A system for locating magnetic material in a patient, the system comprising: a magnetic probe comprising a probe body having a drive coil and a sense coil; a power module in electrical communication with the magnetic probe to supply current to the drive coil of the magnetic probe; a sense module in electrical communication with the magnetic probe to receive signals from the sense coil of the magnetic probe; and a processing module in electrical communication with the power module and the sense module, wherein the processing module generates a waveform that controls the supply of current from the power module to the magnetic probe; wherein the processing module receives a signal from the sense module that indicates the proximity to the magnetic material, characterized in that: the magnetic probe comprises first and second substantially identically sized coils of wire and a third coil substantially midway between and coaxial with the first and second coils, the first and second coils both being a first one of a drive coil and a sense coil, and the third coil being the other one of a drive coil and a sense coil; the first, second and third coils comprise a smaller coil and a larger coil which are coaxial, the larger coil having a larger radius than the smaller coil, either the smaller or larger coil being said drive coil and the other being said sense coil, the smaller and larger coils being separated by an axial distance such that if the coils are closer together than the axial distance then a magnetic coupling between the smaller and larger coils is reduced as the larger coil expands radially, and if the coils are further apart than the axial distance then the magnetic coupling between the smaller and larger coils increases as the larger coil expands radially; and the magnetic probe is constructed from a material having a coefficient of thermal expansion less than or equal to 10 -6 / °C and a Young's modulus of substantially 50 GPa or more. a magnetic probe comprising a probe body having a drive coil and a sense coil; a power module in electrical communication with the magnetic probe to supply current to the drive coil of the magnetic probe; a sense module in electrical communication with the magnetic probe to receive signals from the sense coil of the magnetic probe; and a processing module in electrical communication with the power module and the sense module, wherein the processing module generates a waveform that controls the supply of current from the power module to the magnetic probe; wherein the processing module receives a signal from the sense module that indicates the proximity to the magnetic material, the magnetic probe comprises first and second substantially identically sized coils of wire and a third coil substantially midway between and coaxial with the first and second coils, the first and second coils both being a first one of a drive coil and a sense coil, and the third coil being the other one of a drive coil and a sense coil; the first, second and third coils comprise a smaller coil and a larger coil which are coaxial, the larger coil having a larger radius than the smaller coil, either the smaller or larger coil being said drive coil and the other being said sense coil, the smaller and larger coils being separated by an axial distance such that if the coils are closer together than the axial distance then a magnetic coupling between the smaller and larger coils is reduced as the larger coil expands radially, and if the coils are further apart than the axial distance then the magnetic coupling between the smaller and larger coils increases as the larger coil expands radially; and the magnetic probe is constructed from a material having a coefficient of thermal expansion less than or equal to 10 -6 / °C and a Young's modulus of substantially 50 GPa or more.