Abstract: A method and apparatus are disclosed for applying a low strength, time varying magnetic field to a selected screening region of a host, and for sensing a responsive magnetic field from a retained ferrous body within the screening region. The excitation field coil is designed to create a region of finite magnetic field within the screening region, and a region of near zero magnetic field. The sensors are placed within the region of near zero magnetic field to minimize the effect of the applied field. The effects of the applied field are further reduced by use of a reference sensor, field compensation coils, and a feedback loop. The magnetic gradient measured by the sensors is processed to locate and characterize the ferrous body.