First-order Gradiometer Research Articles

Cancellation technique of external noise inside a magnetically shielded room used for biomagnetic measurements

First-order gradiometers inside a magnetically shielded room (MSR) were used to cancel magnetic-field noise. However, the magnetic field inside a MSR is distorted when the amount of external noise is large. This distortion is caused by the low-pass filter property of the MSR. Therefore, the time constants of the frequency-dependent attenuation of the MSR vary spatially and this variation must be taken into account. To investigate noise cancellation, we used a multichannel superconducting quantum interference device consisting of four gradiometers measuring a source signal and two gradiometers as a reference. To compensate for the different magnitudes of the gradiometer wave forms, which differed because of slight differences in their pickup-coil cancel rates, we calculated a fitting parameter. The noise-cancellation method consisted of two processes: reduction of ambient noise caused by the differences in the cancel rate of the gradiometers and a gradient magnetic field inside the MSR, and cancellation of wave-form distortion caused by the spatial variation of the time constants inside the MSR. This cancellation method provides additional attenuation of over 20–30 dB in addition to the balance (>46 dB) of a first-order gradiometer. However, the remaining noise, especially a spike (<2 pT) at the beginning of a large ambient noise step, could not be completely canceled. This noise was caused by the slight difference between the time constants at the reference sensor position and at the signal sensor position. Except for this noise spike, however, the noise cancellation enabled clear magnetocardiogram wave forms to be measured without being affected by strong external noise.

Distal-proximal somatotopy in the human hand somatosensory cortex: a reappraisal.

The distal-proximal representation of the finger and palm in the first somatosensory cortex was reexamined. Somatosensory evoked magnetic fields (SEFs) were measured with a 37-channel first-order axial gradiometer system. Sensory stimulus comprising a 20-ms vibration at a frequency of 200 Hz was delivered to five successive sites in 3-cm increments along the distal-proximal direction over the volar surface of the right index finger and palm. Using a single dipole model, the sources and the signal strengths of the main peak (M50) of the SEFs were estimated. All of the sources were located in the 3b area. There were no statistically significant differences between the locations of dipoles evoked by stimulation of different sites. The results support those of our previous study using a 122-channel whole-head planar gradiometer system that orderly distal-proximal representation of the hand, as described in monkeys, is blurred in the adult human somatosensory cortex.

High-temperature single-layer SQUID gradiometers with long baseline and parasitic effective area compensation

First-order HTS SQUID gradiometers were fabricated on 30 × 10 mm2 bicrystal substrates. These devices have a baseline of 13 mm, intrinsic balance levels of ~1/700 and a typical gradient sensitivity at 1 kHz of 79 fT cm-1 Hz-1/2. A two-SQUID coupling scheme is discussed that further enhances the device's ability to reject uniform fields.

Highly balanced long-baseline single-layer high-Tc superconducting quantum interference device gradiometer

We describe a direct-current superconducting quantum interference device (SQUID) first-order gradiometer fabricated from a single layer of YBa2Cu3O7 on a 30×10 mm2 bicrystal substrate. The device has a baseline of 13 mm and an intrinsic balance of ∼10−3. The gradient sensitivity at 77 K and 1 kHz is 50 fT/(cmHz) in magnetic shielding and 260 fT/(cmHz) when operated unshielded in our laboratory. An antiparallel two-SQUID coupling scheme is employed to optimize the device’s balance to at least 3×10−5.

Linearity of sensitive YBa2Cu3O7−x dc superconducting quantum interference device magnetometers

The linearity of sensitive superconducting quantum interference device (SQUID) magnetometers fabricated from high critical temperature (high-Tc) superconductors and operated in a direct-coupled flux-locked loop with bias reversal was investigated. The system nonlinearity was determined by applying a sinusoidal test signal and measuring the output signal harmonics using a setup which ensures the elimination of nonlinearities arising from the test signal source and the spectrum analyzer. The experimental setup enables the simultaneous measurement of the harmonic distortion of a single magnetometer and of a first-order electronic gradiometer formed by two individual magnetometers. Test signal amplitudes and frequencies were chosen to simulate typical laboratory interferences. An analysis of the results was performed using analytical equations in which the SQUID’s inherent nonlinear character is taken into account. At signal frequencies above about 0.1% of the system bandwidth, the system nonlinearity was found to be mainly caused by the nonlinear behavior of the SQUID magnetometers, whereas at low frequencies nonlinearities arising from read-out electronics components predominate. For signal peak-to-peak amplitudes up to 1 μT and frequencies up to 0.5 kHz, total harmonic distortions below about −105 dB for a single sensor channel and −100 dB for an electronic gradiometer were obtained.

Sensitivity of foetal magnetocardiograms versus gestation week.

Foetal magnetocardiograms (FMCGs) were measured using a nine-channel SQUID system equipped with first-order gradiometers (60 mm baseline, 20 mm diameter). The system was installed in a magnetically shielded room in a hospital. The white noise level was less than 10 fT/square root of Hz, and FMCGs above 1 pT were detected. These results and the depth to the foetal heart were used to estimate the current dipole. The relationship between the current dipole (Q) and the gestation week (G) was calculated and the average performance was determined as Q = 18G - 295. By using the estimated foetal current dipole, the measurement limit (average value) between depth and gestation weeks was determined. When the depth from the pickup coil to the foetal heart is 50, 60, 70 and 80 mm, the first detectable gestation weeks of FMCGs above 1 pT measured by a first-order gradiometer with 60 mm baseline were determined at 21, 23, 26, and 30 weeks respectively, and the detectable gestation weeks in the case of a 30 mm baseline were determined at 22, 26, 30 and 36 weeks respectively.

Magnetoencephalogram systems developed at KIT

We have developed new systems for magnetoencephalography (MEG). The pick up coil is a coaxial type first-order gradiometer with 50-mm base line. The magnetic field resolution of the system is about 4 fT/Hz/sup 0.5/ or 0.8 (fT/cm)/Hz/sup 0.5/ in white noise region. The unique feature of the system is the gantry-free horizontal dewar, which is fabricated through what we call ship-in-a-bottle approach. Less than 10-liter/day liquid helium consumption for the 100-liter capacity is realized. One of the merits of the horizontal dewar is that a small room is sufficient for installation because of the low height (0.89 m) of the dewar. Another merit is that the patient can be measured in lying position which is more relaxing compared to the conventional vertical type.

Frequency representation in the human hand somatosensory cortex: a reappraisal.

Frequency organization in the human primary somatosensory cortex (SI) was re-examined. Somatosensory evoked magnetic fields (SEFs) were measured with a 37 channel first-order axial gradiometer system. Sensory stimuli comprising a 20ms vibration at frequencies of 50, 100, 200 and 400 Hz were delivered to the volar surface of the tip of the right index finger. Using a single dipole model, the sources of the first main peak (M50) of SEFs were estimated. All of the sources were located in the 3b area. There were no statistically significant differences between the locations of the dipoles evoked by different frequency stimulations. These results support our previous study using a 122 channel whole head planar gradiometer system that systematic frequency organization at the hand area of SI cortex may not be present in humans.

A low-noise integrated SQUID gradiometer for biomagnetic sensors

We developed a low-noise and compact SQUID gradiometer for biomagnetic multichannel applications. The SQUID is based on a double relaxation oscillation SQUID and provides very large flux-to-voltage transfer coefficients, allowing simple flux-locked loop electronics to be used for SQUID operation. The pickup coil is a first-order planar gradiometer integrated on the same wafer as the SQUID and has a baseline of 40 mm. The field gradient noise of the SQUID gradiometer in flux-locked loop mode is around 1 fT/cm/spl radic/Hz at 100 Hz, measured inside a magnetically shielded room.

A high-temperature rf SQUID system for magnetocardiography

A first-order axial electronic gradiometer having a baseline of 10 cm was constructed by assembling two rf SQUID magnetometers with coplanar tank resonators, each having a white magnetic field resolution of about at 77 K. The gradiometer's near-field resolution was about , including the Dewar flask's noise. A peak-to-peak noise level of 3 pT was obtained in the bandwidth 0.016-250 Hz. Magnetocardiographic (MCG) measurements were performed using this bandwidth. Measurements on human subjects have been conducted in a magnetically shielded room of moderate shielding factor. Using the signal either of the lower magnetometer or of the gradiometer, high-quality heart signal traces could be collected, which were suitable for diagnostic use. A team of physicians, assisted by two of the authors, used the equipment over 10 months to perform MCG measurements in a medical study of about 80 clinical patients with cardiac arrhythmia problems and healthy persons. The system's performance was stable over that whole period.

Serial processing of the somesthetic information revealed by different effects of stimulus rate on the somatosensory-evoked potentials and magnetic fields

In order to evaluate information processing in the somatosensory cortex, the effect of two different stimulus rates was investigated by simultaneously recording somatosensory-evoked potentials (SEPs) and magnetic fields (SEFs) in nine healthy adults. During electric stimulation of the median nerve at the wrist, SEFs were recorded with the helmet-shaped whole-head coverage magnetometer array with 122 first-order planar gradiometers while SEPs were simultaneously recorded from seven scalp positions. Interstimulus intervals (ISIs) of 0.9 s and 4 s were compared. In all subjects, N20 as well as its magnetic counterpart, N20m, was clearly demonstrated over the contralateral somatosensory area. Subsequent deflections around 80–200 ms did not make any clear peak and were smaller than those at 20–60 ms (P30m, P40m, N50m and P60m). After 200 ms, SEFs were negligible, whereas SEPs had larger amplitude than those of shorter latencies, constituting a peak around 250 ms (P250). Both SEF and SEP deflections later than 40 ms were decreased in responses at the shorter ISI; this diminution was most prominent for P250. Therefore, it is concluded that the tangential currents in the somatosensory cortex (area 3b) mainly contribute to responses during the first 200 ms after the stimulus, whereas the radially oriented currents (most likely in the crown of the postcentral gyrus) take over for subsequent information processing.

Non Destructive Testing using the SQUID with Integrated Gradiometer

We have used the integrated SQUID gradiometer in an unshielded environment to make eddy current nondestructive testing measurement on a multi-layer aluminum structure. The sensor consists of a niobium dc superconducting quantum interference device (SQUID) and a first-order gradiometer pick up coil on the same substrate. As a demonstration of their capabilities, subsurface defects in a multilayer aluminum structure have been located and mapped using eddy current with no magnetic shielding around the specimen or cryostat.

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.

Double relaxation oscillation superconducting quantum interference devices with gradiometric layout

Double relaxation oscillation superconducting quantum interference devices (DROSs) with a gradiometric signal SQUID and either a reference SQUID or a reference junction will be presented in this article. The devices are user friendly, particularly those with a reference junction. Because of the large flux-to-voltage transfer of ∂V/∂Φ=0.7–1 mV/Φ0, the devices can be operated in a flux locked loop based on direct voltage readout without loss of sensitivity. The typical white flux noise of the DROSs amounts to √SΦ=5–6μΦ0/√Hz, which corresponds to an energy resolution ε=SΦ/2Lsq≃200 h. Coupled to an external planar first-order gradiometer, a white magnetic field sensitivity of √SB<2 fT/√Hz was measured inside a magnetically shielded room.

First-order gradiometer of high Tc rf SQUID

We developed a first-order gradiometer by subtracting the outputs of two sets of rf SQUIDs with 6 cm base distance between two SQUIDs. When the gradiometer was balanced by only tuning a potentiometer, the common mode rejection ratio (CMRR) was about 640. The flux noise spectrum of gradiometer in laboratory without sheilding was in the same order of magnitude as that of a magnetometer in shielding. The gradient resolution of the gradiometer was 53fT/(√Hz·cm).We used a 0.08–45 Hz bandpass filter and 50 Hz, 100 Hz, 150 Hz notch filters to suppress the environmental noise and the interferences of 50 Hz line frequency and its harmonics. The gradiometer could detect the magnetocardiogram (MCG) in laboratory.

Unshielded use of thin-film Nb dc superconducting quantum interference devices and integrated asymmetric gradiometers for nondestructive evaluation

Novel nondestructive evaluation measurements made using niobium dc superconducting quantum interference devices with integrated asymmetric first-order gradiometers are described. Comparative theoretical and experimental studies of their spatial response have been described, and it is shown that the gradiometric response makes operation possible in an unshielded and electromagnetically noisy environment. As a demonstration of their capabilities, subsurface defects in a multilayer aluminum structure have been located and mapped using induced eddy currents at 70 Hz, with no magnetic shielding around the specimen or cryostat.

A whole-head SQUID system for detecting vector components

Abstract We have developed a whole-head magneto-encephalogram system having 65 detection sites for detecting vector components of the magnetic field of the brain. Each site contains three first-order gradiometers, and the three detection coils of the gradiometers are perpendicular to each other. This configuration enables the radial and tangential components at each measuring site to be calculated. The helmet shaped dewar is held on a stand fixed to the floor inside the MSR (magnetically shielded room), and the tilt angle of the dewar is adjusted to fit the head. A square-double washer DC-SQUID having a large transfer function d V /d Φ (of the order of mV/ Φ 0) is used in the gradiometer. The large d V /d Φ enables the low-noise SQUID to be coupled directly to the pre-amplifier without additional positive feedback (APF). The typical noise of the system in the MSR is less than 10 fT/√Hz. Linearized output from the flux locked loop circuit is fed into the amplifier-filter unit, and digitized by an A/D converter. The brain function data from 195 channels are stored and analyzed by the workstation.

SQUID-susceptometry up to 10Tesla: An improved method for magnetization studies on a two-dimensional electron system

We report on a high-sensitive SQUID-susceptometer for magnetization studies on two-dimensional electron systems (2DES). It consists of a low-noise thin-film dc SQUID and a wire-wound first-order gradiometer and is operated inside a superconducting 10 T-magnet. At zero magnetic field the system's flux noise level is frequency-independent and only 40×10−6Φ0/√Hz, which is almost the noise level of the bare SQUID. In magnetic fields up to 10 T the sensitivity can be better than 10−13 J/T at frequencies above 1 kHz. With this setup, we have studied the de Haas–van Alphen effect on a gated single layer 2DES revealing spin splitting and the enhanced Lande factor.

High-sensitive superconducting magnetometry on a two-dimensional electron gas up to 10 Tesla

We report on new magnetization studies on a two-dimensional electron system (2DES) revealing spin splitting of the Landau levels. For this, we have built a high-sensitive susceptometer consisting of a low-noise thin-film dc superconducting quantum interference device (SQUID) with a multiturn input coil and a wire-wound first-order gradiometer. The system noise level is only 40×10−6 Φ0/√(Hz) down to a frequency of a few Hz in unshielded environment. In background fields up to 10 T, the system exhibits significant low-frequency noise. At frequencies above 1 kHz, however, the SQUID sensitivity is barely affected and we have reached a value of about 10−14 J/T at 1 T and better than 10−13 J/T at 10 T. With this, we have studied the de Haas–van Alphen effect for a tunable 2DES starting from zero carrier density.

Processing and characterization of high T/sub c/ magnetic shields and flux transformers

The use of high T/sub c/ SQUID (HTSQUID) magnetometers for magnetocardiography (MCG) and nondestructive testing (NDT) requires effective shielding from magnetic noise fields. The authors have investigated the shielding characteristics of high-T/sub c/ superconducting (HTS) material and developed a compact, portable system consisting of several HTS components. A process for fabricating these components out of Bi-Pb-Sr-Ca-Cu-O has been developed. Tubes and disks of various sizes have been formed by cold isostatic pressing (CIP) and sintered at reduced oxygen pressures. The HTS tubes exhibit shielding factors of up to 10/sup 3/ at low frequencies. A novel technique for fabricating superconducting flux transformers (specifically, first-order gradiometers) has also been developed. The flux transfer efficiencies of these first-order gradiometers have been evaluated. The tubes, gradiometers, and flux focusers have been incorporated into an HTSQUID setup. The authors find that the low-frequency noise rejection obtained in their setup is very promising for potential MCG and NDT applications in an unshielded environment.