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Mini-sensor measures magnetic fields of the brain

Especially interesting for
  • neurologists
  • users of therapeutic magnetic nanoparticles

The weak magnetic fields of the human brain require advanced measurement technology. For this reason, only SQUIDs (co-developed by PTB) have been used for their measurement so far. A sensor the size of a sugar cube and developed at NIST (USA) can now measure weak magnetic fields without relying on superconductivity. A test performed at PTB's worldwide unique measurement facility demonstrated the suitability of this new type of sensor for practical applications.

Four CSAM magnetic field sensors positioned above the motor and sensory
brain regions on the head of a volunteer. The cubic sensors are as easy to handle as electrodes. Insertion: Dimension of the sensor and of the coil attached to the sensor for a local modulation of the field (view of the cube from above).

At present, the Superconducting Quantum Interference Devices (SQUIDs) are the most sensitive magnetic field sensors. However, they require complex cooling in a cryogenic vessel which must be regularly filled with liquid helium. But helium is becoming scarcer all around the world, so that the users of SQUIDs technologies are becoming increasingly worried about the diminishing resources. Furthermore, the arrangement of the SQUIDs inside the thermal insulation of the cryogenic vessel is fixed. Small and flexible sensors would be ideal for biomedical applications which require them to be easily attached to the head.

A typical application is the so-called MEG (magneto-encephalogram), which measures the very weak magnetic fields that are generated by the brain while it is working. Such an MEG recording is important, for instance, to diagnose epilepsy or for fundamental research in neurology. Another example of a possible application is the measurement of the fields of magnetic nanoparticles which are administered to patients for therapeutic purposes and whose distribution inside the body has to be precisely monitored.

The new sensor type developed at NIST is well suited for such applications, especially as it works at room temperature. The so-called Chip-scale Atomic Magnetometer (CSAM) essentially consists of a cell filled with rubidium gas and associated microoptics. The interaction between the electron spin of the rubidium atoms and a magnetic field serves as a highly sensitive measure of the field strength. These sensors – which are no larger than a sugar cube – can be positioned on the patient's skin wherever needed, similar to electrodes.

The practical suitability of this type of sensor has now been tested in Berlin – in PTB's “magnetically most quiet room on Earth”. Compared to the “gold standard”, the SQUID, its noise is higher by a factor of 5 to 10. This is, however, compensated by the fact that the sensor can be positioned very close to the source. In addition, its scope of application is much wider. One can reasonably expect that the new sensor type will find its place in magnetical measurement technology requiring high-sensitivity – perhaps not only for biomagnetical investigations.

Scientific publication

T. Sander-Thömmes, J. Preusser, R. Mhaskar, J. Kitching, L. Trahms, S. Knappe: Magnetoencephalography with a chip-scale atomic magnetometer. Biomedical Optics Express 3, 981–990 (2012)