November 02, 2007

Ultrasensitive prototype device approaches gold standard for magnetic field detection

A tiny sensor that can detect magnetic field changes as small as 70 femtoteslas-equivalent to the brain waves of a person daydreaming-has been demonstrated at the National Institute of Standards and Technology (NIST). The sensor could be battery-operated and could reduce the costs of non-invasive biomagnetic measurements such as fetal heart monitoring. The device also may have applications such as homeland security screening for explosives.

Described in the November issue of Nature Photonics,* the prototype device is almost 1000 times more sensitive than NIST's original chip-scale magnetometer demonstrated in 2004 and is based on a different operating principle. Its performance puts it within reach of matching the current gold standard for magnetic sensors, so-called superconducting quantum interference devices or SQUIDs. These devices can sense changes in the 3- to 40-femtotesla range but must be cooled to very low (cryogenic) temperatures, making them much larger, power hungry, and more expensive.

The NIST prototype consists of a single low-power (milliwatt) infrared laser and a rice-grain-sized container with dimensions of 3 by 2 by 1 millimeters. The container holds about 100 billion rubidium atoms in gas form. As the laser beam passes through the atomic vapor, scientists measure the transmitted optical power while varying the strength of a magnetic field applied perpendicular to the beam. The amount of laser light absorbed by the atoms varies predictably with the magnetic field, providing a reference scale for measuring the field. The stronger the magnetic field, the more light is absorbed.

The new NIST mini-sensor could reduce the equipment size and costs associated with some non-invasive biomedical tests. (The body's electrical signals that make the heart contract or brain cells fire also simultaneously generate a magnetic field.) The NIST group and collaborators have used a modified version of the original sensor to detect magnetic signals from a mouse heart.** The new sensor is already powerful enough for fetal heart monitoring; with further work, the sensitivity can likely be improved to a level in the 10 femtotesla range, sufficient for additional applications such as measuring brain activity, the designers say.

To make a complete portable magnetometer, the laser and vapor cell would need to be packaged with miniature optics and a light detector. The vapor cell can be fabricated and assembled on semiconductor wafers using existing techniques for making microelectronics and microelectromechanical systems (MEMS). This design, adapted from a previously developed NIST chip-scale atomic clock, offers the potential for low-cost mass production.

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It is not just much smaller than a SQUID, but also operates at much higher temperatures, at around 150 °C. Currently the complete device is a few millimetres on each side. "The small size and high performance of this sensor will open doors to applications that we could previously only dream of," Kitching says.

Kitching and colleagues made the new magnetometers through photolithography, the same process used to make computer chips. "You can make very large numbers of the devices in parallel on a single wafer [of silicon]," Kitching says. "That will reduce the cost."

The new devices are cheaper and easier to operate versions of SQUIDs. SQUIDS have been used for brain imaging.

Most of the magnetoencephalography (MEG) helmet array bulk is for cooling

MEG has been in development since the 1960s but has been greatly aided by recent advances in computing algorithms and hardware, and promises good spatial resolution and extremely high temporal resolution (better than 1 ms); since MEG takes its measurements directly from the activity of the neurons themselves its temporal resolution is comparable with that of intracranial electrodes. MEG's strengths complement those of other brain activity measurement techniques such as electroencephalography (EEG), positron emission tomography (PET), and fMRI whose strengths, in turn, complement MEG. Other important strengths to note about MEG are that the biosignals it measures do not depend on head geometry as much as EEG does (unless ferromagnetic implants are present) and that it is completely non-invasive, as opposed to PET and possibly MRI/fMRI.

There are cheap brain wave (mind machine interface devices) readers that are $200-600 and use electroencephalography (EEG, detect electrical activity of the brain) or EMG (electromyograph)devices and work with Xbox and PS3's