introduced into a blood vessel or the like using well known techniques when the portion of the outer sheath 1 provided with the cage 11 has reached a predetermined reference position the cage 11 is extended so as to lock the outer sheath 1 at this position the core 20 carrying the head 21 is then moved relative to the outer sheath 1 by moving within the envelope 10 acoustic signals are emitted by the acoustic transducer 23 arranged on the head 21 the acoustic signals are received by the acoustic sensors 12 on the outer sheath 1 since the sensors 12 are arranged at different positions a difference in the time of flight of a signal emitted by a single source 23 is obtained from this difference a position of the transducer 23 relative to the sensors 12 can be determined since the sensors 12 are locked to a known predetermined reference position by means of the cage 11 the actual position of the head 21 and thus of the measuring tip 22 can be determined it should be noted here that the number of sensors is not limited to two with an increased number of sensors an even higher positional accuracy can be achieved fig2 shows an advantageous modification of the embodiment described before in detail with reference to fig1 the same reference signs denote the same parts as in fig1 so that a repetition of this description can be omitted according to fig2 the head 21 is provided with an additional transducer 25 for emitting acoustic signals with this arrangement the angle of inclination of the head 21 relative to the outer sheath 1 can be determined for this purpose the signals emitted by the transducers 23 and 25 must be distinguishable by the sensors 12 distinction.