122-CHANNEL SQUID INSTRUMENT FOR INVESTIGATING THE MAGNETIC SIGNALS FROM THE HUMAN BRAIN

A 122-channel d.c. SQUID magnetometer with a helmet-shaped detector ar ray covering the subject's head has been operational in the Low Temper ature Laboratory of the Helsinki University of Technology since June 1 992 The new system allows simultaneous recording of magnetic activity all over the head. The probe employs 122 planar first-order thin-film gradiometers in dual units with two exactly orthogonal channels at 61 measurement sites. The performance of the device is analyzed and compa red with more conventional axial gradiometer arrays by considering sig nal-to-noise ratios, spatial sampling theory, confidence intervals for the estimated equivalent current dipole positions, and information-th eoretical channel capacity. The signal-to-noise ratio and the resoluti on of the planar and axial arrays with the same number of channels are found practically equal. The number of channels and their spacing in our new Neuromag-122 system are found fully adequate for neuromagnetic measurements. An example of whole cortex recordings of auditory evoke d brain activity is presented and analyzed.