Development of a highly balanced gradiometer for fetal magnetocardiography

Activation of the cardiac muscle is associated with ion transport through the membrane of the cardiac cell. This gives rise to the strongest electrophysiological signals in human body: the cardiograms. Segments of the cardiogram are related to the contraction of the hearts chambers. In fetal magnetocardiography, the magnetic field of a fetal heart is recorded in the vicinity of the maternal abdomen. This magnetic field, however, is extremely weak and can only be recorded by means of Superconducting QUantum Interference Devices (SQUIDs). To reach the superconducting state the SQUIDs are cooled to about - 270oC. Typically, fetal magnetocardiograms are recorded inside magnetically shielded rooms in order to reduce the influence of the environmental magnetic interference. As an alternative to magnetic shielding, higher-order gradiometers can be used. Ideally, a higher-order gradiometer is insensitive to the lower-order gradients of the magnetic field that originate from relatively remote noise sources. In practice, however, it is difficult to manufacture a gradiometer such that its sensitivity to these lower-order gradients is eliminated completely. The residual sensitivity to the lower-order gradients is referred to as imbalance. Balancing a gradiometer reduces imbalance and, thus, improves the environmental interference suppression. The objective of the work presented in this thesis is to design a highly-balanced higherorder gradiometer that enables measurements of fetal magnetocardiograms in unshielded environment. This includes optimization of the geometry of the gradiometer with respect to signal-to-noise ratio, design of a magnetic coils set for gradiometer balancing and gradiometer balancing experiments.