Unshielded Multichannel Magnetocardiographic Mapping of Small Animals.

Since the eighties, multichannel magnetocardiographic mapping (MMCG) has been increasingly used as a clinical method for non-invasive three-dimensional (3D) imaging and localization of electrophysiological phenomena in patients with coronary artery disease or cardiomyopathy, and cardiac arrhythmias [1]. From clinical practice, the need has raised for experimental models, aimed to investigate and interpret the electrogenetic mechanisms underlying abnormal MMCG patterns. Furthermore, contactless MMCG technology seems ideal to non-invasively follow-up cardiac conditions of small intact animals, especially those genetically altered with cardiomyopathy, during chronic experimental studies. So far no data are available on MMCG of small animals hearts. This study was aimed to test the feasibility of unshielded MMCG of small intact animals, with a commercially available multichannel instrumentation for clinical application. Method: A 36-channel DC-SQUID system designed for clinical application in unshielded laboratories (sensitivity: 20 fT/Hz) 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. The latter system was also used to record a 9-point mini-map [2]. The distance between the dewar and the animal anterior chest wall was kept to the minimum (5-10 mm). 20 animals of different breeds (rabbits, guinea pigs, rats and hamsters), which body weight ranged 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 Current Reconstruction (CR) models were used for 3D localization and imaging of cardiac sources. Results: In rabbits, guinea pigs 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. Source localization, during the whole QRS interval, properly localized the heart position in the chest, with ECD, EMD and CR models. Discussion: 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, compatible with detection of both atrial and ventricular magnetic fields, was about 250 grams.