All-optical magnetic field sensors promise MRI system alternative
Fabricated using optical fibers and a newly developed polymer-nanoparticle composite that is sensitive to magnetic fields, the sensors can detect the brain’s magnetic field, which is 100 million times weaker than the magnetic field of earth. The inexpensive and compact magnetic sensors, say the researchers, could offer an alternative to the large and costly magnetic resonance imaging (MRI) systems currently used to map brain activity by avoiding the expensive cooling or electromagnetic shielding required by MRI machines.
“A portable, low-cost brain imaging system that can operate at room temperature in unshielded environments,” says researcher member Babak Amirsolaimani, “would allow real-time brain activity mapping after potential concussions on the sports field and in conflict zones where the effect of explosives on the brain can be catastrophic.”
In addition to brain activity, the researchers showed that the new sensor can detect the weak magnetic pattern of a human heartbeat – demonstrating the technology’s potential as a simple replacement for electrocardiography, or ECG, tests commonly performed to detect heart problems – and has the capability to detect magnetic fluctuations that change every microsecond from an area as small as 100 square microns.
“The all-optical design of the sensor means it could be fabricated inexpensively on a silicon photonics chip,” says Amirsolaimani, “making it possible to produce a system that is almost as small as the sensor’s 10-micron-diameter optical fiber. Multiple sensors could then be used together to provide high spatial resolution brain mapping.”
The new sensors, say the researchers, could help scientists better understand the activity of the brain and diseases of the brain such as dementia and Alzheimer’s. In addition, they may also be useful for measuring the magnetic fields used to predict volcanic eruptions and earthquakes, identify oil and minerals for excavation, and detect military submarines.
The sensors work by taking advantage of the fact that a magnetic field causes the polarization of light to rotate, with the degree of rotation dependent on the material through which the light passes. The researchers developed a new composite material made of magnetite and cobalt nanoparticles – materials with very high magnetic sensitivity – dispersed in a polymer that imparts a detectible polarization rotation in light when very weak magnetic fields are present.
The researchers detected the polarization rotation using an optical interferometer, which works by splitting laser light into two paths – one of which passes through the highly-sensitive material while the other does not. The polarization of each light path is detected and compared to measure fluctuations in very small magnetic fields.
As noise can easily cover up the desired signal being detected, the researchers used an interferometer setup that eliminated ambient environmental effects such as vibration and temperature fluctuations. The setup kept noise levels very close to the theoretical limit of the optical design, which was key for detecting very weak magnetic fields.
Looking ahead, the researchers next plan to study the long-term stability of the sensors and how well they withstand environmental changes. They also want to fabricate several hundred sensors to make a system for evaluating and imaging the entire magnetic field of a human brain.