Biomagnetometers for unshielded and well shielded environments.

The paper describes magnetometers for operation in well shielded and unshielded environments. For unshielded environments, the noise cancellation is accomplished by spatial filtering using higher-order gradiometers which are formed in software. The theory of the software formation of high-order gradiometers has been successfully tested experimentally to the second-order gradient level, using a seven-channel prototype system. An unshielded, open environment system, designed to operate with third-order noise cancellation is presently under construction. It is a whole head system, consisting of a minimum of 64 sensing channels, and is expected to be operational by late 1991. In the opposite extreme, a magnetometer is being developed for operation in a well shielded environment of a whole body superconducting shield. The high temperature superconducting shield will operate in liquid nitrogen, while the magnetometer detection system will operate in liquid helium. The high level of shielding will allow detection of biomagnetic signals using magnetometers. The whole body high temperature superconducting shielded system is presently under development and a half scale working shield already exists. The complete whole body shielded system is expected to be operational in 1994/95.