Low-Cost Fetal Magnetocardiography: A Comparison of Superconducting Quantum Interference Device and Optically Pumped Magnetometers

Sarah Strand, William Lutter, Janette F Strasburger, Vishal Shah, Oswaldo Baffa, Ronald T Wakai, Sarah Strand, William Lutter, Janette F Strasburger, Vishal Shah, Oswaldo Baffa, Ronald T Wakai

Abstract

Background Fetal magnetocardiography (fMCG) is a highly effective technique for evaluation of fetuses with life-threatening arrhythmia, but its dissemination has been constrained by the high cost and complexity of Superconducting Quantum Interference Device (SQUID) instrumentation. Optically pumped magnetometers (OPMs) are a promising new technology that can replace SQUIDs for many applications. This study compares the performance of an fMCG system, utilizing OPMs operating in a person-sized magnetic shield, to that of a conventional fMCG system, utilizing SQUID magnetometers operating in a magnetically shielded room. Methods and Results fMCG recordings were made in 24 subjects using the SQUID system with the mother lying supine in a magnetically shielded room and the OPM system with the mother lying prone in a person-sized, cylindrical shield. Signal-to-noise ratios of the OPM and SQUID recordings were not statistically different and were adequate for diagnostic purposes with both technologies. Although the environmental noise was higher using the small open-ended shield, this was offset by the higher signal amplitude achieved with prone positioning, which reduced the distance between the fetus and sensors and improved patient comfort. In several subjects, fMCG provided a differential diagnosis that was more precise and/or definitive than was possible with echocardiography alone. Conclusions The OPM-based system was portable, improved patient comfort, and performed as well as the SQUID-based system at a small fraction of the cost. Electrophysiological assessment of fetal rhythm is now practical and will have a major impact on management of fetuses with long QT syndrome and other life-threatening arrhythmias.

Keywords: arrhythmia; electrophysiology; fetal heart; long QT syndrome; magnetocardiography.

Figures

Figure 1
Figure 1
Semilog plots of the power spectrum of the x, y, and z (vertical) components of the environmental magnetic noise. The magnetic field is measured in units of Tesla (T).
Figure 2
Figure 2
A, Photograph of 3D‐printed inverted sensor holder populated with 11 optically pumped magnetometer (OPM) sensors. B, Photograph showing the open end of the cylindrical shield, the sliding patient table, and the OPM sensors. C, OPM recording in cylindrical shield with subject lying prone. D, Superconducting Quantum Interference Device (SQUID) recording in MSR from same subject as in (C) with subject lying on her side. The rhythm strips are 5 seconds in duration. The gray vertical lines are 40 ms apart. MSR indicates magnetically shielded room.
Figure 3
Figure 3
Long QT syndrome waveforms and rhythms. Fetuses #18 (A and B), #19 (C and D), and #22 (E and F) showed QTc prolongation. In (B), the QTc is likely longer than shown; however, the T‐wave termination is obscure because of overlap with the P wave. Fetus #18 showed prominent T‐wave alternans (G and H). Fetus #4 showed complex rhythms, including atrial flutter and torsade des pointes (I and J). The rhythm strips are 5 seconds in duration. The gray vertical lines are 40 ms apart. OPM indicates optically pumped magnetometer; SQUID, Superconducting Quantum Interference Device.
Figure 4
Figure 4
Comparison of optically pumped magnetometer (OPM) and Superconducting Quantum Interference Device (SQUID) rhythm strips in fetuses with tachycardia and bradycardia. Fetus #21 (A and B) showed atrial ectopic tachycardia. Fetus #3 (C and D) showed bradycardia and ectopy because of blocked atrial bigeminy. Fetus #6 (E and F) showed second‐degree AV block with brief episodes of ventricular tachycardia. P waves (asterisks) often show low SNR in the raw tracings. The rhythm strips are 5 seconds in duration. The gray vertical lines are 40 ms apart. AV indicates atrioventricular; SNR, signal‐to‐noise ratio.
Figure 5
Figure 5
Optically pumped magnetometer (OPM) vs Superconducting Quantum Interference Device (SQUID) scatter plot comparisons of (A) PR, (B) QRS, (C) QTc, and (D) signal‐to‐noise ratio (SNR).

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