A customized bed based stand alone array of optically pumped magnetometers for fetal magnetocardiography measurements

Abstract

Fetal magnetocardiography (fMCG) is a non-invasive technique that measures the magnetic fields associated with fetal heart electrical activity outside of the maternal abdomen. fMCG has high temporal precision for measuring fetal heart rate and its variability which reflects fetal neurodevelopment. Free of cryogenics and low-cost sensors called microfabricated optically pumped magnetometers (OPMs) have emerged as an alternate to cryogenic SQUID (Superconducting Quantum Interference Device) systems to record fMCG. Previous research has demonstrated the ability of the OPMs to measure the fMCG at different maternal positions by taking the advantage of the conformal and geometric flexibility of the sensors. In this work, we designed and configured a bed-based stand-alone array of OPMs to obtain serial recordings of fMCG. 72 combined OPM-SQUID recordings were conducted at different gestational ages in 22 pregnant women. We were able to obtain fMCG with similar detectability as the gold standard SQUID from OPM sensors mounted on a novel belly-shape patient interface design with movable sensor holders. While additional translational research is needed, the outcome of this study can further facilitate the development of a non-cryogenic low-cost smaller footprint device to increase the use of OPMs for fetal research and clinical applications.

Keywords: Biomagnetism; Fetus; Magnetocardiography; Optically pumped magnetometers; Pregnancy.

Fig. 1

Stand-alone 14-sensor OPM system (QuSpin Inc., USA) with a customized bed. (A) Small footprint shielded enclosure (Magnetic Sheilds Ltd, UK). (B) Bed with sensor setup. (C) Sensor holder (designed by UAMS and production by Cerca Magnetics Ltd, UK) . The z-axis of each sensor was pointing towards the maternal abdomen and the y-axis was tangential to the mother’s body. (D) Subject lying flat with the abdomen over the sensors. (E) Recording performed inside the shielded enclosure.

Fig. 2

Noise floor in the empty magnetically shielded enclosure.

Fig. 3

Ten-second OPM fMCG data from a pregnant woman at 33 weeks of gestational age: (A) Biomagnetic data before removing maternal cardiac activities (B) Extracted fMCG after signal processing. Blue traces in Fig. 3A correspond to the fetal cardiac activities displayed in Fig. 3B.

Fig. 4

Heart rate extracted from back-to-back OPM and SQUID measurements. (A) Shows the FHR for OPM (blue traces) and SQUID (black traces). (B) Averaged OPM signals. (C) Averaged SQUID signals. Note that measurements were performed back-to-back, we expected to observed FHR difference between recordings.

Fig. 5

FHRV analysis. Pairs of OPM (blue) and SQUID (black) data within GA groups: (A) Heart rate (HR) in bpm (beats per minute). (B) R-R intervals in milliseconds.

Fig. 6

Examples of averaged fMCG waveforms for OPM (blue signals) and SQUID (black signals) recordings within a GA group: (A-B) 30 weeks of GA. (C-D) 31 weeks of GA. (E–F) 35 weeks of (G-H) 37 weeks of GA. Green and red brackets indicate the QT interval and PR interval, respectively.

Fig. 7

Durations of the fetal cardiac time intervals in milliseconds. Pairs of OPM (blue) and SQUID (black) data within a GA groups: (A) P wave. (B) PR interval. (C) QRS complex. (D) QT interval.