A method includes receiving multiple measurements,acquired using a registration tool including anultrasound (US) transducer and a position sensor of aposition-tracking system. The measurements are acquiredby attaching the tool to multiple locations on a patienthead and acquiring position measurements of the positionsensor and respective US measurements of bone tissue atthe locations. First positions of the bone tissue arecalculated based on the position measurement and the USmeasurements obtained using the registration tool. Secondpositions of the bone tissue are identified in ananatomical image of the patient head. The anatomicalimage is registered with a coordinate system of theposition tracking system, by correlating the firstpositions and the second positions, to enable tracking amedical instrument, which is inserted into the patienthead and includes another position sensor of theposition-tracking system, using the anatomical imageregistered with the position-tracking system.
