Construction and tests of a heart scanner based on superconducting sensorscooled by small stirling cryocoolers

At the University of Twente, a heart scanner has been designed and construc ted that uses superconducting devices (superconducting quantum interference devices (SQUIDs)) to measure the magnetic field of the heart. A key featur e is the elimination of liquid cryogens by incorporating cryocoolers. In th e design, two coolers are operated in counter-phase to reduce the mechanica l interference. In addition to the application of ferromagnetic shields aro und the compressors, the magnetic cooler interference is reduced by placing the SQUID magnetometers coplanar with respect to the coolers. In this way, the cooler noise was reduced to a level below the intrinsic sensor noise: 0.16 pT/root Hz. A temperature of 60 K was realised with a cool-down time o f about 2 h. The corresponding heat load to the coolers is roughly 0.9 W. M agnetocardiograms were recorded inside a magnetically shielded room. (C) 20 01 Elsevier Science Ltd. All rights reserved.