Abstract
An array of 21 first-order gradiometers based on zero-field optically-pumped magnetometers is demonstrated for use in magnetoencephalography. Sensors are oriented radially with respect to the head and housed in a helmet with moveable holders which conform to the shape of a scalp. Our axial gradiometers have a baseline of 2 cm and reject laser and vibrational noise as well as common-mode environmental magnetic noise. The median sensitivity of the array is 15.4 fT/Hz1/2, measured in a human-sized magnetic shield. All magnetometers are operated independently with negative feedback to maintain atoms at zero magnetic field. This yields higher signal linearity and operating range than open-loop operation and a measurement system that is less sensitive to systematic and ambient magnetic fields. All of the system electronics and lasers are compacted into one equipment rack which offers a favorable outlook for use in clinical settings.
Highlights
Measuring DC magnetic fields with high precision is important for biomedical imaging applications such as magnetoencephalography (MEG) [1]
This is the first demonstration of a system designed for MEG based on Optically-pumped magnetometer (OPM) to employ gradiometers arrayed radially, which allows for the rejection of systematic noise to a large degree
Despite a 10 GHz spread in cell resonance frequencies and considerable laser and mechanical noise, our gradiometers were able to consistently operate with noise floors below 25 fT/Hz1/2 with a median of 15.4 fT/Hz1/2 in a magnetic shield equipped for human trials
Summary
Measuring DC magnetic fields with high precision is important for biomedical imaging applications such as magnetoencephalography (MEG) [1]. Neuronal signals inside the brain create small magnetic fields that penetrate the skull largely undisturbed and these fields can be detected by an array of sensitive magnetometers placed centimeters to millimeters from the surface of the scalp. Superconducting Quantum Interference Devices (SQUIDs) have become the predominant tool for MEG applications [4, 5], where roughly 300 low-temperature SQUID sensors are placed into a head-shaped liquid helium dewar. The non-cryogenic nature is one attractive feature of OPMs, which eliminates the need for frequent replenishing of helium, and allows for (2020) 7:11 conformal placement of the sensors within millimeters of the scalp, independent of head size and shape. A series of simulation studies predicted clear improvements in signal-tonoise ratio of OPMs compared to SQUIDs due to the closer proximity, despite the higher noise floor of OPMs [6,7,8,9]
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