Apparatus is provided that includes a tissue anchor, which (a) includes a helical tissue-coupling element which has a distal tissue-penetrating tip at a distal end of the tissue anchor, and is configured to be advanced into soft tissue, and (b) is shaped so as to define a longitudinal channel extending to the distal end of the tissue anchor. The apparatus further includes (a) a depth-finding tool, which includes a radiopaque bead, which is shaped so as to define a hole therethrough, and which is positioned within and axially moveable along the channel so as to serve as a marker indicating a depth of penetration of the tissue-coupling element into the soft tissue, and (b) and a shaft that is removably positioned within the channel, such that the shaft passes through the hole of the bead, and such that the bead is axially moveable along the shaft and along the channel.