Squid Gradiometers in Real Environments

Use of SQUIDs for stationary magnetic field detection (e.g., in MagnetoEncephaloGraphy, MEG) is discussed. Such devices operate in the presence of environmental noise and must be provided with shielding and/or noise cancellation techniques. First, shielded and unshielded environments are characterised and some specific noise sources are examined in detail. Then, gradiometers of various orders and their construction by hardware and software methods are described, and the gradiometer errors (represented by common mode and eddy current vectors) are analysed. Noise cancellation by high-order spatial gradiometers is examined in detail, and the gradiometer performance in shielded and unshielded environments is evaluated experimentally using a whole cortex MEG system. Successful operation of high-order gradiometers in unshielded environments is demonstrated on examples of human MEG.experiments. Adaptive noise cancellation also is examined, and frequency independent and frequency dependent methods are described. The adaptive and gradiometer noise cancellation performances are compared, and it is shown that the adaptive methods are effective only under special circumstances when the noise character is time independent, while the gradiometers are quite universal and work well even when the noise character is changing.