Development of a biomagnetic measurement system for brain research

The tail of the dewar of the authors´ system has a special shape that makes it possible to measure the brain activity of anyone lying on a bed regardless of head shape or size. It allows the parietal, one side of the temporal and the occipital regions to be measured simultaneously. In the system, 128 axial first-order gradiometers are mounted at intervals of about 26 mm. The diameter and baseline length of the pickup coil are 14 mm and 50 mm, respectively. The authors use square-shaped double-washer DC-SQUIDs with additional positive feedback to improve the flux resolution. System reliability is improved by hermetically sealing the SQUID in a ceramic package and including a heater resistor to eliminate trapped flux. To decrease the liquid helium evaporation rate, output cables are made with flexible printed circuits. The read-out electronics are based on the Direct Offset Integration Technique, which is a highly simplified feedback method. The white noise level measured in a magnetically shielded room is less than 10 fT//spl radic/Hz above 5 Hz. This system has successfully been applied to measuring auditory evoked fields.<>