Squid system for meg and low field magnetic resonance

A SQUID magnetometer system was developed for measuring sustained brain activity by magnetoencephalogmphy (DC-MEG) and to record the free precession decay of protons (FPD) of the human brain at very low fields. The SQUID system has a white noise level of about 4 fT/root Hz. To generate the MR signal, two magnetic fields are used: a static polarisation field of a few mT and a static detection field of a few microtesla. To test the spectral resolution of the system, we measured the FPD of protons in distilled water having a spectral line width of about 156 mHz with an instrumental resolution of 2 mHz. The proton resonance line width of the human brain was found to be about 3.0 Hz. Using the same SQUID system we recorded a DC-MEG signal with an amplitude of about 1.5 pT upon motor stimulation. On the basis of these data, we discuss the possibility of detecting a shift of the resonance line due to the superposition of the neuromagnetic field generated by sustained brain activity.