Superconducting Quantum Interference Device Sensor Research Articles

SQUID magnetoneurography: an old-fashioned yet new tool for noninvasive functional imaging of spinal cords and peripheral nerves.

We are engaged in the development and clinical application of a neural magnetic field measurement system that utilizes biomagnetic measurements to observe the activity of the spinal cord and peripheral nerves. Unlike conventional surface potential measurements, biomagnetic measurements are not affected by the conductivity distribution within the body, making them less influenced by the anatomical structure of body tissues. Consequently, functional testing using biomagnetic measurements can achieve higher spatial resolution compared to surface potential measurements. The neural magnetic field measurement, referred to as magnetoneurography, takes advantage of these benefits to enable functional testing of the spinal cord and peripheral nerves, while maintaining high spatial resolution and noninvasiveness. Our magnetoneurograph system is based on superconducting quantum interference devices (SQUIDs) similar to the conventional biomagnetic measurement systems. Various design considerations have been incorporated into the SQUID sensor array structure and signal processing software to make it suitable for detecting neural signal propagation along spinal cord and peripheral nerve. The technical validation of this system began in 1999 with a 3-channel SQUID system. Over the course of more than 20 years, we have continued technological development through medical-engineering collaboration, and in the latest prototype released in 2020, neural function imaging of the spinal cord and peripheral nerves, which could also be applied for the diagnosis of neurological disorders, has become possible. This paper provides an overview of the technical aspects of the magnetoneurograph system, covering the measurement hardware and software perspectives for providing diagnostic information, and its applications. Additionally, we discuss the integration with a helium recondensing system, which is a key factor in reducing running costs and achieving practicality in hospitals.

Single-trial classification of evoked responses to auditory tones using OPM- and SQUID-MEG

Objective. Optically pumped magnetometers (OPMs) are emerging as a near-room-temperature alternative to superconducting quantum interference devices (SQUIDs) for magnetoencephalography (MEG). In contrast to SQUIDs, OPMs can be placed in a close proximity to subject’s scalp potentially increasing the signal-to-noise ratio and spatial resolution of MEG. However, experimental demonstrations of these suggested benefits are still scarce. Here, to compare a 24-channel OPM-MEG system to a commercial whole-head SQUID system in a data-driven way, we quantified their performance in classifying single-trial evoked responses. Approach. We measured evoked responses to three auditory tones in six participants using both OPM- and SQUID-MEG systems. We performed pairwise temporal classification of the single-trial responses with linear discriminant analysis as well as multiclass classification with both EEGNet convolutional neural network and xDAWN decoding. Main results. OPMs provided higher classification accuracies than SQUIDs having a similar coverage of the left hemisphere of the participant. However, the SQUID sensors covering the whole helmet had classification scores larger than those of OPMs for two of the tone pairs, demonstrating the benefits of a whole-head measurement. Significance. The results demonstrate that the current OPM-MEG system provides high-quality data about the brain with room for improvement for high bandwidth non-invasive brain–computer interfacing.

Development of Ultra-Low Field Magnetic Resonance Imaging System at 1 mT

Ultra-low field (ULF) magnetic resonance imaging (MRI) is considered an innovative MRI technique that can be combined with magnetoencephalography (MEG). In this study, a 1-mT ULF-MRI system without a pre-polarization technique aiming at combining an MEG system is developed. The intensity of the static magnetic field B0 is decided considered to be 1 mT because of the limitation of the magnetic field exposed to our MEG sensor made of superconducting quantum interference devices (SQUIDs). A square-shaped (750 mm × 750 mm) B0-coil based on a Merritt-type configuration is fabricated. The inhomogeneity of B0 is less than 0.07 % in the ±125 mm area. Further, an induction coil is employed to detect the magnetic resonance signal instead of the SQUID sensors of the MEG to confirm the feasibility of realizing MRI measurement at 1 mT without the pre-polarization technique. Relaxation time measurements and MR imaging of a phantom made of a CuSO4 aqueous solution are performed. Reasonable longitudinal and transverse relaxation times are obtained, and clear shapes of the phantom are obtained using the gradient- and spin-echo pulse sequences with this ULF-MRI system. These results indicate the potential of the proposed 1-mT ULF-MRI to effectively integrate ULF-MRI and MEG systems without pre-polarization.

Highly Sensitive Tunable Magnetometer Based on Superconducting Quantum Interference Device

In the present article, experimental results regarding fully integrated superconducting quantum interference devices (SQUID), including a circuit to tune and optimize the main sensor device characteristics, are reported. We show the possibility of modifying the critical current of a SQUID magnetometer in liquid helium by means of a suitable heating circuit. This allows us to improve the characteristics of the SQUID sensor and in particular to optimize the voltage-magnetic flux characteristic and the relative transfer factor (responsivity) and consequently to also improve the flux and magnetic field noise. It is also possible to reset the SQUID sensor in case of entrapment of magnetic flux, avoiding taking it out of the helium bath. These results are very useful in view of most SQUID applications such as those requiring large multichannel systems in which it is desirable to optimize and eventually reset the magnetic sensors in a simple and effective way.

Development of Magnetocardiograph without Magnetically Shielded Room Using High-Detectivity TMR Sensors.

A magnetocardiograph that enables the clear observation of heart magnetic field mappings without magnetically shielded rooms at room temperatures has been successfully manufactured. Compared to widespread electrocardiographs, magnetocardiographs commonly have a higher spatial resolution, which is expected to lead to early diagnoses of ischemic heart disease and high diagnostic accuracy of ventricular arrhythmia, which involves the risk of sudden death. However, as the conventional superconducting quantum interference device (SQUID) magnetocardiographs require large magnetically shielded rooms and huge running costs to cool the SQUID sensors, magnetocardiography is still unfamiliar technology. Here, in order to achieve the heart field detectivity of 1.0 pT without magnetically shielded rooms and enough magnetocardiography accuracy, we aimed to improve the detectivity of tunneling magnetoresistance (TMR) sensors and to decrease the environmental and sensor noises with a mathematical algorithm. The magnetic detectivity of the TMR sensors was confirmed to be 14.1 pTrms on average in the frequency band between 0.2 and 100 Hz in uncooled states, thanks to the original multilayer structure and the innovative pattern of free layers. By constructing a sensor array using 288 TMR sensors and applying the mathematical magnetic shield technology of signal space separation (SSS), we confirmed that SSS reduces the environmental magnetic noise by -73 dB, which overtakes the general triple magnetically shielded rooms. Moreover, applying digital processing that combined the signal average of heart magnetic fields for one minute and the projection operation, we succeeded in reducing the sensor noise by about -23 dB. The heart magnetic field resolution measured on a subject in a laboratory in an office building was 0.99 pTrms and obtained magnetocardiograms and current arrow maps as clear as the SQUID magnetocardiograph does in the QRS and ST segments. Upon utilizing its superior spatial resolution, this magnetocardiograph has the potential to be an important tool for the early diagnosis of ischemic heart disease and the risk management of sudden death triggered by ventricular arrhythmia.

Development of SQUID based TDEM system and its utilization for field survey at Tumallapalle, Andhra Pradesh, India

This paper describes the development of low Tc SQUID (Superconducting Quantum Interference Device) based TDEM (Time Domain Electro-Magnetic) system for geophysical applications. The SQUID system has been used to perform central loop TDEM sounding measurements near Tumallapalle Uranium mine, Cuddapah (dt), AP, India, where the terrain consists of a thin conducting layer below which low grade Uraniferous Phosphatic Siliceous Dolostone is present. Locating the thin conducting layer which is covered by a thick and highly resistive layer is a challenging task due to the induction of weak eddy currents. In order to enhance the resolution of the target located at larger depth, it is essential to apply larger transmitter moment and use a sensitive receiver. In this work, we combine the use of sensitive SQUID sensor and transmitter loop with larger size to achieve better target resolution. The survey has been performed using transmitter loop sizes of 100 m × 100 m and 400 m × 400 m. In order to compare the performance of the SQUID based TDEM system, the same experiments have also been repeated using induction coil as receiver which has been built for this work. In this article, we describe in detail the results obtained from the central loop TDEM measurements.

Error Calibration for Full Tensor Magnetic Gradiometer Probe Based on Coordinate Transformation Method

In order to calibrate the zero offset error and triaxial non-orthogonal error of full tensor magnetic gradiometer (FTMG) probe composed of superconducting quantum interference device (SQUID) magnetometers, we propose a new calibration method in this article. With the help of the triaxial Helmholtz coil, we can generate a magnetic field with known parameters. By establishing the objective functions containing the error parameters for the outputs of the SQUIDs and solving them with the least-squares fitting algorithm, the included angle between the SQUID sensor and the Helmholtz coil can be obtained, then all the required error parameters can be calculated by coordinate transformation method. Compared with the traditional method, this method does not need to rotate the SQUID magnetometers-based probe to collect the data set for calibration, thus avoiding the influence of motion induced noise on the calibration results. In order to verify the effectiveness of the proposed method, we adopted the widely used ellipsoid fitting method as a comparison method and conducted field comparison experiments. The results show that the proposed calibration method is better than the traditional ellipsoid fitting calibration method under the same experimental environment. The root mean square errors of five independent magnetic gradient tensor components are reduced to 0.39, 0.35, 0.41, 0.31 and 0.57 nT/m from 1630.55, 756.59, 3097.56, 1051.09 and 1245.64 nT/m by the proposed method.

High-Tc rf SQUIDs with Large Flux Focusing

The high-Tc rf SQUID (superconducting quantum interference device) sensor, in which the rf SQUID was placed face to face with superconductor flux concentrator/dielectric resonator, was designed and fabricated. The SQUID sensor was made up of a SQUID washer (with an area of 9.6 mm2 and an inductance of 150 pH) and a flux focusing/dielectric resonator (with an area of 100 mm2 and an resonant frequency varied from 430 to 650 MHz). The author conducted a test to prove that with a white flux noise spectral density of SΦ 1/2=3×10-5Φ0/ Hz1/2 and field sensitivity of BN100 fT/Hz1/2, this flux focusing/dielectric resonator-type rf SQUID sensor can work stably where electromagnetic interference is smaller.

SQUID microscopy for mapping vector magnetic fields

We mounted a vector-scanning superconducting quantum interference device (SQUID) sensor on a commercial SQUID microscope and successfully improved the sensitivity and spatial resolution of the sensor. Our proposed vector 3D SQUID sensor used multilayered niobium (Nb)-based technology; we realized three SQUID sensors in the structure of a vector pickup coil system on a single chip. The vector pickup coil system was built with three pickup coils that were orthogonal to one another to obtain the X, Y, and Z components of a magnetic field vector. To improve both sensitivity and spatial resolution, we attempted to reduce the inner diameter by increasing the number of windings of the pickup coils using the multilayered Nb process. The design value for the dc SQUID sensors was either the critical current density Jc = 320 A cm−2 for two Josephson junctions (JJs) or the critical current Ic = 12.8 μA for the 2 μm × 2 μm JJs. To measure the current–voltage (I–V) and voltage–flux (V–Φ) characteristics of a sensor, we constructed a homemade measurement system. The fundamental characteristics of our SQUID sensors were in good agreement with the design parameters. We mounted our vector SQUID sensor on a commercial scanning SQUID microscope (SQM2000, Seiko Instrument Inc.) with a single flux-locked loop channel to test one of the three channels. We repeated the measurements three times to obtain the X, Y, and Z componential images of the magnetic field from a single vortex, and we synthesized the 3D mapping of the magnetic field vectors from a single vortex. We proved that our sensor mounted on a SQUID microscope was suitable for measuring the X, Y, and Z components of a vector magnetic field.

Readiness of Magnetic Nanobiosensors for Point-of-Care Commercialization

Nanobiosensors contribute to point-of-care (POC) efforts to make routine biodiagnostics more accessible to patients with respect to both expense and convenience. However, devices whose operation is based on magnetic phenomena appear delayed in their progress toward POC commercialization despite their promise of better sensitivity, as compared to devices based on optical, mechanical, or electrochemical phenomena. This review aims to elucidate the technical challenges preventing magnetic nanobiosensors from reaching market readiness. The following types of magnetic nanobiosensor operation are reviewed: giant magnetic impedance (GMI), superconducting quantum interference device (SQUID), anisotropic magnetoresistance, giant magnetoresistance (GMR), resonant coil, Hall effect, and microcantilever. In particular, a careful comparison of each type in terms of their advantages, disadvantages, recently overcome challenges, and sensitivities will be presented. For example, a disadvantage of GMI sensors, and certain others reviewed here, is the fact that ferromagnetic materials used in their construction directly impact the biosensing event since magnetic nanoparticle (MNP) labels are involved in the strategy. Other challenges associated with use of MNP labels will be addressed. Furthermore, some of the more interesting state-of-the-art magnetic nanobiosensor efforts will be discussed in order to provide an overview of target analytes and sample media under consideration. This review identifies GMR sensors as poised to dominate the market owing to their good sensitivity and ease of use. On the other hand, SQUID sensors, at their current stage of development, are revealed as unsuitable for POC applications due to their high operational cost and unwieldy instrumentation. Magnetics experts endeavoring to progress the field toward commercialization will find this review indispensable.

The step-edge type HTS rf SQUID coupling with a LC circuit in ultra-low field NMR system

The step-edge type high temperature superconductor (HTS) radio frequency (rf) superconducting quantum interference devices (SQUIDs) were fabricated from YBa2Cu3O7−δ thin films with a field sensitivity of 66 fT/Hz1/2. Even though the sensitivity of the rf SQUID sensor used in the experiments was degraded to 77 fT/Hz1/2 in the magnetic shielded room (MSR), the performance of the rf SQUID sensor was good enough for the ultra-low field (ULF) nuclear magnetic resonance (NMR) experiments at a measurement field of 61.5 μT. With the help of a strictly controlled fast switching circuit and a coupled LC circuit which acted as flux transformer, the signal-to-noise ratio (SNR) of the rf SQUID-based ULF NMR system with different operating points was measured. The optimum SNR was 40 dB which increased significantly by coupling an input coil closely beneath the rf SQUID sensor. The extracted 1/T1 value for pure water sample at 23 ∘C showed the rf SQUID based ULF NMR was an effective system for ULF NMR researches. All above, the direct coupled method and the control circuit show a great potential in the future rf SQUID-based ULF NMR &MRI system.

3D nano-bridge-based SQUID susceptometers for scanning magnetic imaging of quantum materials

We designed and fabricated a new type of superconducting quantum interference device (SQUID) susceptometers for magnetic imaging of quantum materials. The 2-junction SQUID sensors employ 3D Nb nano-bridges fabricated using electron-beam lithography. The two counter-wound balanced pickup loops of the SQUID enable gradiometric measurement and they are surrounded by a one-turn field coil for susceptibility measurements. The smallest pickup loop of the SQUIDs were 1 μm in diameter and the flux noise was around 1 μФ0/√Hz at 100 Hz. We demonstrate scanning magnetometry, susceptometry and current magnetometry on some test samples using these nano-SQUIDs.

Remote Sensing of Ciliary Beating Using a SQUID Gradiometer

Beating of ependymal cilia in the mammalian brain ventricle causes cerebrospinal fluid circulation. Beating of ependymal cilia is studied in vitro in many laboratories but has not been monitored non-invasively in live animals. We have developed a new method for remote sensing of the magnetic flux density changes associated with the reciprocal movement of a magnetic particle bound to cilia in the lateral ventricle of a newborn rat brain with superconducting quantum interference device (SQUID). We detected an oscillatory signal (10–20 Hz and ca. 10 pT amplitude) from a small neodymium magnetic particle attached to cilia with the SQUID above the specimen; the distance between the cilia to the SQUID sensor was 12 mm. This successful detection provides a new sensing platform that enables the non-invasive measurement of beating of ependymal cilia from a live animal.

Superconducting Quantum Interferometers for Nondestructive Evaluation.

We review stationary and mobile systems that are used for the nondestructive evaluation of room temperature objects and are based on superconducting quantum interference devices (SQUIDs). The systems are optimized for samples whose dimensions are between 10 micrometers and several meters. Stray magnetic fields from small samples (10 µm–10 cm) are studied using a SQUID microscope equipped with a magnetic flux antenna, which is fed through the walls of liquid nitrogen cryostat and a hole in the SQUID’s pick-up loop and returned sidewards from the SQUID back to the sample. The SQUID microscope does not disturb the magnetization of the sample during image recording due to the decoupling of the magnetic flux antenna from the modulation and feedback coil. For larger samples, we use a hand-held mobile liquid nitrogen minicryostat with a first order planar gradiometric SQUID sensor. Low-Tc DC SQUID systems that are designed for NDE measurements of bio-objects are able to operate with sufficient resolution in a magnetically unshielded environment. High-Tc DC SQUID magnetometers that are operated in a magnetic shield demonstrate a magnetic field resolution of ~4 fT/√Hz at 77 K. This sensitivity is improved to ~2 fT/√Hz at 77 K by using a soft magnetic flux antenna.

Detection of magnetic nanoparticles with a large scale AC superconducting susceptometer

Magnetic nanoparticles (MNPs) are being used in several applications in medicine such as hyperthermia, magnetic particle imaging, in vitro and in vivo bioassay, and still there are many other possibilities for use of these particles to come as research progress in this field. One crucial step of its use is the detection of these particles when present in a certain tissue. For in vitro bioassay, the sample can be harvested and placed inside the detector in optimal conditions to favor sensitivity. However, for in vivo human measurements the system must be noninvasive and conform to the anatomic restrictions requiring sensitive detectors and dedicated setups. In this study, we detect nanoparticles with an AC biosusceptometer having an excitation homogeneous magnetic field with 145 μT, provided by a set of rectangular large Rubens coils driven at 10 Hz. The magnetization induced in the sample was detected by a second-order axial gradiometer (20 mm in diameter and 40 mm of baseline) coupled to an RF Superconducting Quantum Interference Device (SQUID) model 330X (BTi). The MNPs used were manganese ferrite-based surface-coated with citric acid dissolved in water at various concentrations. The colloid is stable at physiological conditions. X-ray diffraction confirmed the spinel structure and using Scherrer’s relation revealed a particle size of 17.3 nm. The magnetization curve showed a typical superparamagnetic behavior with a specific saturation magnetization of 51.2 emu g−1. The stock solution of nanoparticles had a concentration of 23.17 mg ml−1, corresponding to 1.7 × 1015 NPs ml−1. Measurements were made in a volume of 30 ml with 20 × 103–100 × 103 dilutions of the stock solution of nanoparticles and performed at distances of 1.1, 1.5 and 2.5 cm from the top of the sample vial to the closest coil of the gradiometer. The limits of detection were 8.1 × 109 NP ml−1, 9.5 × 109 NP ml−1 and 11.0 × 109 NP ml−1 for the distances above. These values suggest that the technique might have interesting applications in the real-time in vivo detection of nanoparticles after systemic injections. The present setup can be improved by using more sensitive SQUID sensors and less noisy cryostats.

Suppression of Baseline Wander Artifact in Magnetocardiogram Using Breathing Sensor

Measurements of magnetocardiogram (MCG) are commonly contaminated by different types of artifacts like breathing, power line interference and subject’s movement, out of which artifact due to breathing is unavoidable. This breathing artifact is mainly caused by movement of the chest of the subject during respiration and the consequent changes of the distance between the superconducting quantum interference device (SQUID) sensors (used for recording the MCG signal) and the subject (lying in a supine position under the cryostat housing the SQUID sensors); because of this, the measured MCG signal is a superposition of the true MCG signal of cardiac origin and the breathing related artifact. In most previous works, the suppression of baseline wander artifact was implemented by using digital filters, wavelet based technique, empirical mode decomposition (EMD) and ensemble empirical mode decomposition (EEMD). We present here a novel approach for effective suppression of breathing related baseline wander artifacts by acquiring the MCG data contaminated by the breathing artifact simultaneously with the experimental recording of the actual breathing pattern of the subject using a temperature sensor. The temperature sensor monitors the very slight change in temperature that occurs near the nostril during inhalation and exhalation, and this data is used for the subsequent elimination of the breathing artifact from the contaminated MCG by using the regression approach. We illustrate the method and compare its performance with other conventional approaches for the suppression of baseline wander in the measurement of MCG using single channel data. This is the first time that a breathing sensor has been employed for the suppression of breathing related baseline wander artifact. Experimental results reveal the superiority of the proposed approach in suppression of the breathing related base-line wander artifact with the important advantage that the algorithm used for the suppression uses the actual breathing data derived from an experimental input.

Nondestructive Evaluation Using a High-<italic>T</italic>cSQUID Microscope

We report the application of a scanning high- $T_{{\rm{c}}}$ SQUID (superconducting quantum interference device) microscope with a ferromagnetic flux guide for the nondestructive evaluation of weld seams and wear tracks and scars on austenitic stainless steel plates as well as measurement of magnetic stray fields distribution above patterned by electron lithography 30-nm-thick cobalt films. A soft magnetic amorphous Vitrovac foil was used to guide the flux from the samples, which were held at room temperature, to the liquid-nitrogen-cooled SQUID-sensor and back. The flux guide passes through a hole in the $\text{1 mm}\times \text{2.5 mm}$ pick-up loop of the high- $T_{{\rm{c}}}$ SQUID sensor, thereby providing improved coupling of the magnetic flux from the object to the SQUID. In order to avoid the influence of the SQUID biasing the magnetic field on the object under investigation, a modulation and feedback coil was coupled to the pick-up loop of the SQUID directly and beyond the ferromagnetic flux guide. Such decoupling of feedback coil from soft magnetic flux antenna ensures that the high- $T_{{\rm{c}}}$ SQUID microscope does not disturb the sample magnetization during image recording. The SQUID microscope can be used to measure the spatial distribution of the z -component of the stray field above a specimen without mechanical contact to it.

Initial Results from SQUID Sensor: Analysis and Modeling for the ELF/VLF Atmospheric Noise.

In this paper, the amplitude probability density (APD) of the wideband extremely low frequency (ELF) and very low frequency (VLF) atmospheric noise is studied. The electromagnetic signals from the atmosphere, referred to herein as atmospheric noise, was recorded by a mobile low-temperature superconducting quantum interference device (SQUID) receiver under magnetically unshielded conditions. In order to eliminate the adverse effect brought by the geomagnetic activities and powerline, the measured field data was preprocessed to suppress the baseline wandering and harmonics by symmetric wavelet transform and least square methods firstly. Then statistical analysis was performed for the atmospheric noise on different time and frequency scales. Finally, the wideband ELF/VLF atmospheric noise was analyzed and modeled separately. Experimental results show that, Gaussian model is appropriate to depict preprocessed ELF atmospheric noise by a hole puncher operator. While for VLF atmospheric noise, symmetric α-stable (SαS) distribution is more accurate to fit the heavy-tail of the envelope probability density function (pdf).

Development of SQUID magnetometer with direct-feedback noise cancellation for magnetocardiogram without magnetically shielded room

Superconducting quantum interference device (SQUID) magnetometers have been widely used to perform biomagnetic measurements. In biomagnetic measurements such as magnetocardiograms (MCGs) and magnetoencephalograms, a magnetically shielded room (MSR) is used for noise reduction. However, because the MSR is expensive and heavy, the environments in which it can be used are restricted. In addition, MCG measurements obtained outside the MSR have a poor signal-to-noise ratio. We developed a wide dynamic range SQUID magnetometer that uses the flux-quanta counting (FQC) method with direct-feedback noise cancellation for MCG measurements and does not require the MSR. The FQC method is used to expand the dynamic range of the SQUID magnetometers to enable MCG measurements outside the MSR in the presence of large magnetic noise. The noise cancellation system uses two SQUID sensors; one for sensing and another one for reference. Both the sensing and reference signals are fed back to the sensing SQUID. This makes it possible to reduce the magnetic noise. We demonstrated the dynamic range, noise spectrum of the developed SQUID system, and MCG waveforms measured outside the MSR when the noise cancellation was set to ON or OFF. The noise cancellation factor of the developed SQUID system ranged from 10 to 20 dB. In MCG waveforms with noise cancellation, the QRS complex and T wave were clearly observed. The results show that the developed SQUID system clearly measures MCG waveforms in a condition involving large environmental magnetic noise.

Gradiometers based on superconducting quantum interference device for nondestructive testing

A prototype of gradiometer for detection and analysis of magnetic signals that are generated by defects in metal structures and materials in the presence of external magnetic bias is based on dc-current superconducting quantum interference device (SQUID). A prototype of single-channel SQUID gradiometer that contains a fiberglass nonmagnetic cryostat, measurement probe with the SQUID sensor and magnetic flux transformer (second-order axial gradiometer), SQUID electronics, and software for control of SQUID gradiometer is studied. The prototype exhibits stable operation under laboratory conditions in the absence of additional magnetic shielding. Upgrade of the SQUID sensors and remaining elements of the prototype of magnetometer is planned for application in nondestructive testing.