Magnetocardiography with high-TC Gradiometers Working in Unshielded Environments

The noninvasive measurement of magnetic heart activity based on high-T_C DC SQUIDs, could be a tool for investigating cardiac electrophysiological properties, if such a system is able to work in unshielded environments. For the realization of such a system we use thin film planar SQUID gradiometers with bicrystal or step-edge Josephson junctions. Even for a small baseline of approximately 4 mm (limited by substrate dimensions), the field gradent resolution is in the range required for clinical analysis. We use a portable system which consists of a glass fiber cryostat with a measuring unit where we can place up to four sensors, different read-out electronics, and signal filtering methods. Firstly, measurements inside a magnetically shielded room were used to show the capability of our magnetocardiography (MCG) system in medical applcations. The system works in an unshielded environment also, without additional field compensation. We demonstrate a measurement of a magnetocardiogram in a real clinical environment, and discuss the possibilties as well as the limitations of this system in magnetocardiography.