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Development of a High-Tc SQUID-Based Magnetometer System for MCG Measurement
Shinya KURIKI Hiroshi OYAMA Amane HAYASHI Satoru HIRANO Tomoaki WASHIO Mizushi MATSUDA Koichi YOKOSAWA
IEICE TRANSACTIONS on Electronics
Publication Date: 2002/03/01
Print ISSN: 0916-8516
Type of Manuscript: INVITED PAPER (Special Issue on Superconductive Electronics)
Category: SQUIDs and Their Applications
MCG, SQUID, high-Tc superconductor, active compensation, magnetically shielded room,
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We describe here development of a multichannel high-Tc SQUID magnetometer system for measurement of cardiac magnetic fields, aiming at future application of diagnosis of heart diseases. Two types of direct-coupled SQUID magnetometers were fabricated and used: single pickup coil magnetometer having flux dams to suppress the shielding current that would induce flux penetration and the consequent low-frequency noise, and double pickup coil magnetometer having no grain boundary junctions and flux dams on the pickup coil. The superconducting film of both the magnetometers had holes and slots, leaving 5 µm-wide strip lines, to suppress trapping and penetration of magnetic flux vortices in environmental fields. We studied different schemes of active shielding to reinforce the efficiency of field-attenuation of magnetically shielded room (MSR). A feedback-type compensation using a normal detection coil wound around the wall of MSR and a selective cancellation of 50 Hz noise by means of adaptive filter were developed. Such combination of passive and active shielding, based on the use of simple MSR, would be suitable in a practical low-cost magnetometer system for clinical MCG examination. We fabricated a liquid nitrogen cryostat that could contain up to 20 magnetometer-capsules at 4 cm separation in a flat bottom, with a distance of 16 mm between the air and liquid nitrogen. The cryostat was set in a gantry, which had rotational, vertical and horizontal freedoms of movement, in a moderate-shielding MSR that was combined with the developed active shielding. Measurements of MCG were performed for normal subject using eight magnetometers operating simultaneously.