Active noise compensation for multichannel magnetocardiography in an unshielded environment

Dept. of Appl. Phys., Twente Univ., Enschede, Netherlands
IEEE Transactions on Applied Superconductivity (Impact Factor: 1.24). 07/1995; 5(2):2470 - 2473. DOI: 10.1109/77.403091
Source: IEEE Xplore


A multichannel high-Tc-SQUID-based heart scanner for unshielded environments is under development, Outside a magnetically shielded room, sensitive SQUID measurements are possible using gradiometers. However, it is difficult to realize large-baseline gradiometers in high-Tc materials, Therefore, the authors developed two active noise compensation techniques. In the Total Field Compensation technique, a Helmholtz type coil set is placed around the sensors. One magnetometer is used as a zero detector controlling the compensation current through the coil set. For Individual Flux Compensation, the reference signal is sent to the separate SQUIDs (or their flux transformer circuits) to compensate the local environmental noise fluxes, The latter technique was tested on low-Tc rf-SQUID magnetometers, each sensor set to a field resolution SQUID magnetometers, i.e. 0.1 pTRMS/√Hz. The authors were able to suppress the environmental disturbances to such an extent that magnetocardiograms could be recorded in an ordinary environment. Here the two suppression techniques are described and experimental results are presented.

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Available from: Horst Rogalla, Nov 09, 2015
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    ABSTRACT: A multichannel high-Tc dc-SQUID based heart-magnetometer is currently under development in our laboratory. Since this system has to be simple to use, the cooling of the device should be established by means of a turn-key apparatus incorporating a cryocooler. Because of its magnetic interference, the cooler has to be separated from the SQUID unit. Therefore, an interface between the cooler and the SQUIDs is needed. Possibilities are a gas flow system or a conductive strip. A prototype closed-cycle gas flow system has been constructed and tested, in which helium gas is cooled by a Leybold Heraeus RG 210 Gifford-McMahon cryocooler. Then it is transported through a gas line of 2.5 meter length, and after that through a glass-epoxy heat exchanger on which the SQUIDs can be installed. With this system a temperature of 30 K can be established in about 2 hours (depending on the gas flow rate). Based on the results obtained with this configuration, a smaller system was designed incorporating two Signaal Usfa UP 7058 Stirling cryocoolers. Compared to the prototype the dimensions were reduced by roughly a factor 5. In this paper the construction of the prototype system (incorporating a gas flow) is shortly described and some experimental data are presented. However, we mainly focus on the design of the follow-up system based on the two Stirling cryocoolers. We consider different approaches for interfacing the SQUID unit and the coolers, paying attention to thermodynamic aspects as well as magnetic interference.
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    ABSTRACT: A heart scanner based on high-Tc SQUIDs is currently under development at the University of Twente. It is intended to be used in standard clinical environments without a magnetically shielded room. In order to make the application simple to use, the SQUIDs will be cooled by small cryocoolers, thus realizing a turnkey apparatus. The aimed field resolution is 50 fTRMS Hz−12 in a measuring band of 0.1–100 Hz. The mechanical cooler interference is reduced by incorporating two coolers and operating them in counter phase. The magnetic cooler interference is reduced by positioning the coolers and the SQUIDs in a coplanar arrangement, and by separating the SQUIDs from the cold tips with a solid conducting thermal interface. A design is presented in which a temperature of 55 K is expected with a cool-down time of less than 1 h.
    Preview · Article · Mar 1997 · Cryogenics
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    ABSTRACT: A modular, 9-channel high-T<sub>c</sub> SQUID system for magnetocardiography (MCG) was developed and tested in an unshielded environment. Galvanically-coupled magnetometers made from Y-Ba-Cu-O films, with intrinsic white noise levels as low as 70 fT/√Hz, are used as SQUID sensors. In an unshielded environment, a noise level of about 1 pT/√Hz for each channel was achieved using an active noise compensation system. A new digital planar gradiometer is proposed. First magnetocardiograms recorded in an unshielded environment are presented
    No preview · Article · Jul 1997 · IEEE Transactions on Applied Superconductivity
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