Gradiometer Coil Research Articles

Disposable sample holder for high temperature measurements in MPMS superconducting quantum interference device magnetometers

A sample holder for high temperature (300 K<T<800 K) measurements in superconducting quantum interference device magnetometers is presented. It is fabricated using aluminum foil and it is appropriate for samples in either solid or powder form. The holder is homogeneous for the gradiometer coil, and this results in a contribution to the background signal that is below the instrument noise at any field (<10(-9) A m2 at mu(0)H=200 mT). Further it is inexpensive and simple to fabricate, and it can be considered as a disposable sample holder that avoids eventual contamination between different samples.

A 30-Channel SQUID Vector Biomagnetometer System Optimized for Reclining Subjects

We propose an LTS-SQUID biomagnetometer system equipped with vector magnetometers for the purpose of measuring the evoked magnetic field from the spinal cord. In case of measurement around the neck, the number of sensors to be arranged is inevitably limited by the small observation area. To acquire more information on the magnetic field distribution in a limited area, we introduced a vector magnetometer, which has an axial-type gradiometric pick-up coil and two planar-type gradiometric pick-up coils located orthogonal to each other, which detects not only the magnetic field component radial to the body but also the tangential components. The cryostat was designed to put the SQUID sensors close to the back or back of the neck of the subject sitting in a reclining position, so that the subject can relax and hold the position throughout a long-time measurement. The fluctuation caused by the subject's movement can be suppressed. We installed 10 vector magnetometers arranged in a zigzag pattern in the cryostat and successfully detected the biomagnetic signals from the neck of the subject.

A new type of gradiometer for the receiving circuit of magnetic induction tomography (MIT)

Magnetic induction tomography (MIT) is a low-resolution imaging modality which aims at the three-dimensional (3D) reconstruction of the electrical conductivity in objects from alternating magnetic fields. In MIT systems the magnetic field perturbations to be detected are very small when compared to the excitation field (ppm range). The voltage which is induced by the excitation field in the receiver coils must be suppressed for providing sufficient dynamic range. In the past, two very efficient strategies were proposed: adjusted planar gradiometers (PGRAD) and the orientation of a receiver coil with respect to the excitation coil such that the net magnetic flow is zero (zero flow coil, ZFC). In contrast to the PGRAD no voltage is induced in the ZFC by the main field. This is advantageous because two comparatively high voltages in the two gradiometer coils can never be subtracted perfectly, thus leaving a residual voltage which is prone to drift. However, a disadvantage of the ZFC is the higher susceptibility to interferences from far RF sources. In contrast, in the gradiometer such interferences are cancelled to a high degree. We developed a new type of gradiometer (zero flow gradiometer, ZFGRAD) which combines the advantages of ZFC and PGRAD. All three systems were compared with respect to sensitivity and perturbation to signal ratio (PSR) defined as the ratio of the signal change due to a magnetic perturbation field at the carrier frequency and the signal change due to shifting a metallic sphere between two test points. The spatial sensitivity of the three systems was found to be very similar. The PSR of the ZFGRAD was more than 12 times lower than that of the ZFC. Finally, the feasibility of image reconstruction with two arrays of eight excitation coils and eight ZFGRAD, respectively, was shown with a single-step Gauss–Newton reconstructor and simulated measurement data generated for a cylindrical tank with a spherical perturbation. The resulting images show a clear, bright feature at the correct position of the sphere and are comparable to those with PGRAD arrays.

Automated and versatile SQUID magnetometer for the measurement of materials properties at millikelvin temperatures

We present the design and construction of a SQUID-based magnetometer for operation down to temperatures T≃10mK, while retaining the compatibility with the sample holders typically used in commercial SQUID magnetometers. The system is based on a dc-SQUID coupled to a second-order gradiometer. The sample is placed inside the plastic mixing chamber of a dilution refrigerator and is thermalized directly by the He3 flow. To measure the magnetic moment, the sample is moved through the gradiometer coils by lifting the whole dilution refrigerator insert. A home-developed software provides full automation and an easy user interface.

The noise immunity of gradiometer coils for14N NQR land mine detection: Practical limitations

Magnetic resonance measurements in the field, such as for land mine detection using14N nuclear quadrupole resonance (NQR), must deal with large levels of environmental radio-frequency interference. One approach to minimize the level of interference which enters the NQR receiver is the use of a coil with no magnetic dipole moment. Such a coil is, ideally, sensitive only to spatial gradients of the magnetic fields and is referred to as a gradiometer. It is straightforward to estimate the amount of reduction one can expect for an ideal gradiometer. Here it is shown that for14N NQR land mine detection in practice, the ultimate level of interference received can be expected to be significantly greater than what one would expect solely due to these spatial gradients. This is due to the fact that it is quite difficult to construct an ideal gradiometer.

A Superconducting Quantum Interference Device Detection Coil Design with Enhanced Sensitivity for Deep Brain Activation

We propose a new combination coil system to be used in magnetoencephalography (MEG). This system is capable of detecting magnetic signals produced by brain activation located in deep regions (40 mm to 60 mm beneath the head skin) while minimizing brain noises that arise from shallow regions (10 mm to 30 mm beneath the head skin). The system has two coils at each measurement location around the head, a magnetometer coil and a second-order gradiometer coil, which have different dependences of magnetic sensitivity to the distance between the coils and brain activation sources. The magnetic signals detected by one coil multiplied by a weighting coefficient is subtracted from those detected by the other coil so that the magnetic sensitivity for the combination coil is more enhanced for deep sources than for shallow ones. We designed a practical configuration of the combination coil system by optimizing the weight and the base line length of the gradiometer. We improved the signal-to-noise ratio for typical source and noise depths by sixteen times in comparison with a conventional magnetometer coil. By optimizing parameters, we obtained magnetic sensitivity that was 3.6 times higher on average for deep brain signal sources than for shallow brain noise sources.

Adapted eddy current excitation using a dual gradiometric coil system

Adapted eddy current excitation using a dual gradiometric coil system

Three dimensionally configured SQUID vector gradiometers for biomagnetic measurement

We proposed a composite superconducting quantum interference devices (SQUIDs) gradiometer with an axial-type gradiometric pick-up coil and two planar-type gradiometric pick-up coils. Three gradiometers are built into a single module in orthogonal position to each other and the pick-up coils of two planar-type gradiometers are concentric. Therefore, the sensor can detect magnetic fields elicited by action currents in a living body as a quasi three-dimensional vector at a specific point. The outward form of the sensors is identical to our conventional axial-type gradiometric SQUID magnetometer. Therefore, the new sensor can be mounted on our conventional sensor array without large modification. We applied the new sensors to the SQUID magnetometer system for the spinal cord evoked magnetic fields and successfully measured the magnetic fields from the spinal cords from small animals.

An alternating current superconductor susceptometric system to evaluate liver iron overload

An ac susceptometric system to quantify liver iron overload composed of a second order axial gradiometer coil coupled to a rf superconducting quantum interference device detector and a large field coil array is presented. A homogeneous ac magnetizing field with low frequency (7.7 Hz) and low intensity (114 μT) is used. Preliminary measurements over a group of 34 normal individuals and 20 patients with iron overload show the ability of the instrument to perform the measurement and to distinguish normal and pathological individuals. The diamagnetic signature of the surrounding tissues is minimized using a special water bag on the torso. In summary it was shown that with a relatively simple instrumentation it was possible to build a superconducting susceptometer dedicated to quantify in vivo iron concentrations, which is clinically important information in the assessment and management of patients with liver iron overload, mainly those who regularly receive blood transfusion.

Simulation of non-contact measurement of the electrical impedance using an anatomical model.

The measurement of the impedance of biological tissue can be a non-invasive method to find new data of diagnostic relevance. A system for a non-contact measurement of the electrical impedance of biological tissue is presented. The system consists of an excitation coil and two sensing coils, an upper and a lower coil. If the two sensing coils are coupled it can be used as a gradiometer coil. Numerical experiments with focus on the eddy currents in the tissue and on the detection of the small changes of the signal are carried out to calculate the fields, eddy current distributions and induced voltages. Hereby tests with different frequencies of the excitation current and different conductivities of a tissue block are used. Then the homogeneous tissue block is replaced with a fraction of the arm of an anatomical model which contains different tissue classes.

Fabrication of highly balanced directly coupled YBa2Cu3Oy gradiometers and their noise properties in an unshielded environment

We fabricated directly coupled gradiometers made of YBa2Cu3Oy thin films on bicrystal substrates. The parasitic effective area of the gradiometric pickup coil was found to be comparable with that of the superconducting quantum interference devices (SQUIDs) in each gradiometer. A gradiometer balance of below 10−3 was achieved by selecting the coupling direction of the SQUID so as to cancel the parasitic effective area of the pickup coil by the effective area of the SQUID itself. The noise of the gradiometers measured in an unshielded environment decreased in proportion to the improvement in the gradiometer balance; however, the noise of the gradiometer with a balance of 0.03% was limited by the gradient-field component of the environmental noise.

Automation of SQUlD nondestructive evaluation of steel plates by neural networks

This paper presents a method for automation of SQUID nondestructive evaluation (NDE) using neural networks, exempting the need for a trained technician, necessary to most of the usual NDE methods. An LTS-SQUID susceptometer, with a 16 mm diameter planar concentric gradiometer, was used to image flaws in steel samples from the bottom of an oil storage tank. Natural and artificial corrosion pits of various sizes were present in the samples, and a vertical magnetic field of 0.5 mT was applied by a superconducting magnet concentric with the gradiometer coils. A finite element model was used to simulate the magnetic signals due to the flaws, yielding training sets for the artificial neural networks. A neural system composed of two cascaded networks was developed to preprocess and analyze the magnetic signals. The first network removes a distortion that occurs in the experimental magnetic signal, and the second network detects the presence of flaws, and also assesses their severity. The trained neural networks were successfully tested with the experimental data obtained with the SQUID system.

Gradiometer pick-up coil design for a low field SQUID-MRI system.

We describe the use of liquid helium-cooled (4.2 K) gradiometer coils and a DC superconducting quantum interference device (SQUID) preamplifier to improve the SNR of magnetic resonance imaging (MRI) at 0.01 T. Gradiometer windings are used both to reduce lossy interactions with the MRI system's room temperature magnet and gradient coils and also to reject interference from more distant sources, which reduces the need for RF shielding. We have tested both axial and planar (figure-of-eight) gradiometer configurations. The figure-of-eight gradiometer has a more rapid fall-off in sensitivity with increasing distance from its windings than the axial gradiometer, but this is compensated for by reduced lossy interactions and improved interference rejection. We have used the system to image the human arm.

Theoretical sensitivity limits of the compensation method for magnetization characteristics measurement of HT c superconducting materials using LT c SQUID 2nd order gradiometer

This article deals with theoretical analysis of a fast compensation method of measuring magnetization characteristics using superconductive quantum 2nd order gradiometer suitable for measuring material samples with low magnetic susceptibility in an alternating low-frequency magnetic field. Special attention is paid to measuring magnetization characteristics of HT c superconductors at a temperature of 78K. Transmission and noise properties of the measuring system consisting of LT c SQUID 2nd order gradiometer and magnetization coil system with the sample of HT c superconductive material placed in liquid nitrogen in an external case under an LHe cryostat of a gradiometer are studied. The maximum sensitivity of the system given by noise properties of a superconductive quantum gradiometer, noise parameters of the magnetization magnetic field and environmental noise in the measuring environment is determined. It follows from a theoretical analysis that the spectral density limit of an input equivalent noise magnetic moment of a system placed in a magnetically shielded room with parameters achieved in rooms designed for measuring biomagnetic fields can reach the order of 10 −13 Am 2 Hz −1/2. Under conditions typical of unshielded magnetic laboratories for measuring weak magnetic fields the order of (10 −8÷10 −11) Am 2 Hz −1/2 can be achieved theoretically. The measuring process is outlined and parameters achieved with systems in an unshielded environment and examples of measured characteristics of HT c superconductors in an alternating magnetization magnetic field with the frequency 0.1 Hz are cited. The accessibility of a sample for manipulation, general arrangement and a simple measuring process allow quick scanning and recording of magnetization characteristics (a set of five amplitude phased hysteretic curves in a time of ∼2.5 min).

Surface and Gradiometer Coils near a Conducting Body: The Lift-off Effect

The use of surface coils in magnetic resonance is widespread. Examples include MRI, detection of subsurface aquifers by NMR, and, more recently, landmine detection by nuclear quadrupole resonance. In many of these cases a finite-sized sample to be examined is contained within a larger medium that is a poor electrical conductor, and eddy currents induced by the RF fields provide a loss mechanism that reduces the effective quality factorQof the transmitter and receiver coils. Here the losses induced in a circular surface coil (a horizontal loop antenna) separated a distance from a dissipative medium are calculated and compared to measurements. It is shown that often the overall efficiency of the coil for magnetic resonance can be improved by displacing the coil away from the conducting medium a prescribed “lift-off” distance. The use of a gradiometer as a surface coil is also examined, and it is shown by theory and experiment that in certain circumstances such a gradiometer can be more efficient than a conventional surface coil for inspection of conducting media.

Combined mapping of human auditory EEG and MEG responses

Auditory electric and magnetic P50(m), N1(m) and MMN(m) responses to standard, deviant and novel sounds were studied by recording brain electrical activity with 25 EEG electrodes simultaneously with the corresponding magnetic signals measured with 122 MEG gradiometer coils. The sources of these responses were located on the basis of the MEG responses; all were found to be in the supratemporal plane. The goal of the present paper was to investigate to what degree the source locations and orientations determined from the magnetic data account for the measured EEG signals. It was found that the electric P50, N1 and MMN responses can to a considerable degree be explained by the sources of the corresponding magnetic responses. In addition, source-current components not detectable by MEG were shown to contribute to the measured EEG signals.

MCG simulations with a realistic heart-torso model.

Magnetocardiograms (MCG's) simulated with a high-resolution heart-torso model of an adult subject were compared with measured MCG's acquired from the same individual. An exact match of the measured and simulated MCG's was not found due to the uncertainties in tissue conductivities and cardiac source positions. However, general features of the measured MCG's were reasonably represented by the simulated data for most, but not all of the channels. This suggests that the model accounts for the most important mechanisms underlying the genesis of MCG's and may be useful for cardiac magnetic field modeling under normal and diseased states. MCG's were simulated with a realistic finite-element heart-torso model constructed from segmented magnetic resonance images with 19 different tissue types identified. A finite-element model was developed from the segmented images. The model consists of 2.51 million brick-shaped elements and 2.58 million nodes, and has a voxel resolution of 1.56 x 1.56 x 3 mm. Current distributions inside the torso and the magnetic fields and MCG's at the gradiometer coil locations were computed. MCG's were measured with a Philips twin Dewar first-order gradiometer SQUID-system consisting of 31 channels in one tank and 19 channels in the other.

Integrated low-temperature superconductor SQUID gradiometers for nondestructive evaluation

We describe an eddy current nondestructive evaluation system using a low-temperature superconductor magnetic field sensor in an electromagnetically unshielded environment. The sensor comprises a niobium dc superconducting quantum interference device (SQUID) integrated with a first-order gradiometric pickup coil to reject spatially uniform interference fields but remain sensitive to flaw induced fields. We demonstrate its use in locating and mapping subsurface flaws in a multilayered aluminum structure using an eddy current excitation field at a frequency of 70 Hz, with no magnetic shielding around the specimen or cryostat.

Fetal magnetocardiography: Methods for rapid data reduction

Fetal magnetocardigraphy (fMCG) provides a unique method for noninvasive observations of the fetal heart. Electrical currents generated by excitable tissues within the fetal heart yield measurable external magnetic fields. Measurements are performed with superconducting quantum interference devices inductively coupled to magnetometer or gradiometer coils, and the resulting signals are converted to digital form in the data acquisition system. The measured fields are usually contaminated by fetal and maternal movements (usually respiration), other physiological fields such as skeletal muscle contraction, the maternal cardiac signal, and environmental electromagnetic fields. Sensitivity to relatively distant sources, both physiological and environmental, is substantially reduced by the use of magnetic gradiometers. Other contaminants may be removed by proper signal conditioning which may be automatically applied using “black box” algorithms that are transparent to the user and highly efficient. These procedures can rapidly reduce the complex signal plus noise waveforms to the desired fMCG with minimal operator interference.

Image and sample geometry effects in SQUID magnetometers

This review concerns the interpretation of the signal provided by SQUID magnetometers designed for measurements of magnetization and magnetic susceptibility. ‘‘Image effect’’ refers to the influence of superconducting elements in the vicinity of the sample and detection coil. Several exact and approximate methods are presented for calculation of how the device sensitivity G(r) varies with position r of a unit dipole sample in the presence of a superconducting shielding tube. In particular, the relative decrease of G(r) is approximately given by the third power of the ratio of the detection coil and shield diameters, and the effect of the shield’s finite length is found to be negligible in practical situations. A survey of complex detection coils (Helmholtz, saddle-type, gradiometer, and second derivative coil) includes calculation of the optimum spacing of the Helmholtz pair in the presence of a superconducting cylinder. The image effects due to the properties of the SQUID circuit and the field-dependent effects in high-field SQUID magnetometers are also discussed. Change in the instrument calibration due to sample size, shape, and location is considered for arbitrary samples as well as for specific cases of small, medium, and very long samples. A spherical harmonic expansion of G(r) makes it possible to derive simple formulas describing sample geometry effects for medium-sized samples with regular geometries (thin rod, cylinder, and rectangular parallelepiped). The results are compared to published experimental data.