Active shielding to reduce low frequency disturbances in direct current near biomagnetic measurements

Measurements of dc near biomagnetic fields are disturbed by low frequency n oise that is not reduced sufficiently by most of the magnetically shielded rooms or gradiometers. For this reason an active shielding system has been developed at the Biomagnetic Center of the University of Jena. This work de scribes the principle of the active shielding system and demonstrates its p roperties concerning the attenuation of disturbing fields, frequency range, and some applications in biomedical measurements. We achieved a reduction of external low frequency magnetic fields by more than 50 dB and an attenua tion of the field gradient by about 25 dB. This active shielding enables me asurements of near dc biomagnetic fields in investigations of periinfarct d epolarizations after ischemic stroke and spreading depression in migraine p atients. (C) 1999 American Institute of Physics. [S0034-6748(99)01405-7].