Apparatus, including a sheath, consisting of a lumen having a sheath distal end which is configured to be inserted into a human patient. A magnetic structure is fixedly attached to the sheath distal end. The apparatus includes a probe, having a probe distal end which is configured to be inserted through the lumen into the human patient. The probe includes a magnetic transducer which is disposed in the probe distal end and which is configured to generate a signal in response to a magnetic field. The apparatus further includes a processor which is configured to sense a change in the signal due to proximity of the magnetic structure to the transducer.
