Technique for measuring the ac susceptibility of portions of the human body or other large objects

Measurement of the ac magnetic susceptibility of a large, weakly magnetic object has advantages over dc techniques in an unshielded environment having substantial magnetic fluctuations at low frequencies. We discuss the principles and practical details of ac techniques using both large- and small-diameter field coils with a second-order gradiometer and rf SQUID in a standard biomedical dewar. It is possible to make measurements in which the magnetic field of the susceptibility signal at the gradiometer is as small as 5×10−9 of the applied field, which was about 0.04 mT in these experiments. This sensitivity is sufficient to be of use in determining the susceptibility of tissues inside the human body.