A 16-channel SQUID-device for biomagnetic investigations of small objects

Biomagnetic investigations in basic physiological research using animals require measurement devices different from commercial biomagnetometers used in human investigations. Two major problems have to be tackled in the design of such biomagnetometers. First, the spatial sampling needs to be much higher. Second, the distance between pick-up coils and the sources needs to be much shorter in order to compensate the worse signal-to-noise ratio (SNR) due to the smaller pick-up coils. We designed and built a 16-channel biomagnetic measurement system meeting these design criteria. The pick-up coil diameter of this new biomagnetometer is 6.7 mm, thus allowing 16 channels on an area of 3.2x3.2 cm2. The pick-up coils are located 3 mm above the dewar outer bottom, hence the closest distance to the cortical surface can be a few millimetres. We provide as an example of first measurements performed with the new biomagnetometer investigations of epileptic spikes in adult rabbits by simultaneous magnetoencephalogram (MEG) and electrocorticogram (ECoG) recordings. The high SNR of the recorded MEG and the simultaneously detected electric potentials allow investigations of the spatio-temporal pattern of neuronal processes of epileptiform spikes with signal strengths of about 3.5 pT.