Kwon-Kyu Yu

64-channel magnetocardiogram system based on double relaxation oscillation SQUID planar gradiometers

We developed a magnetocardiogram (MCG) system measuring tangential field components using planar gradiometers. The SQUID insert has 64 planar gradiometers, of which 32 measure the x-component and the other 32 measure the y-component, where the chest surface is in the x and y plane. The SQUID sensor is a double relaxation oscillation SQUID type with high flux-to-voltage transfers. Thus, a room-temperature dc preamplifier with a modest input voltage noise could be used to detect the SQUID output voltage directly. The pickup coils are first-order planar gradiometers with a baseline of 4 cm, and they were connected to the input coils using a superconductive bonding of Nb wires. The sensor coverage area of the 64 channels is 190 mm in diameter, and the average boil-off rate of liquid He is 3.6 L/d in everyday operation of the SQUID system. The SQUID operation of the 64 channels was done by digital controls and the average noise of the MCG system was 4.6 fT Hz−1/2 at 100 Hz, inside a magnetically shielded room.

Fabrication and magnetocardiography application of the second-order superconducting quantum interference device gradiometer made from a single-layer of YBa2Cu3O7 film

We have fabricated single-layer, second-order high Tc superconducting quantum interference device (SQUID) gradiometers on SrTiO3 substrates and investigated their noise properties and performance in magnetocardiography. The gradiometer consists of three parallel pickup loops that are directly coupled to a step-edge junction SQUID, in such a way that the coupling polarity of two side loops is opposite to that of the center loop. The overall size of the device is 17.6 mm×6 mm with a baseline of 5.8 mm. The measured gradient noises are 0.45 and 0.84 pT/cm2/√Hz at 1 Hz for the shielded and the unshielded cases, respectively, which correspond to equivalent field noises of 150 and 280 fT/√Hz, respectively. In spite of the short baseline of 5.8 mm, the high common-mode rejection ratio of the gradiometer, 103, allowed us to record a magnetocardiogram of a human subject, which demonstrates the feasibility of the design in biomagnetic studies.

A high-sensitivity magnetocardiography system with a divided gradiometer array inside a low boil-off Dewar

We fabricated a low-noise 64-channel first-order axial gradiometer system for measuring magnetocardiography (MCG) signals. The key technical features of the system are the compact structure of the gradiometer, division of the sensor array plate, direct mounting of the sensor plates into the Dewar bottom, reduced neck diameter of the liquid He Dewar, and compact readout electronics. To make the refill interval of liquid He longer, the distance between the compensation coil of the gradiometer and the input coil pads of the superconducting quantum interference device (SQUID) was reduced to 20 mm. By using direct ultrasonic bonding of Nb wires between the pickup coil wires and input coil pads, the superconductive connection structure became simple. The baseline of the first-order gradiometer is 70 mm, a little longer than for typical conventional axial gradiometers, to provide a larger signal amplitude for deep sources. The 64-channel gradiometer array consists of four blocks, and each block is fixed separately onto the bottom of the Dewar. The neck diameter of the He Dewar (192 mm) is smaller than the bottom diameter (280 mm) in which the gradiometers are distributed. The average boil-off rate of the Dewar is 3 l per day when the 64-channel system is in operation every day. Double relaxation oscillation SQUIDs (DROSs) having large flux-to-voltage transfer coefficients were used to operate SQUIDs via compact electronics. The magnetically shielded room (MSR) has a wall thickness of 80 mm, and consists of two layers of permalloy and one layer of aluminum. When the 64-channel system was installed inside the MSR, the field noise level of the system was about 3.5 fTrms Hz−1/2 at 100 Hz. MCG measurements with high signal quality were done successfully using the developed system. In addition to the parameter analysis method, we developed software for the three-dimensional imaging of the myocardial current on a realistic image of the heart based on the anatomical image of the torso.

Two-dimensional NMR spectroscopy of 13C methanol at less than 5 μT

Two-dimensional (2D) spectroscopy is one of the most significant applications of nuclear magnetic resonance (NMR). Here, we demonstrate that the 2D NMR can be performed even at a low magnetic field of less than 5μT, which is ten times less than the Earths magnetic field. The pulses used in the experiment were composed of circularly polarized fields for coherent as well as wideband excitations. Since the excitation band covers the entire spectral range, the simplest two-pulse sequence delivered the full 2D spectrum. At 5μT, methanol with (13)C enriched up to 99% belongs to a strongly coupled regime, and thus its 2D spectrum exhibits complicated spectral correlations, which can be exploited as a fingerprint in chemical analysis. In addition, we show that, with compressive sensing, the acquisition of the 2D spectrum can be accelerated to take only 45% of the overall duration.

Strong pulsed excitations using circularly polarized fields for ultra-low field NMR

A pulse, which is produced by a single coil and thereby has a linear polarization, cannot coherently drive nuclear spins if the pulse is stronger than the static field B0. The inaccuracy of the pulse, which arises from the failure of the rotating wave approximation, has been an obstacle in adopting multiple pulse techniques in ultra-low field NMR where B0 is less than a few μT. Here, we show that such a limitation can be overcome by applying pulses of circular polarization using two orthogonal coils. The sinusoidal nutation of the nuclear spins was experimentally obtained, which indicates that coherent and precise controls of the nuclear spins can be achieved with circularly polarized pulses. Additional demonstration of the Carl-Purcell-Meiboom-Gill sequence verifies the feasibility of adopting multiple pulse sequences to ultra-low field NMR studies.

Toward a brain functional connectivity mapping modality by simultaneous imaging of coherent brainwaves

Matching the proton-magnetic-resonance frequency to the frequency of a periodic neural oscillation (e.g., alpha or gamma band waves) by magnetic resonance imaging techniques, enables direct visualization of brain functional connectivity. Functional connectivity has been studied by analyzing the correlation between coherent neural oscillations in different areas of the brain. In electro- or magneto-encephalography, coherent source reconstruction in a source-space is very tricky due to power leaking from the correlation among the sources. For this reason, most studies have been limited to sensor-space analyses, which give doubtful results because of volume current mixing. The direct visualization of coherent brain oscillations can circumvent this problem. The feasibility of this idea was demonstrated by conducting phantom experiments with a SQUID-based, micro-Tesla NMR/MRI system. We introduce an experimental trick, an effective step-up of the measurement B-field in a pulse sequence, to decouple the magnetic resonance signal from the strong magneto-encephalographic signal at the same frequency.

High-T/sub C/ SQUID magnetometers for low noise measurements of magnetocardiograms

YBCO dc superconducting quantum interference device (SQUID) magnetometers based on bicrystal junctions have been fabricated for magnetocardiograph measurements. The pickup coil of the device was designed employing 16 parallel loops with 50-/spl mu/m-wide lines. Noise performance was improved by applying a modification in the coupling line between pickup loop and SQUID washer. We could obtain optimized SQUID magnetometer design having magnetic field noise of about 38 fT/Hz/sup 1/2/ at 1 Hz and about 20 fT/Hz/sup 1/2/ at 100 Hz with an 1/f corner frequency of 2 Hz, measured inside a magnetically-shielded room. Demonstration of magnetocardiograph measurements showed quite promising results with high signal-to-noise ratio of 82 with a moderate magnetic shielding.

Comparison of Magnetocardiograms Measured Using Different SQUID Pickup Coil Configuration

We fabricated multichannel magnetocardiogram (MCG) systems of 1 magnetometer type and 2 planar gradiometer types, and compared the MCG data measured using different pickup coil systems. The magnetometer type is a 61-channel magnetometer system measuring vertical component of MCG fields. The magnetometers were integrated magnetometer structure with the pickup coil and SQUID fabricated on the same wafer. The two planar gradiometer systems are 64-channel planar gradiometer systems of first-order with a baseline of 4 cm, and of second-order with a baseline of 5 cm. The planar gradiometer systems are measuring field components tangential to the chest surface. The MCG waveforms and field distributions were measured on the same subject using the 3 MCG systems of different pickup coil types. The measured field distributions and waveforms were transformed into data of other pickup coil types, and the comparison between the measured and transformed results were done.

Optical transmission modules for multi-channel superconducting quantum interference device readouts.

We developed an optical transmission module consisting of 16-channel analog-to-digital converter (ADC), digital-noise filter, and one-line serial transmitter, which transferred Superconducting Quantum Interference Device (SQUID) readout data to a computer by a single optical cable. A 16-channel ADC sent out SQUID readouts data with 32-bit serial data of 8-bit channel and 24-bit voltage data at a sample rate of 1.5 kSample/s. A digital-noise filter suppressed digital noises generated by digital clocks to obtain SQUID modulation as large as possible. One-line serial transmitter reformed 32-bit serial data to the modulated data that contained data and clock, and sent them through a single optical cable. When the optical transmission modules were applied to 152-channel SQUID magnetoencephalography system, this system maintained a field noise level of 3 fT/√Hz @ 100 Hz.

64-Channel Second-Order Planar Gradiometer System for Magnetocardiograms

A 64-channel planar gradiometer system was developed to measure magnetocardiogram (MCG) signals. The planar gradiometers are second-order planar gradiometers with a baseline of 50 mm, fabricated from Nb thin films on Si wafers using thin film processes. The SQUIDs were fabricated from Nb/Al-oxide/Nb junctions, and a double relaxation oscillation SQUID (DROS) scheme was used to have a large flux-to-voltage transfer, typically about 1 mV/Phi0. The planar gradiometers were arranged to measure magnetic field components tangential to the chest surface, which has a field distribution peak just above the current dipole. The diameter of the sensor coverage area is 190 mm, which was proved to be large enough to provide MCG field distribution in a single measurement. Inside a thin-wall shield room, the white noise level of the planar gradiometer system is about 8 fTradicHz. The 64-channel system was operated in a thin-wall shield room, and MCG signals were measured.