Apparatus for determining the position of an object within a body of a subject includes at least one acoustic wave generator, adapted to direct a first acoustic wave toward the body at a first frequency. An acoustic tag is adapted to be fixed to the object, the tag including a shell defining a cavity therein and a medium contained within the shell, such that responsive to incidence thereon of the first acoustic wave, the tag emits a second acoustic wave at a second frequency, different from the first frequency. One or more detectors are adapted to detect the second acoustic wave and to generate signals responsive thereto. A signal processor is coupled to process the signals so as to determine coordinates of the object in the body. <；IMAGE>；
