NIST’s atom-based magnetic sensor, about the size of a sugar cube, can measure human brain activity. Inside the sensor head is a container of 100 billion rubidium atoms (not seen), packaged with micro-optics (a prism and a lens are visible in the center cutout). The light from a low-power infrared laser interacts with the atoms and is transmitted through the grey fiber-optic cable to register the magnetic field strength. The black and white wires are electrical connections. Image: Knappe/NIST |
A miniature atom-based magnetic sensor developed by the NIST
has passed an important research milestone by successfully measuring human
brain activity. Experiments reported this week (week of April 16, 2012) verify
the sensor’s potential for biomedical applications such as studying mental
processes and advancing the understanding of neurological diseases.
NIST and German scientists used the NIST sensor to measure
alpha waves in the brain associated with a person opening and closing their
eyes as well as signals resulting from stimulation of the hand. The
measurements were verified by comparing them with signals recorded by a SQUID
(superconducting quantum interference device). SQUIDs are the world’s most
sensitive commercially available magnetometers and are considered the
“gold standard” for such experiments. The NIST mini-sensor is
slightly less sensitive now but has the potential for comparable performance
while offering potential advantages in size, portability and cost.
The study results indicate the NIST mini-sensor may be
useful in magnetoencephalography (MEG), a noninvasive procedure that measures the
magnetic fields produced by electrical activity in the brain. MEG is used for
basic research on perceptual and cognitive processes in healthy subjects as
well as screening of visual perception in newborns and mapping brain activity
prior to surgery to remove tumors or treat epilepsy. MEG also might be useful
in brain-computer interfaces.
MEG currently relies on SQUID arrays mounted in heavy
helmet-shaped flasks containing cryogenic coolants because SQUIDs work best at
4 degrees above absolute zero, or minus 269 degrees Celsius. The chip-scale
NIST sensor is about the size of a sugar cube and operates at room temperature,
so it might enable lightweight and flexible MEG helmets. It also would be less
expensive to mass produce than typical atomic magnetometers, which are larger
and more difficult to fabricate and assemble.
“We’re focusing on making the sensors small, getting
them close to the signal source, and making them manufacturable and ultimately
low in cost,” says NIST co-author Svenja Knappe. “By making an
inexpensive system you could have one in every hospital to test for traumatic
brain injuries and one for every football team.”
The mini-sensor consists of a container of about 100 billion
rubidium atoms in a gas, a low-power infrared laser and fiber optics for
detecting the light signals that register magnetic field strength—the atoms
absorb more light as the magnetic field increases. The sensor has been improved
since it was used to measure human heart activity in 2010. NIST scientists
redesigned the heaters that vaporize the atoms and switched to a different type
of optical fiber to enhance signal clarity.
The brain experiments were carried out in a magnetically
shielded facility at the Physikalisch Technische Bundesanstalt (PTB) in Berlin, Germany,
which has an ongoing program in biomagnetic imaging using human subjects. The
NIST sensor measured magnetic signals of about 1 picotesla (trillionths of a
tesla). For comparison, the Earth’s magnetic field is 50 million times stronger
(at 50 millionths of a tesla). NIST scientists expect to boost the
mini-sensor’s performance about tenfold by increasing the amount of light
detected. Calculations suggest an enhanced sensor could match the sensitivity
of SQUIDS. NIST scientists are also working on a preliminary multi-sensor
magnetic imaging system in a prelude to testing clinically relevant
applications.
