Electronic Gradiometer Research Articles

An electronic second-order gradiometer for biomagnetic applications in clinical shielded rooms

A modular 55-channel magnetometer for biomagnetic studies in clinical shielded rooms is described. Seventy-six superconducting quantum interference device (SQUID) magnetometers in three levels are used to electronically form first-order or second-order gradiometer configurations. The magnetometers are completely integrated on 7.2-mm*7.2-mm chips. Additional positive feedback is used to increase the gradient of the V- Phi (voltage-flux) characteristics to dV/d Phi approximately=1 mV/ Phi /sub 0/, and a novel scheme called bias current feedback reduces the bias current sensitivity to theta Phi / theta I/sub b/ approximately=0.02 Phi /sub 0// mu A. This allows one to couple the SQUIDs directly to simplified feedback electronics without helium-temperature impedance-matching circuits or flux modulation techniques. The magnetometer noise level is typically 1.4 fT/ square root Hz at 1 kHz and 3.2 fT/ square root Hz at 1 Hz, measured inside a Pb/Cryopern shield. A seven-channel prototype module has been tested successfully inside a clinical shielded room. A typical first-order gradiometer noise level of 2.2 fT/ square root THz at 1 kHz and 7 fT/ square root Hz at 2 Hz has been achieved. >

A YBa/sub 2/Cu/sub 3/O/sub 7/ thin film SQUID gradiometer for measurements in unshielded space

Low-noise RF superconducting quantum interference device (SQUID) washers fabricated from YBa/sub 2/Cu/sub 3/O/sub 7/ epitaxial thin films have been used to construct a first-order electronic gradiometer operating at 77 K and suitable for biomagnetic measurements. Mechanical adjustment of one SQUID's plane in a gradiometric setup, which consisted of two SQUIDs separated by a vertical 6-cm base, made it possible to attenuate signals due to far DC magnetic field sources by three orders of magnitude. A magnetic field resolution of 280 fT/ square root Hz above 2 Hz was attained through the use of large thin-film and bulk flux focusers. The fine structure of human-heart magnetocardiograms could be recorded in unshielded space. In a shielded room, magnetoencephalograms were obtained. The system was used to obtain new data on the acoustically evoked cortical response.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Magnetoencephalography using high temperature rf SQUIDs

We have developed high-critical-temperature radio-frequency Super conducting QUantum Interference Devices (SQUIDs) with step-edge grain-boundary Josephson junctions and large flux focusers. These planar devices were fabricated from epitaxial YBa2Cu3O7 films and operated in the magnetometer and first-order gradiometer configurations while immersed in liquid nitrogen. At the temperature of 77K, we have attained a magnetic field resolution for the magnetometer better than 200 fT/Hz1/2 down to less than 1 Hz, i.e., over the low signal frequency range important for medical diagnostics. The results to date show a high promise for biomagnetic diagnostics. For the first time, we recorded the evoked responses from human brains using a high-temperature magnetometer and a first-order electronic gradiometer channel simultaneously. These results were obtained in a magnetically shielded room. An improvement in the magnetic field resolution by another order of magnitude is possible and probable.