Abstract: Apparatus (10) is provided for use with a biologically-compatible-fluid pressure source (16), the apparatus (10) including an elongate carrier (26), adapted to be inserted through a proximal opening (18) of a body lumen (20), and a piston head (30) coupled to a distal portion of the carrier (26). The piston head (30) is adapted to form a pressure seal with a wall of the lumen (20) after the carrier (26) has been inserted into the lumen (20), and to be advanced distally through the body lumen (20) in response to pressure from the fluid pressure source (16). The apparatus (10) is configured to facilitate distal advancement of the piston head (30) by facilitating passage of fluid out of the lumen (20) from a site within the lumen (20) distal to the piston head (30). The apparatus (10) additionally includes an optical system (32), coupled to the carrier (26) in a vicinity of the distal portion, the optical system (32) having distal and proximal ends.