In future a new magnetic sensor the size of a sugar cube
might simplify the measurement of brain activity. In the magnetically shielded
room of Physikalisch-Technische Bundesanstalt (PTB) the sensor has passed an
important technical test: Spontaneous as well as stimulated magnetic fields of
the brain were detected. This demonstrates the potential of the sensor for
medical applications, such as, the investigation of brain currents during
cognitive processes with the aim of improving neurological diagnostics. The
main advantage of the new sensor developed by NIST in the USA over the
conventionally used cryoelectronics is its room temperature operation
capability making complicated cooling obsolete. The results have recently been
published in the journal “Biomedical Optics Express”.
The magnetic field sensor is called Chip-scale Atomic
Magnetometer (CSAM) as it uses miniaturized optics for measuring absorption
changes in a Rubidium gas cell caused by magnetic fields. The CSAM sensor was
developed by NIST (National Institute of Standards and Technology), which is
the national metrology institute of the USA. In this cooperation between
PTB and NIST each partner contributes his own particular capabilities. PTB’s
staff has long standing experience in biomagnetic measurements in a unique
magnetically shielded room. NIST contributes the sensors, which are the result
of a decade of dedicated research and development.
Up to now the measurement of very weak magnetic fields was
the domain of cryoelectronic sensors, the so called superconducting quantum
interference device (SQUID). They can be considered as the “gold
standard” for this application, but they have the disadvantage to operate
only at very low temperatures close to absolute zero. This makes them expensive
and less versatile compared to CSAMs. Even though at present CSAMs are still
less sensitive compared to SQUIDs, measurements with a quality comparable to
SQUIDs, but at lower costs, might eventually become reality. Due to the cooling
requirements, SQUIDs have to be kept apart from the human body by a few
centimeters. In contrast to that, CSAMs can be attached closely to the human
body. This increases the signal amplitude as the magnetic field from currents
inside the human bodydecays rapidly with increasing distance.
An important application is the measurement of the magnetic
field distribution around the head, which is called magnetoencephalography
(MEG). It enables the characterization of neuronal currents. Such
investigations have gained importance during the last few years for
neurologists and neuroscientists. Objective indicators of psychiatric disorders
as well as age dependent brain diseases, are urgently needed for the support of
today’s clinical diagnostics.
Already in 2010 scientists from NIST and PTB had
successfully tested the performance of an earlier version of the present CSAM
by measurements of the magnetic field of the human heart. For the present study
the sensor was positioned about 4 mm away from the head of healthy subjects. At
the back of the head, the magnetic fields of alpha waves were detected, a basic
brain rhythm which occurs spontaneously during relaxation. In another
measurement the brain fields due to the processing of tactile stimuli were
identified. These fields are extremely weak and the CSAM result was validated
by a simultaneous MEG measurement relying on the established SQUID technology.