so dimensioned in axial direction that in a plane through the axis they do show directivity an excitation device is provided which successively excites groups of adjacently arranged elements and time delays are provided for delaying the transmitted and received pulses for the elements of a group that the differences in travel times are compensated an adder is provided for the summation of echo pulses and a device is provided for visually displaying the part of the examined heart surrounding the catheter application_number US 46854474 A description the catheter shown in fig1 consists of a cylindrical body 1 containing a number of circumferentially arranged piezoelectric crystals or elements 2 which are insulated from each other and resonate in radial direction at the frequency of the ultrasonic waves these crystals have such a length that the sound is substantially confined to a plane perpendicular to the longitudinal axis of the cylindrical body 1 fig2 schematically shows the operation of the device during a first cycle during this cycle the crystals k1 k4 are excited the crystals may be considered to be line source transmitters which emit sound waves in all radial directions the distances from the crystals to a plane v1 positioned perpendicular to the center line of the group of crystals vary namely for the crystals k1 and k4 the distance is longer than for the crystals k2 and k3 when the crystals are excited simultaneously the echo pulses.