Patent ID: 7414404

Claim:
An apparatus for detecting a metal object beneath a surface of a human or animal body comprises transmitter means for generating a magnetic field in the vicinity of the metal object, thereby to generate a secondary magnetic field, and detection means for detecting the secondary magnetic field, wherein the detection means include a first gradiometer having first and second gradiometer coils wound in opposite polarity to one another in series in a figure-of-eight configuration about a clearance region for providing a first output signal representative of a first direction, y, to the metal object, a second gradiometer having first and second gradiometer coils wound in opposite polarity to one another in series in a figure-of-eight configuration about a clearance region arranged substantially orthogonal to the first gradiometer for providing a second output signal representative of a second direction, x, to the metal object, the second direction being substantially orthogonal to the first direction and in the same plane as the first direction, an outer coil and an inner coil arranged to be substantially concentric with one another and each of which is arranged to provide an output signal representative of secondary magnetic field strength in a third direction, z, which is substantially orthogonal to the plane of the first and second directions, and processing means for comparing the output signal generated by the inner coil with the output signal generated by the outer coil so as to provide an indication of depth, d, of the metal object beneath the surface, the inner and outer coils and the clearance regions of the first and second gradiometers being arranged about a common axis in the third direction to define an access channel within which a device is received, for marking the surface of the body, the apparatus being capable of simultaneously indicating the depth of the metal object beneath the surface of the body and marking the surface there-above.