Lowering Threshold for Detecting Very Weak Magnetic Signals Paves Way for New Diagnostic Options

Magnetic field sensors built by physicists at Saarland University are breaking sensitivity records and paving the way for an entire range of probable new applications, from non-contact measurements of the electrical activity in the human brain or heart to spotting ore deposits or archaeological remains buried underground.

With the magnetometer installed inside a cylinder, even the magnetic field strength of leaves can be measured: Uwe Hartmann demonstrates this capability using a dried lotus leaf. (Image credit: Oliver Dietze)

Professor Uwe Hartmann and his research team have created a system that enables them to detect weak magnetic signals across large distances in regular environments (no shielding, no vacuum, no low temperatures), regardless of the presence of many sources of interference. Their system can sense signal strengths much below a billionth of a tesla—about a million times smaller than the Earth’s magnetic field—and can be used to sense biomagnetic signals in the human body or geophysical occurrence.

The study team will be displaying their efforts at Hannover Messe from April 1st (Hall 2, Stand B46) and are seeking partners with whom they can advance their technology for real-world applications.

If doctors want to observe a patient’s heart to check whether it is beating erratically, they first need to fix electrodes to the patient’s wrists, chest, and ankles. Similarly, when attempting to measure the electrical activity of the brain. The patient first needs to be wired up before their brain’s electrical activity can be recorded. But when things need to occur quickly, this can mean that medical staff end up wasting valuable time. It would be much easier if a device like a metal detector was present that could be swept over the patient’s head or body but would still provide consistent results. Thus far, non-contact medical diagnostic procedures have been unsuccessful as they are just not suitable for daily use. Sensors that are sufficiently sensitive to measure the biomagnetic fields generated by the human body need to function in very cautiously regulated settings. They have to be properly shielded from external sources of interference, have to be operated in a vacuum or at unfeasibly low temperatures of below -200 °C.

Currently, however, Professor Uwe Hartmann and his team of experimental physicists at Saarland University have been successful in building magnetic field sensors that can work under usual ambient environments while still being able to sense extremely low-level signals, such as the weak biomagnetic fields generated by a number of the body’s functions. “You could say the precision of our technique is like being able to locate a grain of sand in a mountain range. We can detect over relatively large distances magnetic fields that are approximately a million times weaker than the Earth’s magnetic field—just a few picotesla, that is a millionth of a millionth of a tesla,” explains Uwe Hartmann. Thus far, sensors functioning under usual ambient settings have been able to sense magnetic fields that are about a thousand times smaller than the Earth’s magnetic field. The real test, however, was not the hardly detectable magnitude of the signals themselves.

The main problem when measuring these tiny signals in a normal environment is being able to cleanly separate the signals from the multitude of interference signals that are inevitably present.

Uwe Hartmann, Professor, Saarland University.

There are all kinds of factors that produce noise, or that falsify the weak signal that the physicists are concerned about. Sources of interference include the Earth’s magnetic field, moving traffic, electrical devices, signals from the body’s other organs, or even from solar storms. Hartmann’s research team has been inspecting magnetometers (magnetic field sensors) for years and they have effectively designed these devices for a complete range of applications.

“Over the last few years, we have managed to boost the sensitivity and selectivity of our magnetometers. The sensitivity that our sensors now demonstrate is the result not only of our continuous sensor-development work, but particularly the improvements in our data processing software,” he explains.

Hartmann and his team have been a part of a number of projects where their focus was on sifting out interference signals from measurement data. The scientists in Saarbrücken have, for instance, created a smart sensor cable wherein the magnetometers are linked to one another in a network. Many of these systems are presently being tried out as components in airport traffic management systems. In another application, the sensors have been used for remote monitoring of perimeter fencing. Here, the system has to be able to differentiate and identify all of the various factors that cause measurable variations in the magnetic field. The research team thus conducted several tests in which they mimicked changes to the magnetic field, such as those that take place when the fence vibrates or when it is struck, and allocated the resulting signal patterns to the matching sources. The physicists have modeled the signal patterns mathematically, decoded the results into algorithms, and used these to program the analyzer—a process that is being constantly tweaked as progressively comprehensive data becomes available.

We use this information to teach the system and to continuously expand its capabilities. It can recognize typical signal patterns and automatically assign them to different sources of interference. We are now in a position where we can assign measurement data and signal patterns very precisely to their respective causes.

Uwe Hartmann, Professor, Saarland University.

While the research being performed by Professor Hartmann and his team is basically standard research, there are a broad range of probable applications for these extremely sensitive magnetometers. They could, for instance, be used for diagnostic purposes in neurology or cardiology, where they could complement existing methods such as electrocardiography (ECG) or electroencephalography (EEG). Another prospective area of use is in geophysical sensing when probing for mineral deposits, crude oil, or archaeological remains.

The researchers will be highlighting their work at Hannover Messe where they will be seeking commercial partners, mainly companies in the medical technology domain, with whom they can progress their technology for real-world applications.

The team will be showcasing the sensitivity of their sensors in Hall 2 (Stand B46) by detecting unexpected examples of magnetic objects in the immediate surroundings.

The study project was funded by the European Union, the German Research Foundation (DFG), and the Federal Ministry of Education and Research (BMBF).

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