Characteristics of magnetocardiograms measured using various pickup coil structures

Abstract

Magnetocardiograms (MCG) measured using multichannel SQUID sensors of different pickup coil configuration have different magnetic signal waveforms and field distributions. In order to compare MCG data measured using different multichannel systems and to make standard format of data, we fabricated and characterized three different MCG systems each having different structure of pickup coils. Pickup coils are integrated magnetometer, planar first-order gradiometer with baseline 4cm, planar second-order gradiometer with baseline 5cm. The magnetometer system measures normal component of MCG fields with 61 channels, and the two planar gradiometer systems measure tangential components of MCG fields each with 64 channels. The SQUIDs are double relaxation oscillation SQUID type fabricated using Nb/Al-oxide/Nb thin film process on Si wafers. From the measured tangential field maps, normal component waveforms and field maps were reconstructed using minimum norm estimation method. The average correlation values over one cardiac cycle between the measured Bz waveforms and reconstructed Bz waveforms were higher than 90% both for the first-order and second-order gradiometers, meaning that we can transform reliably the tangential measurement results to the conventional normal measurement scheme.