A SQUID based AC susceptometer with a magnetising field improving the sensitivity profile

Presents an in vivo SQUID susceptometer using a nonhomogeneous magnetising field which is null at the sensing coil and increases towards the patient position with a constant gradient plus a cubic term at large distances. This shape of the magnetising field enhances the signal due to internal organs with respect to the signal due to superficial tissue. A high signal-to-noise ratio is achieved by operating the susceptometer in a lock-in mode with an AC magnetising field. A negative feedback control is used to compensate the signal due to the magnetising field. A rejection of the applied field with a relative residual of 1/spl times/10/sup -7/ is reached. Preliminary measurements have been performed on phantoms of known magnetic susceptibility. A double layer phantom was investigated in order to study the advantage of using a nonhomogeneous field compared to a uniform field. The authors found that, if the nonhomogeneous field was used, the relative error in susceptibility, when considering the sample as homogeneous, was 2.5 times smaller than with the homogeneous field.