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Los Alamos National Laboratory Research Quarterly, Spring 2003
SQUID Magnetometry
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chart showing measurable brain fields

Just above the skull, the brainŐs magnetic fields can be as small as 10 femtoteslas. Measurable brain fields are produced by an Ňevoked response,Ó that is, the electrical activity produced by the brain in response to stimuli such as sounds or light flashes. To measure brain fields, SQUIDs must be shielded from the ambient magnetic fields of Earth, power lines, and other sources, or the ambient fields must be canceled electronically or by computer programs. Even the steel in a car or a screwdriver has a magnetic effect. Also shown are the magnitudes of other biomagnetic fields. Note that the heartŐs magnetic fields above the chest are typically 100 to 1,000 times stronger than brain fields above the head. Thus, it is much easier to measure heart rhythms than it is to measure brain fields.

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