A better way to measure magnetic fields could make fetal heart problems easier to detect

The electrical fields produced within the body are a powerful diagnostic tool. Clinicians routinely use these signals to measure the function of the brain, heart, nerves, and muscles, providing insights that are impossible to gather with other tools.

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But this approach has limitations. For example, electrical signals from fetal hearts are hard to gather because they overwhelmed by the mother’s signals. This makes some fetal heart conditions particularly difficult to diagnose.

There is another way to study the body’s electrical activity, however—by measuring the magnetic field it produces. Because magnetic fields decay quickly over short distances, this makes it much easier to separate a fetal signal from the mother’s.

But magnetometers with the required sensitivity rely on superconducting technology that has to be cooled to the temperature of liquid helium. The insulation this requires prevents these devices from getting close to the target organ. Consequently, the magnetic signals have always been weak and hard to interpret.

What’s needed is a room-temperature magnetometer that can be placed within millimeters of the target and is sensitive enough to measure the magnetic signals of interest.

Today, that now looks possible thanks to the work of Kasper Jensen at the University of Copenhagen in Denmark and colleagues, who have measured various diagnostic signals from a fetal-size heart using a room-temperature magnetometer. The work has the potential to revolutionize the measurement of biomagnetic fields and could help doctors diagnose fetal heart conditions that are otherwise undetectable.

The device that makes this work is known as an optically pumped magnetometer. It consists of a small flask of atomic gas, in this case cesium atoms. The spin of each cesium atom is highly sensitive to ambient magnetic fields, which makes them useful measuring tools.

To start with, the spin of all the atoms has to be lined up in the same direction. This is done with polarized laser light. When the laser is switched off, the spins begin to drift according to the local magnetic field. Measuring the spins again a short while later shows how they have changed, revealing the strength and direction of the local field.

In recent years, various groups have begun to use optically pumped magnetometers to study biomagnetic fields. But many of these attempts have been frustrated. The narrow bandwidth of the magnetometers prevents them from picking up all the desired signals.

In many devices, the atoms have to be heated to several hundred degrees Celsius and must therefore be insulated and separated from the target. Since magnetic field strength drops dramatically over short distances, this can have a significant impact on the utility of the devices.

Jensen and co get around these problems with a small, optically pumped magnetometer that has relatively wide band sensitivity and works at body temperature. That means the device can be placed on or within a few millimeters of the target organ.

The team put the device through its paces by using it to measure the magnetic field associated with beating guinea pig hearts that had been isolated in the lab. These are about the size of human fetal hearts and so offer a good test.

The approach shows promising results. Jensen and co say they clearly detected the heartbeat along with a wide variety of diagnostic characteristics.

In a normal heart, the muscle contraction that is the signature of a “heartbeat” is triggered by the passage of electrical waves across the surface of the heart. Several waves are involved, and these cause the synchronized contraction of different parts of the heart.

Cardiologists label these waves with the letters P, Q, R, S, and T. The timing between them is an important indicator of heart function.

One signal of particular interest in fetal hearts is the Q-T interval. When this is prolonged, it is indicative of a serious problem. However, electrocardiograms cannot be used to detect this in fetal hearts.

Jensen and co say their new technique can detect this problem. To show how, they used drugs to induce a prolonged Q-T interval in the guinea pig hearts. They  say the optically pumped magnetometer clearly picked up the diagnostic signs.

That’s interesting work with significant implications. Prolonged Q-T intervals occur in 1 of every 2,500 births, and detecting them early is important. The new technique should be able to do just that.

“Based on our measurements on the guinea-pig heart, we conclude that real-time detection of the heartbeat of a human fetus at gestational age of 18-22 weeks, where the heart-sensor distance is estimated to be ≥ 5 cm, should be possible,” say Jensen and co.

This sets up an exciting future. The next stage will be to test the technique in humans and then specifically in pregnant women. It also has the potential to measure other magnetic fields in the body, such as those produced by the brain and the nervous system. Get ready to welcome a new form of diagnostic tool.

Ref: https://arxiv.org/abs/1806.10954 : Magnetocardiography on an isolated animal heart with a room-temperature optically pumped magnetometer

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