Recording brain activities in unshielded Earth’s field with optically pumped atomic magnetometers

Abstract

Understanding the relationship between brain activity and specific mental function is important for medical diagnosis of brain symptoms, such as epilepsy. Magnetoencephalography (MEG), which uses an array of high-sensitivity magnetometers to record magnetic field signals generated from neural currents occurring naturally in the brain, is a noninvasive method for locating the brain activities. The MEG is normally performed in a magnetically shielded room. Here, we introduce an unshielded MEG system based on optically pumped atomic magnetometers. We build an atomic magnetic gradiometer, together with feedback methods, to reduce the environment magnetic field noise. We successfully observe the alpha rhythm signals related to closed eyes and clear auditory evoked field signals in unshielded Earth’s field. Combined with improvements in the miniaturization of the atomic magnetometer, our method is promising to realize a practical wearable and movable unshielded MEG system and bring new insights into medical diagnosis of brain symptoms.

Interference in Atomic Magnetometry

“…In addition to the above approach of detecting the interference, we show that it is relatively simple and intuitive to observe the interference effect in practical applications. This is because the practical magnetic fields, for example, biomagnetic fields [17,38,39] and NMR fields generated by organic compounds, [35][36][37] naturally have components along different axes to the magnetometers. As a typical example, we demonstrate the interference in NMR signals recorded with atomic magnetoemeters and shall see the interference effect interprets NMR asymmetric spectra, that is, large differences of the amplitudes of resonant lines, which are greatly distorted from the conventional NMR prediction [21,22,35] and have never been well understood before.…”

“…A promising application is using arrays of high-sensitivity atomic magnetometers to monitor the faint magnetic-field signals from human brain activities. [17,38,39] The weak brain magnetic field (on the order of 100 fT) generated by neural currents spatially distributes around the magnetic sensor and its components along different directions probably interface with each other when recorded with atomic magnetometers, resulting in the systematic error of the actual brain field amplitudes. If such interference occurs and without the prior knowledge of interference in magnetometers, it would reduce the accuracy in locating the brain electrophysiological symptom.…”

“…[32][33][34] However, as we will show in this work, the quasi-steady approximation results in an important systematic effect of measuring magnetic field and could cause misdiagnosis for a wide range of magnetometer applications, for example, in NMR [35][36][37] and in biomagnetism. [17,38,39] For example, it has been extensively demonstrated that the NMR signals recorded with atomic magnetometers uncommonly differ from the expected values with a few tens of percent distortion. [28][29][30][35][36][37] This poses a rather puzzling situation and limits the accuracy of determining the structural information and molar concentration of NMR samples.…”

“…[28][29][30][35][36][37] This poses a rather puzzling situation and limits the accuracy of determining the structural information and molar concentration of NMR samples. [40] Moreover, without the prior knowledge of the hidden systematic effect, it would reduce the accuracy of atomic magnetometers in locating the magnetic field sources, such as for diagnosing the brain electrophysiological symptom [17,38,39] and defects in Li-ion battery cells. [41] Therefore, there is a pressing need for a subtle analysis of the relevant physical processes and achieving a precise detection model in atomic magnetometry.…”

Active Magnetic-Field Stabilization with Atomic Magnetometer

“…For these experiments, regardless of the usage of comagnetometer, which is for suppressing the magnetic-field noise, an environment with lower magnetic-field noise is beneficial for further reducing the systematic errors related to magnetic-field variations. Furthermore, a magnetic-field noise level of about tens of fT/Hz and a wide range of operational magnetic field covering Earth’s magnetic field is attractive for ultralow-field nuclear magnetic resonance [ 52 , 53 ] and for recording bio-magnetic signals from human brain activities [ 54 , 55 ].…”