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Modular optically-pumped magnetometer system
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Abstract
We describe a modular optically-pumped magnetometer (OPM) system which enables fast prototyping and testing of new measurement schemes. Quick reconfiguration of self-contained laser and sensor modules allow easy construction of various array layouts. The modularity of this system enables scaling of shared light-source arrays, and development of methods for high density array management for magnetic imaging and sensing in both medical and industrial fields. We demonstrate the OPM system in a first-order axial gradiometer configuration with a magnetic field gradient sensitivity of 10.4 . To illustrate the capabilities of this system, we measured alpha-rhythms from the brain of a human participant, and assessed the magnetometer sensitivity both with single sensor channels and in a differential gradiometer configuration.
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Recent developments in performance and practicality of optically-pumped magnetometers (OPMs) have enabled new capabilities in non-invasive brain function mapping through magnetoencephalography. In particular, the lack of cryogenic operating conditions allows for more flexible placement of sensor heads closer to the brain, leading to improved spatial resolution and source localisation capabilities. Through recording visually evoked brain fields (VEFs), we demonstrate that the closer sensor proximity can be exploited to improve temporal resolution. We use OPMs, and superconducting quantum interference devices (SQUIDs) for reference, to measure brain responses to flash and pattern reversal stimuli. We find highly reproducible signals with consistency across multiple participants, stimulus paradigms and sensor modalities. The temporal resolution advantage of OPMs is manifest in a twofold improvement, compared to SQUIDs. The capability for improved spatio-temporal signal tracing is illustrated by simultaneous vector recordings of VEFs in the primary and associative visual cortex, where a time lag on the order of 10–20 ms is consistently found. This paves the way for further spatio-temporal studies of neurophysiological signal tracking in visual stimulus processing, and other brain responses, with potentially far-reaching consequences for time-critical mapping of functionality in healthy and pathological brains.
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.
We demonstrate the use of a hybrid ³He/⁸⁷ magnetometer to measure absolute magnetic fields in the pT range. The measurements were undertaken by probing time-dependent ³He magnetisation using ⁸⁷Rb zero-field magnetometers. Measurements were taken to demonstrate the use of the magnetometer in cancelling residual fields within a magnetic shield. It was shown that the absolute field could be reduced to the 10 pT level by using field readings from the magnetometer. Furthermore, the hybrid magnetometer was shown to be applicable for the reduction of gradient fields by optimising the effective ³He T2 time. This procedure represents a convenient and consistent way to provide a near zero magnetic field environment which can be potentially used as a base for generating desired magnetic field configurations for use in precision measurements.
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Optically Pumped Magnetometers (OPMs) have emerged as a viable and wearable alternative to cryogenic, superconducting MEG systems. This new generation of sensors has the advantage of not requiring cryogenic cooling and as a result can be flexibly placed on any part of the body. The purpose of this review is to provide a neuroscience audience with the theoretical background needed to understand the physical basis for the signal observed by OPMs. Those already familiar with the physics of MRI and NMR should note that OPMs share much of the same theory as the operation of OPMs rely on magnetic resonance. This review establishes the physical basis for the signal equation for OPMs. We re-derive the equations defining the bounds on OPM performance and highlight the important trade-offs between quantities such as bandwidth, sensor size and sensitivity. These equations lead to a direct upper bound on the gain change due to cross-talk for a multi-channel OPM system.
In recent years, substantial progress has been made in developing a new generation of magnetoencephalography (MEG) with a spin-exchange relaxation free (SERF)-based atomic magnetometer (AM). An AM employs alkali atoms to detect weak magnetic fields. A compact AM array with high sensitivity is crucial to the design; however, most proposed compact AMs are potassium (K)- or rubidium (Rb)-based with single beam configurations. In the present study, a pump-probe two beam configuration with a Cesium (Cs)-based AM (Cs-AM) is introduced to detect human neuronal magnetic fields. The length of the vapor cell is 4 mm, which can fully satisfy the need of designing a compact sensor array. Compared with state-of-the-art compact AMs, our new Cs-AM has two advantages. First, it can be operated in a SERF regime, requiring much lower heating temperature, which benefits the sensor with a closer distance to scalp due to ease of thermal insulation and less electric heating noise interference. Second, the two-beam configuration in the design can achieve higher sensitivity. It is free of magnetic modulation, which is necessary in one-beam AMs; however, such modulation may cause other interference in multi-channel circumstances. In the frequency band between 10 Hz and 30 Hz, the noise level of the proposed Cs-AM is approximately 10 f T/Hz1/2, which is comparable with state-of-the-art K- or Rb-based compact AMs. The performance of the Cs-AM was verified by measuring human auditory evoked fields (AEFs) in reference to commercial superconducting quantum interference device (SQUID) channels. By using a Cs-AM, we observed a clear peak in AEFs around 100 ms (M100) with a much larger amplitude compared with that of a SQUID, and the temporal profiles of the two devices were in good agreement. The results indicate the possibility of using the compact Cs-AM for MEG recordings, and the current Cs-AM has the potential to be designed for multi-sensor arrays and gradiometers for future neuroscience studies.
Magnetoencephalography (MEG) is a sophisticated tool which yields rich information on the spatial, spectral and temporal signatures of human brain function. Despite unique potential, MEG is limited by a low signal-to-noise ratio (SNR) which is caused by both the inherently small magnetic fields generated by the brain, and the scalp-to-sensor distance. The latter is limited in current systems due to a requirement for pickup coils to be cryogenically cooled. Recent work suggests that optically-pumped magnetometers (OPMs) might be a viable alternative to superconducting detectors for MEG measurement. They have the advantage that sensors can be brought to within ~4 mm of the scalp, thus offering increased sensitivity. Here, using simulations, we quantify the advantages of hypothetical OPM systems in terms of sensitivity, reconstruction accuracy and spatial resolution. Our results show that a multi-channel whole-head OPM system offers (on average) a fivefold improvement in sensitivity for an adult brain, as well as clear improvements in reconstruction accuracy and spatial resolution. However, we also show that such improvements depend critically on accurate forward models; indeed, the reconstruction accuracy of our simulated OPM system only outperformed that of a simulated superconducting system in cases where forward field error was less than 5%. Overall, our results imply that the realisation of a viable whole-head multi-channel OPM system could generate a step change in the utility of MEG as a means to assess brain electrophysiological activity in health and disease. However in practice, this will require both improved hardware and modelling algorithms.
We have developed a four-channel optically pumped atomic magnetometer for magnetoencephalography (MEG) that incorporates a passive diffractive optical element (DOE). The DOE allows us to achieve a long, 18-mm gradiometer baseline in a compact footprint on the head. Using gradiometry, the sensitivities of the channels are < 5 fT/Hz1/2, and the 3-dB bandwidths are approximately 90 Hz, which are both sufficient to perform MEG. Additionally, the channels are highly uniform, which offers the possibility of employing standard MEG post-processing techniques. This module will serve as a building block of an array for magnetic source localization.
Flash differential scanning calorimetry was used to study the glass transition temperature Tg of polycarbonate ultrathin films. The investigation was made as a function of film thickness from 22 to 350 nm and over a range of cooling rates from 0.1 to 1000 K/s. Polycarbonate spin cast films were floated on a layer of grease on the calorimetric chip. The results show a greatly reduced glass temperature for the thinnest films relative to the macroscopic value. We also observed that the magnitude of the glass temperature reduction decreases as the cooling rate increases with the highest cooling rates showing little thickness dependence of the Tg. Dynamic fragility and activation energy at Tg were found to decrease with decreasing film thickness. The results are discussed in the context of literature reports for supported and freely standing polycarbonate films. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014
We present a novel formulation for SQUID operation, which enables us to evaluate and compare the sensitivity and applicability of different devices. SQUID magnetometers for low-frequency applications are analyzed, taking into account the coupling circuits and electronics. We discuss nonhysteretic and hysteretic single-junction rf SQUIDs, but the main emphasis is on the dynamics, sensitivity, and coupling considerations of dc-SQUID magnetometers. A short review of current ideas on thin-film, dc-SQUID design presents the problems in coupling and the basic limits of sensitivity. The fabrication technology of tunnel-junction devices is discussed with emphasis on how it limits critical current densities, specific capacitances of junctions, minimum linewidths, conductor separations, etc. Properties of high-temperature superconductors are evaluated on the basis of recently published results on increased flux creep, low density of current carriers, and problems in fabricating reliable junctions. The optimization of electronics for different types of SQUIDs is presented. Finally, the most important low-frequency applications of SQUIDs in biomagnetism, metrology, geomagnetism, and some physics experiments demonstrate the various possibilities that state-of-the-art SQUIDs can provide.
We report on the measurement of somatosensory-evoked and spontaneous magnetoencephalography (MEG) signals with a chip-scale atomic magnetometer (CSAM) based on optical spectroscopy of alkali atoms. The uncooled, fiber-coupled CSAM has a sensitive volume of 0.77 mm³ inside a sensor head of volume 1 cm³ and enabled convenient handling, similar to an electroencephalography (EEG) electrode. When positioned over O1 of a healthy human subject, α-oscillations were observed in the component of the magnetic field perpendicular to the scalp surface. Furthermore, by stimulation at the right wrist of the subject, somatosensory-evoked fields were measured with the sensors placed over C3. Higher noise levels of the CSAM were partly compensated by higher signal amplitudes due to the shorter distance between CSAM and scalp.
The ever-increasing demand for high-capacity rechargeable batteries highlights the need for sensitive and accurate diagnostic technology for determining the state of a cell, for identifying and localizing defects, and for sensing capacity loss mechanisms. Here, we leverage atomic magnetometry to map the weak induced magnetic fields around Li-ion battery cells in a magnetically shielded environment. The ability to rapidly measure cells nondestructively allows testing even commercial cells in their actual operating conditions, as a function of state of charge. These measurements provide maps of the magnetic susceptibility of the cell, which follow trends characteristic for the battery materials under study upon discharge. In particular, hot spots of charge storage are identified. In addition, the measurements reveal the capability to measure transient internal current effects, at a level of μA, which are shown to be dependent upon the state of charge. These effects highlight noncontact battery characterization opportunities. The diagnostic power of this technique could be used for the assessment of cells in research, quality control, or during operation, and could help uncover details of charge storage and failure processes in cells.
A novel experimental scheme enabling the investigation of transient exotic spin couplings is discussed. The scheme is based on synchronous measurements of optical-magnetometer signals from several devices operating in magnetically shielded environments in distant locations (≳ 100 km). Although signatures of such exotic couplings may be present in the signal from a single magnetometer, it would be challenging to distinguish them from noise. By analyzing the correlation between signals from multiple, geographically separated magnetometers, it is not only possible to identify the exotic transient but also to investigate its nature. The ability of the network to probe presently unconstrained physics beyond the Standard Model is examined by considering the spin coupling to stable topological defects (e.g., domain walls) of axion-like fields. In the spirit of this research, a brief (∼2 hours) demonstration experiment involving two magnetometers located in Kraków and Berkeley (∼9000 km separation) is presented and discussion of the data-analysis approaches that may allow identification of transient signals is provided. The prospects of the network are outlined in the last part of the paper.
The Hanle effect is briefly reviewed. It is shown that optical pumping can be used to create alignment of states. The combined use of Hanle effect and optical pumping then permits one to measure magnetic fields as small as 10−9 G.
Zero-field level crossing resonances have been observed on the ground state of 87Rb. The width, a few microgauss, and the signal to noise ratio, about 2.5 × 103, allow the measurement of 10-9 gauss fields.
Alkali-metal magnetometers use the coherent precession of polarized
atomic spins to detect and measure magnetic fields. Recent advances have
enabled magnetometers to become competitive with SQUIDs as the most
sensitive magnetic field detectors, and they now find use in a variety
of areas ranging from medicine and NMR to explosives detection and
fundamental physics research. In this thesis we discuss several
developments in alkali-metal atomic magnetometry for both practical and
fundamental applications. We present a new method of polarizing the
alkali atoms by modulating the optical pumping rate at both the linear
and quadratic Zeeman resonance frequencies. We demonstrate
experimentally that this method enhances the sensitivity of a potassium
magnetometer operating in the Earth's field by a factor of 4, and we
calculate that it can reduce the orientation-dependent heading error to
less than 0.1 nT. We discuss a radio-frequency magnetometer for
detection of oscillating magnetic fields with sensitivity better than
0.2 fT/ Hz , which we apply to the observation of nuclear magnetic
resonance (NMR) signals from polarized water, as well as nuclear
quadrupole resonance (NQR) signals from ammonium nitrate. We demonstrate
that a spin-exchange relaxation-free (SERF) magnetometer can measure all
three vector components of the magnetic field in an unshielded
environment with comparable sensitivity to other devices. We find that
octadecyltrichlorosilane (OTS) acts as an anti-relaxation coating for
alkali atoms at temperatures below 170°C, allowing them to collide
with a glass surface up to 2,000 times before depolarizing, and we
present the first demonstration of high-temperature magnetometry with a
coated cell. We also describe a reusable alkali vapor cell intended for
the study of interactions between alkali atoms and surface coatings.
Finally, we explore the use of a cesium-xenon SERF comagnetometer for a
proposed measurement of the permanent electric dipole moments (EDMs) of
the electron and the 129Xe atom, with projected sensitivity of
deltade=9x10 -30 e-cm and deltadXe=4x10 -31 e-cm after 100 days of
integration; both bounds are more than two orders of magnitude better
than the existing experimental limits on the EDMs of the electron and of
any diamagnetic atom.
This review paper illustrates the different SQUID based systems used for biomagnetic imaging. The review is divided into nine sections. The first three sections are introductory: section 1 is a short overview of the topic; section 2 summarizes how the biomagnetic fields are generated and what are the basic mathematical models for the field sources; section 3 illustrates the principles of operation of the SQUID device. Sections 4-8 are specifically devoted to the description of the different systems used for biomagnetic measurements: section 4 discusses the different types of detection coils; section 5 illustrates the SQUID sensors specifically designed for biomagnetic applications together with the necessary driving electronics, with special emphasis on high-temperature superconductivity (HTS) SQUIDs, since HTS devices are still in a developing stage; section 6 illustrates the different noise reduction techniques; section 7 describes the different multichannel sensors presently operating; and, finally, section 8 gives a hint of what kind of physiological and/or clinical information may be gathered by the biomagnetic technique. Section 9 suggests some future trends for the biomagnetic technique.
Highly sensitive magnetometers capable of measuring magnetic fields below 1pT have an impact on areas as diverse as geophysical surveying, the detection of unexploded ordinance, space science, nuclear magnetic resonance, health care and perimeter and remote monitoring. Recently, it has been shown that laboratory optical magnetometers, based on the precession of the spins of alkali atoms in the vapour phase, could achieve sensitivities in the femtotesla range, comparable to, or even exceeding, those of superconducting quantum interference devices. We demonstrate here an atomic magnetometer based on a millimetre-scale microfabricated alkali vapour cell with sensitivity below 70fTHz12. Additionally, we use a simplified optical configuration that requires only a single low-power laser. This result suggests that millimetre-scale, low-power femtotesla magnetometers are feasible, and we support this proposition with a simple sensitivity scaling analysis. Such an instrument would greatly expand the range of applications in which atomic magnetometers could be used.
The light absorbed by an atomic vapour in a transverse optical pumping experiment exhibits resonant variations when the static magnetic field is swept around the zero field value corresponding to the crossing of the ground state Zeeman sublevels (Hanle effect). Resonances of the same kind also appear when the transverse pumping is done in an amplitude modulated magnetic field (parametric resonances). These resonances are extremely narrow when observed on alkali vapors and therefore they are of great interest for the detection and measurement of very weak magnetic fields. The characteristics of the two types of resonances are discussed and the advantages of the parametric resonances are emphasized. La lumière absorbée par une vapeur atomique soumise à un pompage optique transversal varie de manière résonnante lorsqu'on balaie le champ magnétique statique au voisinage du point de croisement en champ nul des sous-niveaux Zeeman de l'état fondamental (effet Hanle). Des résonances analogues apparaissent lorsqu'on effectue le pompage optique transversal de la vapeur atomique dans un champ modulé en amplitude (résonances paramétriques). La très grande finesse des raies ainsi observées sur les atomes alcalins les rend très intéressantes pour la détection et la mesure des champs magnétiques très faibles. On discute les propriétés de ces deux types de résonances et on met en évidence les avantages qu'il y a à utiliser les résonances paramétriques.
Several studies have described cranio-cerebral correlations in accordance with the 10-20 electrode placement system. These studies have made a significant contribution to human brain imaging techniques, such as near-infrared spectroscopy and trans-magnetic stimulation. With the recent development of high resolution EEG, an extension of the 10-20 system has been proposed. This new configuration, namely the 10-10 system, allows the placement of a high number (64-256) of EEG electrodes. Here, we describe the cranio-cerebral correlations with the 10-10 system. Thanks to the development of a new EEG-MRI sensor and an automated algorithm which enables the projection of electrode positions onto the cortical surface, we studied the cortical projections in 16 healthy subjects using the Talairach stereotactic system and estimated the variability of cortical projections in a statistical way. We found that the cortical projections of the 10-10 system could be estimated with a grand standard deviation of 4.6 mm in x, 7.1 mm in y and 7.8 mm in z. We demonstrated that the variability of projections is greatest in the central region and parietal lobe and least in the frontal and temporal lobes. Knowledge of cranio-cerebral correlations with the 10-10 system should enable to increase the precision of surface brain imaging and should help electrophysiological analyses, such as localization of superficial focal cortical generators.
We extend our theoretical model based on spatial filtering to determine the ability of a SQUID magnetometer to resolve 2 localized current sources in the brain. We find that in order to resolve 2 separated but coaxial cortical sources, the source-to-pickup coil distance must be comparable to the distance between the 2 sources. The size of the current sources affects the resolving power of a magnetometer, but an anisotropy of 3 in the cortical tissue does not produce a significant effect. The model also provides a solution of the inverse calculation, either to reconstruct the original current source distribution from the measured magnetic field, or to continue the field at the magnetometer inward towards the sources. Both the inverse and inward calculations are limited by the fact that the inverse filter function serves as a high-pass filter, which leads to instabilities at high spatial frequencies, particularly in the presence of noise. The instabilities can be minimized by choosing an appropriate window to attenuate the noise, but this in turn reduces the spatial resolution.
Measurements of the brain's magnetic field, called magnetoencephalograms (MEG's), have been taken with a superconducting magnetometer in a heavily shielded room. This magnetometer has been adjusted to a much higher sensitivity than was previously attainable, and as a result MEG's can, for the first time, be taken directly, without noise averaging. MEG's are shown, simultaneously with the electroencephalogram (EEG), of the alpha rhythm of a normal subject and of the slow waves from an abnormal subject. The normal MEG shows the alpha rhythm, as does the EEG, when the subject's eyes are closed; however, this MEG also shows that higher detector sensitivity, by a factor of 3, would be necessary in order to clearly show the smaller brain events when the eyes are open. The abnormal MEG, including a measurenment of the direct-current component, suggests that the MEG may yield some information which is new and different from that provided by the EEG.
Weak alternating magnetic fields outside the human scalp, produced by alpha-rhythm currents, are demonstrated. Subject ard
magnetic detector were housed in a multilayer magnetically shielded chamber. Background magnetic noise was reduced by signal-averaging.
The fields near the scalp are about 1 x 10-9 gauss (peak to peak). A course distribution shows left-right symmetry for the particular averaging technique used here.
Alkali-metal magnetometers compete with SQUID detectors as the most sensitive magnetic field sensors. Their sensitivity is limited by relaxation due to spin-exchange collisions. We demonstrate a K magnetometer in which spin-exchange relaxation is completely eliminated by operating at high K density and low magnetic field. Direct measurements of the signal-to-noise ratio give a magnetometer sensitivity of 10 fT Hz(-1/2), limited by magnetic noise produced by Johnson currents in the magnetic shields. We extend a previous theoretical analysis of spin exchange in low magnetic fields to arbitrary spin polarizations and estimate the shot-noise limit of the magnetometer to be 2x10(-18) T Hz(-1/2).
The magnetic field is one of the most fundamental and ubiquitous physical observables, carrying information about all electromagnetic phenomena. For the past 30 years, superconducting quantum interference devices (SQUIDs) operating at 4 K have been unchallenged as ultrahigh-sensitivity magnetic field detectors, with a sensitivity reaching down to 1 fT Hz(-1/2) (1 fT = 10(-15) T). They have enabled, for example, mapping of the magnetic fields produced by the brain, and localization of the underlying electrical activity (magnetoencephalography). Atomic magnetometers, based on detection of Larmor spin precession of optically pumped atoms, have approached similar levels of sensitivity using large measurement volumes, but have much lower sensitivity in the more compact designs required for magnetic imaging applications. Higher sensitivity and spatial resolution combined with non-cryogenic operation of atomic magnetometers would enable new applications, including the possibility of mapping non-invasively the cortical modules in the brain. Here we describe a new spin-exchange relaxation-free (SERF) atomic magnetometer, and demonstrate magnetic field sensitivity of 0.54 fT Hz(-1/2) with a measurement volume of only 0.3 cm3. Theoretical analysis shows that fundamental sensitivity limits of this device are below 0.01 fT Hz(-1/2). We also demonstrate simple multichannel operation of the magnetometer, and localization of magnetic field sources with a resolution of 2 mm.
Total-field atomic gradiometer for unshielded portable magnetoencephalography
- M E Limes
- E L Foley
- T W Kornack
- S Caliga
- S Mcbride
- A Braun
- W Lee
- V G Lucivero
- M V Romalis
M. E. Limes, E. L. Foley, T. W. Kornack, S. Caliga,
S. McBride, A. Braun, W. Lee, V. G. Lucivero, and M. V.
Romalis, "Total-field atomic gradiometer for unshielded
portable magnetoencephalography," Jan. 2020.
- M G Bason
- T Coussens
- M Withers
- C Abel
- G Kendall
- P Kruger
M. G. Bason, T. Coussens, M. Withers, C. Abel,
G. Kendall, and P. Kruger, "Non-invasive Current Density Imaging of Lithium-Ion Batteries,"
arXiv:2103.03358 [physics], Jan. 2021.
arXiv:
2103.03358.