High-TcSQUID gradiometer system for magnetocardiography in an unshielded environment (original) (raw)
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Applied Physics Letters, 2000
By employing high-temperature superconducting quantum interference device ͑SQUID͒ magnetometers, we have assembled a second-order gradiometer for magnetocardiography ͑MCG͒ in unshielded environment. With this high-temperature superconductor ͑HTS͒ SQUID system, we demonstrated its diagnostic relevance for MCG in terms of signal-to-noise ratio, spatial resolution, frequency bandwidth, rejection of environmental disturbances, and long-term stability. The electronically balanced gradiometer consists of three HTS radio-frequency SQUIDs with superconducting coplanar resonators, mounted in axial gradiometric arrangement with a baseline of 7.5 cm. The system achieves a common mode rejection for axial homogeneous fields of about 10 4 without any mechanical balancing, and a white noise about 130 fT/ͱHz at 77 K, with an 8ϫ8 mm 2 flux pickup area. MCG maps above volunteers' chests have been recorded in unshielded environment in a bandwidth of about 130 Hz. We showed the influence of several notch filters ͑suppressing the power line frequency͒ on the quality of the MCG signals.
First-order SQUID gradiometer with electronic subtraction for magnetocardiography
The paper presents theoretical and experimental results of the research concerning the design and properties of a 1st-order SQUID gradiometer with electronic subtraction and the recording of the first magnetocardiogram (MCG) performed in Romania, at the Bioelectromagnetism Laboratory of the Faculty of Biomedical Engineering. The MCG recording was possible by using a complex installation composed from a non ferromagnetic shielded room placed in the centre of the large (4 × 4 × 4) m, triaxial Helmholtz coils system. The Helmholtz system ensures the static compensation of the geomagnetic field and the active shielding too, by using the magnetic field negative feedback. Magnetocardiography (MCG) is a noninvasive and risk-free technique for contactless surface mapping of the magnetic fields generated by the electrical activity of the heart. The difficulty of recording of a magnetocardiogram is determined by the very low value of biomagnetic field (10-15-10-12)T. Even the strongest of suc...
Physica C: Superconductivity, 2008
We fabricated a magnetocardiogram (MCG) system having 64 second-order wire-wound gradiometers for operation inside a thin magnetically shielded room (MSR). In order to provide good quality MCG signals in noisy environments using a thin economic shielded room, we used second-order gradiometers with a baseline of 70 mm. In order to reduce the liquid level necessary to cool the sensor into superconducting state, the total length of the gradiometer was reduced to 160 mm by using a special superconducting connection method between pickup coil and input coil. The SQUID sensor is the low-temperature double relaxation oscillation SQUID (DROS) having large flux-to-voltage transfers to simplify the readout electronics. The coverage area of the 64-channel sensor array is about 256 mm. The MSR is consisted of 1 layer of 1.4-mm thick Permalloy and 1 layer of 12-mm thick Al. The thin MSR has shielding factors of about 17 at 0.1 Hz and 300 at 10 Hz. When the second-order gradiometer system was operated inside the thin MSR, the average noise level is about 7 fT/ p Hz at 200 Hz.
Measurement of MCG in Unshielded Environment Using a Second-Order SQUID Gradiometer
IEEE Transactions on Magnetics, 2000
In this paper, we fabricated a low-T C second-order superconducting quantum interference device gradiometer (SQUID) to measure magnetocardiography (MCG) in unshielded environment. The second-order gradiometer consists of the pickup coil and the SQUID. The pickup coil is formed with two single-turn coils and one double-turn coil. The three coils are connected in order of single-, double-, and single-turn coil. The coupling polarity of two single coils is opposite to the double-turn coil. The SQUID is based on double relaxation oscillation SQUID (DROS), which consists of a hysteretic signal SQUID, a reference junction, and shunted a relaxation circuit with a resistor and an inductor. The DROS has ten times larger flux-to-voltage transfer coefficient ( 1 mV/8 0 ) than that of the conventional SQUID. Therefore, DROS could be operated with a simple flux-locked loop circuit. The pickup coil and the SQUID were fabricated on the independent wafers and connected superconductively using an Nb wire. The overall size of the second-order gradiometer is 94 12 mm 2 with a baseline of 35 mm. The average field noise was about 8 fT Hz at 100 Hz with the second-order gradiometer in shielded or unshielded environment. The noise level is low enough to measure MCG signals in the unshielded environment. Finally, we measure the MCG in shielded and unshielded environments by using the second-order gradiometer and compared the signal characteristics measured in both environments.
IEEE Transactions on Instrumentation and Measurement, 2000
We developed an optical pumping magnetometer system for magnetocardiography (MCG). The basic system requirements for measuring MCG are as follows: a 5-µT weak bias field in a magnetically shielded room, a 20-mm cell for spatial resolution, and a 30-Hz bandwidth for heart signals. In order to reduce the environmental magnetic noise, a double-cell gradiometer configuration has been adopted. The measured noise level was ∼10 pT/ √ Hz. By switching current flows through a symmetrical three-axis rectangular Helmholtz coil set, the system is designed to measure three vector components of the magnetic field gradient. We show that this configuration reduces the error in current dipole localization with respect to the usual normal component detection by computer simulation of confidence-region estimation. Based on the simulation, we suggest that the system is applicable for diagnosing myocardial infarction on the anterior wall of a human heart in coronary artery disease.
Multi-Channel HTS rf SQUID Gradiometer System Recording Fetal and Adult Magnetocardiograms
IEEE Transactions on Appiled Superconductivity, 2005
We report on experiments with a multi-channel HTS radio-frequency (rf) SQUID gradiometer for recording fetal and adult magnetocardiograms. Four sensing SQUID magnetometers and two common reference SQUID magnetometers form a 4 channel electronic gradiometer system of either first or second order. The magnetometers consist of HTS step edge SQUIDs and flux concentrators fabricated from YBaCuO thin films, with dielectric substrate resonators serving as tank circuits. With a washer area of 18 mm in diameter, all six magnetometers reached a field sensitivity of 20-30 fT Hz. Each gradiometer channel is formed using two or three such magnetometers with individual readouts in electronic difference. The dc and rf crosstalk between any channel pair was measured. In ordinary operation we did not find any noise contribution from neighboring channels, even though the resonant frequencies of the resonators are closely spaced. In a standard magnetically shielded room, using a first-order gradiometer configuration with an ultra-long baseline of about 20 cm, we demonstrated 4 channel real-time heart signal recordings of a fetus in the 33rd week of gestation.
Adaptive frequency dependent gradiometry applied to SQUID magnetocardiography
IEEE Transactions on Appiled Superconductivity, 2003
In the case of magnetocardiography (MCG), gradiometry is a commonly used method to reduce environmental noise. Here, the balance of the gradiometer is one of the main parameters determining the achievable noise reduction. In earlier work, we reported the use of frequency dependent gradiometer coefficients to effectively balance a gradiometer consisting of signal and reference sensors. In this work, the advantages of adaptive determination of these coefficients during a measurement without perturbing the MCG signal are investigated. MCG measurements were recorded with first and second order high temperature SQUID gradiometers inside and outside magnetic shielding. It will be shown that the heart signal is not distorted by applying our software noise reduction technique. Furthermore, by recalculation of gradiometer coefficients during the MCG measurement, an improved signal-to-noise ratio is achieved. The optimum repetition time for coefficient recalculation is determined.
Japanese Journal of Applied Physics, 2002
Generalized harmonic analysis was applied to a high-temperature superconducting quantum interference device (SQUID) magnetocardiograph signal processing system. The noise frequencies in the signal waveform were calculated by the generalized harmonic analysis (GHA) and the noise components with these frequencies were subtracted from the acquired signal data. Two types of the subtraction procedure were demonstrated: one using the adaptive filtering algorithm and the other a simple subtraction. In both cases, the noise components were reduced. The former system performed rapid calculations but generated an artificial noise in the high-frequency region, because of rough GHA estimation, while in the latter case, the calculation was time-consuming but the noise estimation was carried out accurately, because of the precision of GHA procedure. In the present study, a distinctive noise component at 16 Hz was completely suppressed by the GHA prediction noise reduction process. The proposed sy...