and 40 and goodness of fit between 99.5% and 100%, the average error reduced to 3.2±0.3 mm. The SNR values obtained in this study were also compared with typical clinical values of SNR.

Nonfluoroscopic Localization of an Amagnetic Catheter in a Realistic Torso Phantom by Magnetocardiographic and Body Surface Potential Mapping

This study was performed to evaluate the accuracy of multichannel magnetocardiographic (MCG) and body surface potential mapping (BSPM) in localizing three‐dimensionally the tip of an amagnetic catheter for electrophysiology without fluoroscopy. An amagnetic catheter (AC), specially designed to produce dipolar sources of different geometry without magnetic disturbances, was placed inside a physical thorax phantom at two different depths, 38 mm and 88 mm below the frontal surface of the phantom. Sixty‐seven MCG and 123 BSPM signals generated by the 10 mA current stimuli fed into the catheter were then recorded in a magnetically shielded room. Non‐invasive localization of the tip of the catheter was computed from measured MCG and BSPM data using an equivalent current dipole source in a phantom‐specific boundary element torso model. The mean 3‐dimensional error of the MCG localization at the closer level was 2 ± 1 mm. The corresponding error calculated from the BSPM measurements was 4 ± 1 mm. At the deeper level, the mean localization errors of MCG and BSPM were 7 ± 4 mm and 10 ± 2 mm, respectively. The results showed that MCG and BSPM localization of the tip of the AC is accurate and reproducible provided that the signal‐to‐noise ratio is sufficiently high. In our study, the MCG method was found to be more accurate than BSPM. This suggests that both methods could be developed towards a useful clinical tool for nonfluoroscopic 3‐dimensional electroanatomical imaging during electrophysiological studies, thus minimizing radiation exposure to patients and operators.

Current density estimation on the left ventricular epicardium: A Potential method for ischemia localization

The localization of ischemic regions in the Leart is a poteiitial cliuical applicatiou for magnetocardiography (MCG). Non-invasive localization and deterwination of the sizc of ischemic arcas is clinically of significant importancc. The usc of point-likc sourcc modcls, such äs a singlc currcnt dipolc, is not fcasiblc in ischemia localization äs the ischemic regions can be large. Thus, distributed source models need to be applied in solving the inverse problem. In this study, a discrete equivalent current density was applied on the epicardial surface of the left ventricle (LV). A similar model Las been applied previously, e.g., in locating epicardial current sources [1.2] and in detecting decreased current amplitude in myocardial infarction (MI) patients [3], Triangulated boundary element (BE) torso models, acquired froin magnetic resouance imaging (MRI), were used to model the volumc conductor. The BE modcls also contained ehe outcr surfacc. i.e., the cpicaxdial surface of the LV. The current density estimates were solved frora magnetic field data using Tikhonov regularization. Simulated MCG data with realistic amount of measuremeut noise were used to study different regularization operators. In addition to simulations, current density estimation (CDE) was applied in 13 coronary artcry disoasc (CAD) patients. The calculatcd cstimatcs wcrc comparcd with the clinical rcfcrcncc Information about the LV function.

Magnetocardiographic Pacemapping for Nonfluoroscopic Localization of Intracardiac Electrophysiology Catheters

The purpose of the study was to validate, in patients, the accuracy of magnetocardiography (MCG) for three‐dimensional localization of an amagnetic catheter (AC) for multiple monophasic action potential (MAP) with a spatial resolution of 4 mm2. The AC was inserted in five patients after routine electrophysiological study. Four MAPs were simultaneously recorded to monitor the stability of endocardial contact of the AC during the MCG localization. MAP signals were band‐pass filtered DC‐500 Hz and digitized at 2 KHz. The position of the AC was also imaged by biplane fluoroscopy (XR), along with lead markers. MCG studies were performed with a multichannel SQUID system in the Helsinki BioMag shielded room. Current dipoles (5mm; 10mA), activated at the tip of the AC, were localized using the equivalent current dipole (ECD) model in patient‐specific boundary element torso. The accuracy of the MCG localizations was evaluated by: (1) anatomic location of ECD in the MRI, (2) mismatch with XR. The AC was correctly localized in the right ventricle of all patients using MRI. The mean three‐dimensional mismatch between XR and MCG localizations was 6 ± 2 mm (beat‐to‐beat analysis). The coefficient of variation of three‐dimensional localization of the AC was 1.37% and the coefficient of reproducibility was 2.6 mm. In patients, in the absence of arrhythmias, average local variation coefficients of right ventricular MAP duration at 50% and 90% ofrepolarization, were 7.4% and 3.1%, respectively. This study demonstrates that with adequate signal‐to‐noise ratio, MCG three‐dimensional localizations are accurate and reproducible enough to provide nonfluoroscopy dependant multimodal imaging for high resolution endocardial mapping of monophasic action potentials.

Relation of Magnetocardiographic Arrhythmia Risk Parameters to Delayed Ventricular Conduction in Postinfarction Ventricular Tachycardia

KORHONEN, P., et al.: 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. Multichannel 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.

Non-invasive determination of the activation sequence of the heart: validation by comparison with invasive human data

During cardiac surgery epicardial electrograms were recorded. From these electrograms epicardial activation maps were constructed. Prior to surgery the ECG at the body surface was measured. For each subject an individual volume conductor model was constructed based on MR images. The UDL source model was used to compute the epicardial activation times from the recorded ECGs. The compute activation times were compared to the measured ones. The overall pattern of the activation sequence found by the UDL corresponds well to the actual activation pattern. Epicardial breakthroughs and regions of late activation that were seen in the measured data, occurred at similar sites in the computed data.

Cardiomagnetic Source Imaging Studies with Focal and Distributed Source Models

During the recent years, niagnetocardiographic (MCG) source imaging has received increasing iuterest [l, 2). The ability to locatc currcnt sourccs combincd with prccisc timing of cvcnts is valuable both in basic rcscarch and in clinical studies [3, 4]. Prcsc.ntly, thc MCG is employed at sorae hospitals to test and further develop its clinical use [5, 6|. The BioMag Laboratory at the Helsinki University Central Hospital (HUCH) is equipped with gtate-ofthe-art facilities both for basic research and for clinical applications in functional ncuro and cardiomagnctic studics. Thc cardiac mcasurcmcnt Systems tLc 67clianncl MCG and thc 123-channcl body-surfacc potential mapping (BSPM) equipment have been described in detail elsewhere [7). Combiued with fast data transfer to the Helsinki University of Technology (HUT) and the Signal and image processing methods developed at HUT, the facilities give a good iramework for cardiomagnetic source imaging research. In the following, we describe briefly such studies in association with the ongoing patient studies. First, we outline howindividualized boundary-eleinent torso models are constructed froni inedical image data. We have developed nearly automated methods to segmcnt thc objccts of intorcst from magnctic rcsonancc (MR) or X-ray imagcs, äs well äs a novcl triangulation Software to tessellate the surfaces of the objects. Next, we discuss testing of the ability of MCG to locate point-like sources. Artificial current-dipole sources have been utilized. both in phantom experiinents and in studies with a pacing catheter placed inside the heart during the MCG recordings. In addition. we show rcsults obtaincd with distributcd source modcls, such äs minimum-norm cstimatcs on thc cpicardial surface, and double-layer sources in reconstructing activation sequences on the heart surfaces. In our studies, the golden Standard for the inverse Solutions is provided by clinically validated invasive data recorded before MCG measurements. In many cases, the BSPM was performed simultaneously with thc MCG rccordings, and similar source analysis was carricd out both from thc MCG and thc BSPM data. Methods Patient studies