Magnetocardiography (MCG) – measurement of magnetic signals outside the body due to heart activity – could prove a better option, as magnetic fields are not heavily distorted by this insulating layer. Current MCG systems based on superconducting quantum interference devices (SQUIDs) are expensive, however, and their rigid shape is unsuitable for measurements on the abdomen of a pregnant mother.

A US--German research team is instead investigating the use of microfabricated optically pumped magnetometers (µOPMs), assembled into a flexible sensor array that fits over the mother's abdomen at any stage of pregnancy. The sensors have a footprint of 1 cm2 and a volume of about 2.5 cm3, and do not require cooling. The flexible placement also eases recording of the mother's MCG (mMCG), which is always superimposed on the foetal MCG (fMCG) and must be identified and separated (Phys. Med. Biol. 60 4797).

"Through advanced foetal MCG sensor technology we hope to provide a less expensive tool for evaluating certain types of foetal heart arrhythmia that are mechanically silent and cannot be distinguished with standard echocardiography techniques," explained Tilmann Sander-Thoemmes, from the Physikalisch-Technische Bundesanstalt in Germany.

Heartbeat tracking

Project leader Svenja Knappe, from the University of Colorado, Boulder, and National Institute of Standards and Technology, and co-workers created a multichannel array comprising 25 µOPM sensors inserted into three flexible belt‐shaped holders. They used the array to perform several 300 s measurements on a pregnant (32 weeks) volunteer, with two sensor belts placed on her abdomen and one over her chest. The distance between the sensitive volume of the magnetometer and the skin was approximately 4.5 mm.

The device is a prototype and not all channels were working perfectly, as examining the raw multi‐channel data revealed. The researchers excluded channels with a noise levels greater than 120 pT (about 1 million times weaker than the Earth's magnetic field), and used data from 16 sensors for their analysis. Data recorded from the chest channels showed clear R‐peaks (ventricular contraction of the heart) indicating a heart rate of approximately 60 beats per minute (bpm), due to the mother's heart. Data from abdomen channels exhibited more rapid peak sequences, corresponding to the fMCG signals.

The signal recorded over the abdomen, however, was dominated by the stronger mMCG signal. To reliably extract the foetal heart rate, the fMCG and mMCG signals must be separated. To do this, the researchers used two techniques developed for analysing multi-channel SQUID data: orthogonal‐projection (OP) algorithms and independent‐component analysis (ICA).

After OP processing, visual examination of an abdomen channel signal clearly showed the fMCG as a rapid heartbeat. Eleven fMCG R‐peaks were clearly visible in a 4 s section, indicating a foetal heart rate of more than 150 bpm.

Individual sensors exhibited differences in sensitivity, with a similar range to that of multichannel SQUID systems. The estimated signal‐to‐noise ratio of a typical fMCG signal was 4 (peak‐to‐peak noise of 10 pT; foetal R‐peak height of 40 pT), compared with a typical value of 10 for SQUIDs.

To increase the signal‐to‐noise ratio, it's possible to calculate an average beat, by determining the R-peak and averaging individual beats, using their R-peaks for alignment. The researchers determined the mMCG and fMCG R‐peak positions, and averaged a 30 s data interval containing about 30 maternal and 75 foetal heartbeats.

The mMCG average was calculated from raw data, the fMCG average from OP-processed data. After averaging, the foetal P‐wave could be seen, though the foetal T‐wave did not appear. For optimal clinical benefit, such features should ideally be visible in the raw fMCG data. However (depending on gestational age), such features have not been detected in averaged fMCGs recorded with SQUIDs either.

Consistent results

The researchers also applied the ICA method to separate the fMCG from the mMCG in the same 30 s data interval. Results indicated a maternal heart rate of 60 bpm and a foetal heart rate of 150 bpm, consistent with the values from OP separation. Contour maps showing the strength of the magnetic fields over the abdomen were also similar for the different signal separation methods.

This consistency between the two methods indicates the quality of the raw data. "A result is much better validated if two fairly different methods yield similar outcomes," explained Sander-Thoemmes.

The team concluded that μOPMs provide an attractive alternative to SQUID sensors for measuring foetal heart signals with similar data quality. Importantly, µOPMs operate without cryogenics, as well as allowing reduced distance between the foetal heart and the sensor. They note that while these experiments were conducted in a highly magnetically‐shielded room, optically pumped magnetometers have previously been validated in standard 2‐layer magnetically‐shielded rooms.

"The system used for this work was a laboratory prototype. We hope to develop an inexpensive clinical system that's amenable to standard foetal evaluations," explained Knappe. "At the same time, better sensor placement could enable better heart and brain function assessment of the foetus, and we are performing simulations to predict the [potential] advances. Sensor performance also needs to be improved to take full advantage of this new technology."

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