First-order Gradiometer Research Articles

A modular optically pumped magnetometer system

To address the demands in healthcare and industrial settings for spatially resolved magnetic imaging, we present a modular optically pumped magnetometer (OPM) system comprising a multi-sensor array of highly sensitive quantum magnetometers. This system is designed and built to facilitate fast prototyping and testing of new measurement schemes by enabling quick reconfiguration of the self-contained laser and sensor modules as well as allowing for the construction of various array layouts with a shared light source. The modularity of this system facilitates the development of methods for managing high-density arrays for magnetic imaging. The magnetometer sensitivity and bandwidth are first characterised in both individual channel and differential gradiometer configurations before testing in a real-world magnetoencephalography environment by measuring alpha rhythms from the brain of a human participant. We demonstrate the OPM system in a first-order axial gradiometer configuration with a magnetic field gradient sensitivity of 10fT/cm/Hz at a baseline of 4.5 cm. Single-channel operation achieved a sensitivity of 65fT/Hz . Bandwidths exceeding 200Hz were achieved for two independent modules. The system’s increased temporal resolution allows for the measurement of spinal cord signals, which we demonstrate by using phantom signal trials and comparing with an existing commercial sensor.

Gradient magnetic field measurement using first-order gradiometers equipped with 50 mm diameter pickup coils made out of a 2G high-Tc superconducting tape

Two types of first-order gradiometers equipped with 50 mm diameter pickup coils were made out of an REBaCuO High-Tc superconducting (HTS) flexible tape. The HTS tapes were formed into the HTS gradiometers by ``cut-and-wind'' method without cable jointing.The HTS gradiometers were examined at 77 K for magnetic sensitivity to a gradient field applied by an external coil. The axial-type HTS gradiometer eliminated the magnetic field output of the device induced by a uniform field, and enhanced the magnetic field output induced by a gradient field on the axis of the pickup coil. The planar-type HTS gradiometer made it possible to detect a 20 mm diameter steel cylinder placed beside the pickup coil in a uniform field of 100 µT by detecting the spatial field gradient in the vicinity of a ferromagnetic object.

Ultra-high consistency multichannel SERF atomic magnetometer based on diffractive optics

Multichannel spin-exchange relaxation-free (SERF) atomic magnetometers (AMs) have broad application prospects in array biomagnetic measurement systems because of their high spatial resolution. This study proposed an efficient and high-quality beam-splitting method based on a diffractive optical element (DOE), which enables multichannel SERF AM to maintain ultrahigh consistency with a channel interval of 3.27 mm. Based on the relaxation rate model under zero-field resonance, the minor differences in the magnetic linewidth of each channel were analyzed. The experimental results demonstrated ultra-high consistency of the magnetic field measurement signals between multiple channels, with a nonlinear error of less than 0.1%. The optimal sensitivity for a single channel was 11.9 fT/Hz1/2 @31.5 Hz, and 9.3 fT/Hz1/2 @31.5 Hz in the first-order gradiometer mode. The structure also satisfied the requirements of long-term high stability and high sensitivity, which are expected to further obtain more measurement channels. This method plays a critical role in improving the accuracy of magnetocardiography (MCG) and magnetoencephalography (MEG) systems.

Method for full magnetic gradient tensor detection from a single HTS gradiometer

A new method to measure the full magnetic gradient tensor (FMGT) using a single rotating high-temperature superconducting planar first-order gradiometer is described. Determining the FMGT provides valuable additional information about magnetic sources and their anomalies and, critically, the location of magnetic targets with less ambiguity and with fewer survey lines required. This paper presents both a theoretical description and experimental demonstration of the single sensor system that can, in principle, extract all the magnetic gradient tensor components, with suitable referencing and/or field cancellation procedures. The technique involves rotating the planar gradiometer mounted on an angled plinth about the vertical axis of the sensor package. The technique works by shifting gradient terms to harmonics of the rotating frequency, above the 1/f noise of the device, and allows extraction of all the components in the frequency domain. Additional referencing enables compensation for the non-ideal gradiometer response and provides a more accurate measurement of contaminated gradient components. Laboratory results show tracking of a moving dipole target and these are compared to a modelled response, both with and without field compensation.

Development of series SQUID array with on-chip filter for TES detector

A cold preamplifier based on superconducting quantum interference devices (SQUIDs) is currently the preferred readout technology for the low-noise transition edge sensor (TES). In this work, we have designed and fabricated a series SQUID array (SSA) amplifier for the TES detector readout circuit. In this SSA amplifier, each SQUID cell is composed of a first-order gradiometer formed using two equally large square washers, and an on-chip low pass filter (LPF) as a radio-frequency (RF) choke has been developed to reduce the Josephson oscillation interference between individual SQUID cells. In addition, a highly symmetric layout has been designed carefully to provide a fully consistent embedded electromagnetic environment and achieve coherent flux operation. The measured results show smooth V–Φ characteristics and a swing voltage that increases linearly with increasing SQUID cell number N. A white flux noise level as low as 0.28 μ Φ 0/Hz1/2 is achieved at 0.1 K, corresponding to a low current noise level of 7 pA/Hz1/2. We analyze the measured noise contribution at mK-scale temperatures and find that the dominant noise derives from a combination of the SSA intrinsic noise and the equivalent current noise of the room temperature electronics.

A conformal array of microfabricated optically-pumped first-order gradiometers for magnetoencephalography

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.

Design, implementation and experimental characterisation of a high sensitivity GMI gradiometer with an interference compensation system

The giant magneto-impedance (GMI) effect has been, since its discovery, one of the most promising technologies in the development of magnetic sensors. Features such as high sensitivity, stability and low cost are the main reasons for the development of this type of magnetometers. This study reports the design, implementation and experimental characterisation of a high sensitivity GMI gradiometer, aimed at measuring low-intensity magnetic fields. A first-order gradiometer was designed, based on the impedance phase characteristics of two Co 70 Fe 5 Si 15 B 10 amorphous ribbon-shaped sensors connected to an electronic circuit that converts the magnetic field into an output voltage. The developed prototype can operate not only as gradiometer but also as two separate magnetometers, which allows it to incorporate a compensation system against deviations in the biasing field of the GMI sensors. The experimental results for the gradiometer showed a sensitivity of 99 mV/μT, a measuring range of ±30 μT, a magnetic noise of 5 nT Hz -1/2 at 5 Hz and a resolution of 40 nT within a 500 Hz bandwidth. The compensation system has shown a settling time of 1.6 s for perturbations in the range ±10 μT with a steady-state error <;0.02%. All experiments were performed in an unshielded environment.

Portable Magnetometry for Detection of Biomagnetism in Ambient Environments

We present a method of optical magnetometry with parts-per-billion resolution that is able to detect biomagnetic signals generated from the human brain and heart in Earth's ambient environment. Our magnetically silent sensors measure the total magnetic field by detecting the free-precession frequency of highly spin-polarized alkali metal vapor. A first-order gradiometer is formed from two magnetometers that are separated by a 3 cm baseline. Our gradiometer operates from a laptop consuming 5 W over a USB port, enabled by state-of-the-art micro-fabricated alkali vapor cells, advanced thermal insulation, custom electronics, and laser packages within the sensor head. The gradiometer obtains a sensitivity of 16 fT/cm/Hz$^{1/2}$ outdoors, which we use to detect neuronal electrical currents and magnetic cardiography signals. Recording of neuronal magnetic fields is one of a few available methods for non-invasive functional brain imaging that usually requires extensive magnetic shielding and other infractructure. This work demonstrates the possibility of a dense array of portable biomagnetic sensors that are deployable in a variety of natural environments.

A feasibility study to measure magnetocardiography (MCG) in unshielded environment using first order gradiometer

Measurement of weak magnetic fields generated by the electrical activity of the heart is known as magnetocardiogram (MCG). MCG is usually measured using highly sensitive Superconducting Quantum Interference Devices (SQUIDs) inside a Magnetically Shielded Room (MSR). Since MCG signals have a relatively higher amplitude, researchers have attempted to measure MCG in poorly shielded or even in unshielded environments using higher order gradiometers to cancel excess noise. However, they also attenuate signals of interest, especially, those originating from deeper sources. Here, we report a feasibility study to examine the suitability of use of First Order Gradiometers (FOG) to measure MCG in a totally unshielded environment. Usually, conventional noise reduction methods involve averaging of a large number of cardiac cycles aligned by R-peak time instants derived from a simultaneously recorded electrocardiogram (ECG). In addition to this, here we employ a novel noise reduction approach based on Ensemble Empirical Mode Decomposition (EEMD) technique to manage the fall in Signal to Noise Ratio (SNR) in unshielded environment. The study demonstrated an improvement in SNR of about 18 dB in unshielded setup measured using a four channel low TC-DC SQUID system when tested on eight healthy subjects and one subject with a known cardiac dysfunction. MCG traces were measured across different locations on the thorax and the spatial distribution of cardiac magnetic field is visualized using magnetic field maps. The resulting data have been critically evaluated and compared with those derived from MCG measurements made inside the MSR and encourages its clinical utility.

3D Magnetopneumography Magnetic Dipole Model and Its Application Using Fluxgate Gradiometers

Magnetopneumography (MPG) as a non-invasive method for pneumoconiosis diagnosis has been developed to evaluate the load and spatial distribution of particles within the human lungs through scanning of remanent magnetic fields after magnetization of the subject in a strong direct current field. The measurement of particle spatial distribution is very important for pneumoconiosis diagnosis because localized deposits may be associated with some pathological changes such as pulmonary fibrosis. Previous research found that loads of magnetite particles were proportional to their magnetic dipole moments. The three-dimensional (3D) MPG magnetic dipole model (MDM) proposed in this paper and based on Biot-Savart Law and matrix manipulation provides a means of precise measurement of the particle distribution and load amount. A styrofoam + magnetite powder phantom with magnetization was laid on a nonmagnetic board. Two first-order fluxgate gradiometers with 10-12 T sensitivity were coaxially applied over and under the phantom and used for scanning remanent magnetic fields. This paper provides validation results using 3D MPG MDM through two experiments. The overall error of the simulation results is 0.2-2.7% in the center and 7.28-9.42% in the corners of the subject. Finally, this paper gives clinical experiments with a welder suffering stage-II pneumoconiosis and states that the 3D MPG MDM shows similar results to X-ray chest films in pneumoconiosis diagnosis. The results suggest that the 3D MPG MDM is potentially a reasonable and accurate algorithmic model to inversely track the load amount and distribution of magnetite particles within the lungs. Bioelectromagnetics. 2019;40:472-487. © 2019 Wiley Periodicals, Inc.

A High-Balance Planar SQUID Gradiometer

We have developed and fabricated SQUID gradiometers in a Nb/Al-AlOx/Nb technology using an i-line stepper combined with a mask aligner lithography. The stepper and the aligner are used to fabricate fine-scale patterns such as the Josephson junctions and large-scale patterns such as pickup coil, respectively, on the same wafer with a 4-inch diameter. The SQUID is designed as a first-order gradiometer with a dual-washer configuration. Two sets of multiturn spiral input coils with opposite directions are integrated on the washer and connected to the pickup coil to form a flux transformer. In order to improve the balance of the gradiometer, a fully symmetric configuration is introduced with a mirrored SQUID. The pickup coil is a first-order gradiometric type with a base line of 3.5 cm. The size of each loop is 12.65 mm × 11.65 mm, which results in a gradient magnetic field sensitivity of 0.25 nT/(cm·Φ0). The measured gradient magnetic field noise is 75 fT/(m·√Hz) in the white noise region. Moreover, a sapphire holder is used to fix and package the chip in order to maintain the high intrinsic balance (typical 10−4). It makes the SQUID gradiometer more attractive for an unshielded operation in noisy environments.

SQUID Gradiometer Module for Fetal Magnetocardiography Measurements Inside a Thin Magnetically Shielded Room

Gradiometer module plays a significant role in fetal magnetocardiography (fMCG) system and its performance determines the signal qualities directly. In order to obtain the best signal-to-noise ratio (SNR) of the fMCG signals in a thin magnetically shielded room (tMSR), the baseline of the axial gradiometers need to be optimized. In this paper, direct current superconducting quantum interference devices which is composed of weakly damped Josephson Junctions with βc > 3 and has a flux-to-voltage transfer coefficient of 500 μ V/Φ0 is employed for our gradiometers. Superconducting connectivity technique is utilized to connect input coil and pickup coil. First-order gradiometers with different baselines are thus developed. After the evaluation of the magnetic flux density and gradient flux density inside the tMSR, measurement of fMCG signals are taken with the gradiometers mentioned earlier. The simulated fetal QRS peak signals about 2.5 pT could easily be identified in the realtime gradiometer module outputs with an SNR larger than 11 dB.

Design and fabrication of low-noise superconducting quantum interference device magnetometer

Superconducting quantum interference device (SQUID) is the most sensitive magnetic flux sensor known, which is widely used in biomagnetism, low-field nuclear magnetic resonance, geophysics, etc. In this paper, we introduce a high-sensitivity SQUID magnetometer, which consists of an SQUID and a flux transformer. The SQUID is first-order gradiometer configuration, which is insensitive to interference noise. The flux transformer includes a multi-turn spiral input coil and a large-sized pickup coil. And the input coil is inductively coupled to the SQUID through mutual inductance. We present an SQUID magnetometer fabricated with Nb/Al-AlO&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;/Nb Josephson junction technology on a 4-inch silicon wafer at our superconducting electronics facilities. We develop a fabrication process based on selective niobium etching process consisting of five mask levels. In the first two mask levels, the trilayer is patterned by a dry etch to define base electrode, contact pads, and interconnects. The shunt resistor and a dielectric insulating layer are then deposited and patterned by using lift-off and dry etchant, respectively. Finally, the niobium wiring layer is deposited and patterned by using reactive ion etching to define input, pickup and feedback coils. The measurement of the SQUID magnetometer is performed inside a magnetically shielded room. The operating temperature is realized by immersing the SQUID into the liquid helium (4.2 K). Moreover, a superconducting niobium tube is employed to protect the SQUID from being disturbed by external environments. A homemade readout electronics instrument with low input voltage noise is used to characterize the SQUID magnetometer. The results of low-temperature measurements indicate that the magnetometer has a magnetic field sensitivity of 0.36 nT/Φ&lt;sub&gt;0&lt;/sub&gt; and a white flux noise of 8 μΦ&lt;sub&gt;0&lt;/sub&gt;/√Hz,corresponding to a white field noise of 2.88 fT/√Hz. This kind of SQUID magnetometer is suitable for multi-channel systems, e.g., magnetocardiography, magnetoencephalography, etc. Although the SQUID process development benefits from the rapid advance of semiconductor process technology, the uniformity of the SQUID on one wafer is fluctuated due to the film deposition. Now, we have realized a best SQUID yield of 50% on a 4-inch wafer. In the future, the SQUID chip yield should be improved by well controlling the optimizing process. The device yield is expected to reach as high as 80%.

Computational and Experimental Study of Gradiometer Coil Sensor Design for Single-Sided Magnetic Particle Imaging

This paper presents the design and construction of an induction coil sensor, suitable for single-sided Magnetic Particle Imaging (MPI) scanner. Tomographic images of the magnetic nanoparticles (MNPs) used as tracer need to be formed from the voltage induced in the pickup coil as a result of the particles’ magnetization response when excited by a superimposed magnetic field from the excitation and selection fields. The coil sensor is looped in the form of the solenoid (Maxwell arrangement) and inserted into the coaxial arrangement of the excitation and selection field coils to form the single-sided scanner geometry. A first-order gradiometer is realized by winding two identical coils, each coil having 208 turns, with a baseline distance of 30 mm. Simulation and experimental results revealed that the induced voltage is directly proportional to the excitation field, and inversely proportional to the distance away from the excitation coil. Results obtained showed that the designed gradiometer pickup coil is sufficient for single-sided MPI scanner, to be applied in the sentinel lymph node biopsy (SLNB) for breast cancer detection.

Optimization of pick-up coils for weakly damped SQUID gradiometers**Project supported by the Key Project of Shanghai Zhangjiang National Innovation Demonstration Zone of the Special Development Fund, China (Grant No. 2015-JD-C104-060) and the National Natural Science Foundation of China (Grant No. 61741122).

The performance of a superconducting quantum interference device (SQUID) gradiometer is always determined by its pick-up coil geometry, such as baseline and radius. In this paper, based on the expressions for the coupled flux threading a magnetometer obtained by Wikswo, we studied how the gradiometer performance parameters, including the current dipole sensitivity, spatial resolution and signal-to-noise ratio (SNR), are affected by its pick-up coil via MatLab simulation. Depending on the simulation results, the optimal pick-up coil design region for a certain gradiometer can be obtained. To verify the simulation results, we designed and fabricated several first-order gradiometers based on the weakly damped SQUID with different pick-up coils by applying superconducting connection. The experimental measurements were conducted on a simple current dipole in a magnetically shielding room. The measurement results are well in coincidence with the simulation ones, indicating that the simulation model is useful in specific pick-up coil design.

Comparison of Axial Wire-Wound and Planar Thin-Film SQUID-Based Gradiometers for Super-Paramagnetic Relaxometry Measurements

Superparamagnetic relaxometry (SPMR) is a new technology used for detection and localization of cells tagged with super-paramagnetic nanoparticles. SPMR is one of the most promising methods for early cancer diagnosis with the potential for submillimeter size tumor resolution. SQUID-based gradiometers are very suitable for such sensitive measurements. Presently, most commonly used SPMR instruments consist of axial wire-wound gradiometers connected to SQUID-sensors placed inside tight superconducting shields. This configuration avoids flux trapping by the Josephson junctions during magnetization. A thin-film planar gradiometer integrated with a SQUID on a single chip traps flux, if it picks up even small portion of a magnetizing field. Thermal cycling is very slow, therefore it is not practical. Special measures must be applied such as newly developed methods of SQUIDs ac de-fluxing. Planar thin-film gradiometers have advantages over axial wire-wound gradiometers because of their much higher balance level, higher reliability, and reproducibility in production, and significant simplification of multichannel systems design. In this paper, we compare experimental results received using axial wire-wound and planar thin-film gradiometers. This comparison is based on SPMR measurements conducted using the seven-channel axial second-order gradiometer system and two systems with different geometries of the thin-film first-order planar gradiometer arrays. We discuss advantages and drawbacks of the two types of gradiometers.

Weakly Damped SQUID Gradiometer With Low Crosstalk for Magnetocardiography Measurement

In a multichannel magnetocardiography system, the problem of crosstalk is serious due to mutual inductances between the pickup coils of adjacent channels. In this paper, a superconducting quantum interference device gradiometer module based on weakly damped Josephson junctions with large $\beta _{c}\;(\approx 5)$ is presented. An external feedback design is also adopted to suppress the crosstalk between different channels. With a special superconducting connection between input coil and pickup coil, an axial first-order gradiometer with baseline of 70 mm and diameter of 18 mm is fabricated. Using a direct readout circuit with equivalent flux noise of $1.8\;\mu \Phi _{{0}}{\rm{/ \surd Hz}}$ , the flux noise of this gradiometer is measured to be $6.5\;\mu \Phi _{{0}}{\rm{/ \surd Hz}}$ in a white noise range, corresponding to a field sensitivity of ${\rm{0.5\;fT/ (cm\bullet \surd Hz)}}$ . The crosstalk rates in external feedback mode and internal feedback mode are both successfully simulated with MAXWELL software. The results by experimental measurements using these gradiometers are in coincidence with the simulated ones; both simulation and experimental results show a substantial suppression of crosstalk in the external feedback design.

Geometry Structure Optimization of Hexagonal Pyramidal Full Tensor Magnetic Gradient Probe

In full tensor magnetic gradient measurement system based on superconducting quantum interference devices (SQUIDs), six first-order planar-type SQUID gradiometers mounted on the six sloping facets of a hexagonal pyramid can form the probe and determine all independent components of the magnetic gradient tensor with the help of triaxial SQUID magnetometers placed near the probe. The geometrical parameters of probe, such as the inclination angle of sloping facets and $x$ -axis orientation in the probe coordinate system o-xyz, seriously impact on the sensitivities for magnetic gradient tensor invariants. We deduced that the invariants’ sensitivities depend on the maximum among five independent elements’ sensitivities. Based on the functional relationship between each independent element’s relative sensitivity and the geometrical parameters, we designed the optimal probe geometry structure with the inclination being 58.7° and the angle of the $x$ -axis relative to the projection of the first gradiometer’s baseline on the xoy plane being ±15°, when the maximum among the relative sensitivities for the five independent elements achieved the minimum. Finally, we generalized three types of geometrical inaccuracies of the gradiometers arising in assembling the probe and evaluated the influences of each type of them on the measurement accuracy for each independent element.

Development of Earth’s Field Nuclear Magnetic Resonance (EFNMR) Technique for Applications in Security Scanning Devices

In this work Earth’s field nuclear magnetic resonance (EFNMR) technique for applications in security scanning devices has been presented. The EFNMR setup has been build based on Magritek low-frequency (0–1 MHz) KEA-2 NMR console. Excepting the NMR console, all components are home-made units, which are a first-order gradiometer RF probe, pre-polarization system, magnetic field shimming system, RF shielding box and passive filters. Our measurements have shown that even in very noisy lab conditions the EFNMR signals of small amounts (~5 ml) of liquid samples could be measured. EFNMR measurements of various liquids have been performed and their spin–lattice T1 and spin–spin T2 relaxation constants have been obtained. A possibility to detect, at the same time, both fluorine and proton resonance has been shown by EFNMR measurements of trifluoroacetic acid. A good spectral resolution of EFNMR setup has been demonstrated by the spectroscopic measurements of heteronuclear J-coupling of trimethyl phosphate and fluorobenzene. Furthermore, we have shown that the EFNMR measurements of T1 and T2 constants as well as heteronuclear J-coupling of liquids enclosed in metallic containers are feasible. Possible applications of the EFNMR technique in security scanners for the detection of illicit and energetic substances are discussed.

A simplified HTc rf SQUID to analyze the human cardiac magnetic field

We have developed a four-channel high temperature radio-frequency superconducting quantum interference device (HTc rf SQUID) in a simple magnetically shielded room (MSR) that can be used to analyze the cardiac magnetic field. It is more robust and compact than existing systems. To achieve the high-quality magnetocardiographic signal, we explored new adaptive software gradiometry technology constructed by the first-order axial gradiometer with a baseline of 80mm, which can adjust its performance timely with the surrounding conditions. The magnetic field sensitivity of each channel was less than 100fT/√Hz in the white noise region. Especially, in the analysis of MCG signal data, we proposed the total transient mapping (TTM) technique to visualize current density map (CDM), then we focused to observe the time-varying behavior of excitation propagation and estimated the underlying currents at T wave. According to the clear 3D imaging, isomagnetic field and CDM, the position and distribution of a current source in the heart can be visualized. It is believed that our four-channel HTc rf SQUID magnetometer based on biomagnetic system is available to detect MCG signals with sufficient signal-to-noise (SNR) ratio. In addition, the CDM showed the macroscopic current activation pattern, in a way, it has established strong underpinnings for researching the cardiac microscopic movement mechanism and opening the way for its use in clinical diagnosis.