When performing biomagnetic measurements, experimentalists are faced with a two-fold problems very weak signals must be measured in an experimental area where there are disturbing fields which are orders of magnitude stronger than those to be detected. Hence, it is necessary not only to use a sufficiently sensitive detector — and SQUIDs are adequate for most tasks — but also to reduce the detected ambient noise below that of the sensor. This goal can be achieved with heavy magnetic shielding: a well-shielded chamber can reduce ambient noise by more than five orders of magnitude at low frequencies (see Chapter 16). On the other hand, a quite satisfactory rejection of disturbing fields can be obtained for many applications by the use of appropriate geometries for the detection coil. In effect, this approach provides what can be called “spatial discrimination,” since it relies on the coil being sensitive to the field from a nearby biomagnetic source and insensitive to the field from distant noise sources.
KeywordsAmbient Noise Spatial Discrimination Pickup Coil Detection Coil Ferromagnetic Object
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