f Multichannel magnetocardiographic measurements with a physical thorax phantom K. Pesola 1"2 U. Tenner 3 J. Nenonen 1"2 P. Endt TM H. Brauer 3 U. Leder s T. Katila 1'2 1Helsinki University of Technology, Laboratory Of Biomedical Engineering, PO Box 2200, 02015 HUT, Finland 2Medical Engineering Centre, BioMag Laboratory, Helsinki University Central Hospital, 00290 Helsinki, Finland 3Department of Electrical Engineering, Technical University of Ilmenau, PO Box 10 0565, 98684 Ilmenau, Germany 4Physikalisch-Technische-Bundesanstalt, 10587 Berlin, Germany SFriedrich-Schiller-University of Jena, Clinic of Internal Medicine III, 07743 Jena, Germany Abstract--Artificial dipolar sources were applied inside a physical thorax phantom to experimentally investigate the accuracy obtainable for non-invasive magnetocardiographic equivalent current dipole Iocalisation. For the measurements, the phantom was filled with saline solution of electrical conductivity 0.21 S m -1. A multichannel cardiomagnetometer was employed to record the magnetic fields generated by seven dipolar sources at distances from 25mm 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 Iocalisations 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 Iocalisation error was found to be 9~8mm, 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. Keywords--Magnetocardiography, Phantom measurement, Boundary element method, Equivalent current dipole Iocalisation Med. Biol. Eng. Comput., 1999, 37, 2-7 J 1 Introduction MAGNETOCARDIOGRAPHY(MCG) IS a completely non-inva- sive method to study electrophysiological activity in the heart (SILTANEN, 1988). In particular, MCG mapping has provided promising results in locating various intracardiac sources, e.g. the origin sites of life-threatening arrhythmias (NENONEN, 1'994). The accuracy of localisation is influenced by the source, by the volume conductor models, and by the calcula- tion method used in solving the inverse problem. The signal- to-noise ratio (SNR) of the MCG data also affects localisation accuracy. Patient movement (e.g. breathing) or inaccuracies related to reference methods (e.g. X-ray or magnetic reso- nance imaging) further complicate the combination of results with cardiac anatomy. Reported clinical MCG localisation accuracies obtained using an equivalent current dipole (ECD) as the source model, range from about 5mm to 30mm (FENICI and MELILLO, 1993, FENIC[ et al., 1998; MOSHAGE et al., 1996; MAKIJ.g, RV! et al., 1993;' NENONEN Correspondence should be addressed to Ms K. Peso/a; email: katja.pesola@hut.fi First received 5 January 1998 and in final form 5 October 1998 9 IFMBE: 1999 et al., 1993; OEFF and BURGHOFF, 1994; STROINK etal., 1996; WEISSMI2ILLER et al., 1992). In these studies, the ECD has been applied to locate ventricular pre-excitation sites asso- ciated with the Wolff-Parkinson-White syndrome, ventricular extrasystolic beats, the origin of ventricular tachycardia, and a non-magnetic stimulation catheter inserted in the heart. The ECD is an idealised, focal source model but has been proven to be useful in locating sources which are confined to a small volume of tissue. To experimentally investigate the accuracy obtainable with magnetocardiographic ECD localisation, a physical thorax phantom and artificial current dipole sources have been applied. Localisation accuracy was thoroughly evaluated in terms of the SNR and in terms of the goodness of fit of the localisations. Validation of the procedure was determined from measured magnetic data with an accurate reference about the position of the sources. The ECD localisation accuracies obtained here correspond to the best possible accuracy obtainable from clinical measurements. Magnetic recordings were performed using the 67-channel cardiomagnetometer in the BioMag Laboratory at Helsinki University Central Hospital (MONTONEN et al., 1998) in the same clinical environment that patient studies are carried out. The phantom used in this study was prepared in co-operation 2 Medical & Biological Engineering & Computing 1999, Vol. 37