An UHF SQUID gradiometer for biomagnetic measurements

Several biomagnetic measurements require sensitivities as high as 10-14 T/√Hz in the frequency band from almost DC to 100 Hz, typically. At these low frequencies SQUID is today the most sensitive known magnetometer. The suppression of uniform external magnetic noise fields can be achieved by gradiometric measurement technique, if the measurement site is properly chosen. Even better suppression can be obtained by a second derivative meter. With a conventional 20 MHz SQUID gradiometer constructed earlier systematic biomagnetic investigations have been performed in an unshielded environment. The sensitivity of this apparatus was not, however, sufficient e.g. for accurate measurement of the foetal magnetocardiograms or of the brain's magnetic field, although both fields could be observed. To enhance the sensitivity a new UHF system was built. (Auth.)