Apparatus for locating a tissue within a body of a subject includes an acoustic tag configured to be fixed to the tissue and adapted, responsive to acoustic waves incident thereon, to return acoustic echoes. Acoustic transducers are placed at respective positions so as to direct the acoustic waves into the body toward the tissue and to receive the acoustic echoes returned from the tag responsive to the acoustic waves, generating first signals responsive to the received echoes. Transducer position sensors are coupled respectively to the acoustic transducers so as to generate second signals indicative of the respective positions of the acoustic transducers in an external frame of reference. A processing unit processes the first signals and the second signals so as to determine coordinates of the acoustic tag in the external frame of reference.