Susceptibility probes for detecting surgical markers

A method of manufacturing a probe for sensing a magnetic marker during a surgical procedure, the method comprising:

• mounting at least two first coils and at least one second coil substantially coaxially on a longitudinal axis of the probe, the at least two first coils being one of sense coils or drive coils and the at least one second coil being the other of a drive coil or sense coil, such that the one or more drive coils are connectable to a current source to generate a driving magnetic field through the one or more drive coils, and the one or more sense coils are connectable to a signal processor for processing one or more sense voltages induced in the respective one or more sense coils to generate an output signal;
• the first coils or the second coil being configured or arranged to minimize e-a baseline voltage component of the one or more sense voltages induced in the respective one or more sense coils that is attributable directly to the driving magnetic field or to output to the signal processor separate sense voltages of the one or more sense voltages from respective two or more sense coils of the one or more sense coils to allow the signal processor to process the one or more sense voltages to minimize a baseline voltage component of the one or more sense voltages that is attributable directly to the driving magnetic field;
• whereby the first coils and the second coil are configured and arranged as a gradiometer for measuring a proximity of the magnetic marker to the probe and the output signal represents a distance between the marker and the probe;
• fitting to the probe a baseline voltage taring device comprising a first elongate conductor such that the first conductor defines a conducting path that extends azimuthally but incompletely around the longitudinal axis of the probe juxtaposed with the one or more sense coils, and is connectable to the current source or another current source to generate a balancing magnetic field in a vicinity of the one or more sense coils, such that in use the balancing magnetic field induces a balancing voltage in the one or more sense coils which at least partially offsets the baseline voltage component;
• whereby the one or more sense voltages are at least predominantly attributable to a response field generated by the marker in response to the driving field, which corresponds to the proximity of the marker.

A method of setting up a probe for sensing a magnetic marker for use in surgery, the probe comprising:

• at least two first coils and at least one second coil which are arranged substantially coaxially on a longitudinal axis of the probe as a gradiometer for measuring a proximity of the magnetic marker to the probe;
• the at least two first coils being one of either sense coils or drive coils, the at least one second coil being the other of a drive coil or sense coil;
• the one or more drive coils being adapted for connection to a current source to generate a driving magnetic field, and the one or more sense coils being adapted for connection to a signal processor for processing a sense voltage induced in the one or more sense coils to generate an output signal;
• the first coils or the second coil being configured or arranged to minimize a baseline voltage component of the one or more sense voltages induced in the respective one or more sense coils that is attributable directly to the driving magnetic field or to output to the signal processor separate sense voltages of the one or more sense voltages from respective two or more sense coils of the one or more sense coils to allow the signal processor to process the one or more sense voltages to minimize a baseline voltage component of the one or more sense voltages that is attributable directly to the driving magnetic field;
• a baseline voltage taring device comprising a first elongate conductor which defines a conducting path extending partially around the longitudinal axis of the probe juxtaposed with the one or more sense coils, and is connectable to the current source or another current source to generate a balancing magnetic field in the vicinity of the one or more sense coils;
• the method comprising adjusting an azimuthal length of the conducting path around the longitudinal axis of the probe to control the balancing magnetic field to induce a balancing voltage across the one or more sense coils which at least partially offsets the baseline voltage component.