Multichannel Magnetocardiographic Mapping of Small Animals in an unshielded laboratory

Contactless multichannel magnetocardiographic mapping (MMCG) is increasingly used in humans for non-invasive study of ventricular repolarization (VR) in patients with coronary artery disease, cardiomyopathy, and for three-dimensional (3D)localization and imaging of cardiac arrhythmogenic substrates. From clinical practice with MMCG, the need has raised for experimental investigations aimed to investigate and interpret the electrogenetic mechanisms underlying abnormal patterns. Furthermore, MMCG could be used to non-invasively study specific problems in animal models, especially in genetically altered mice with cardiomyopathy. So far no study had been reported on MMCG of small animals. This study was aimed to test the feasibility of unshielded MMCG of small intact animals. Method: A 36-channel DC-SQUID MMCG system designed for clinical application in unshielded laboratories (sensitivity: 20 fT/Hz½) was used for simultaneous MMCG from a 36-point grid, covering the area of 20 cm x 20 cm. Alternatively MMCG was performed sequentially with a 9-channel system (CardioMag Imaging Inc., USA). The latter system was also used to record a 9-point mini-map. 10 animals of different breeds (rabbits, rats and hamsters; body weight between 200 and 2000 grams) were studied, to define the minimum size still compatible with adequate MMCG imaging of cardiac magnetic fields and to define the limit of the method for contactless electrophysiologic source localization. Equivalent current dipole (ECD), Effective Magnetic Dipole (EMD) and distributed currents (DC) models were used in the inverse calculations for 3D localization of cardiac sources. Results: In rabbits and rats, reproducible imaging of both atrial and ventricular magnetic fields providing localization of cardiac sources, was possible after averaging 120 seconds of MMCG. Different breed-related patterns of VR were found in rats, which allowed breed differentiation of apparently identical rats on the basis of MMCG. Cardiac magnetic fields of hamsters were much weaker, thus a magnetocardiographic signal-to-noise ratio adequate for reproducible source localization was achievable only with ventricular signals. Conclusions: Contactless MMCG in small animals is feasible, even in an unshielded laboratory, with both 36 and 9-channels systems designed for clinical recordings. In smaller animals (rats and hamsters), a 9-channel system can be sufficient to detect the whole cardiac magnetic field distribution, with a single recording. The minimum animal weight to detect and study both atrial and ventricular magnetic fields was about 400 grams