A probe assembly for use in a surgical procedure is described. The probe assembly can include a sensor which can be implanted in a patient responsive to external electromagnetic fields for generating position data, an insertion tool for the sensor, which can be used to manipulate the sensor and to deliver it to the desired implantation site and electronic circuitry configured to handle a signal which can interact with the sensor. The electronic circuitry and the insertion tool can have mating formations by which they can be assembled together. The electronic circuitry can be a field generator for generating an electromagnetic field for the sensor.