Apparatus is provided for use with a biologically-compatible-
fluid pressure source, the apparatus including an elongate carrier, adapted to be inserted through a proximal opening of a body lumen, and a
piston head coupled to a
distal portion of the carrier. The
piston head is adapted to form a pressure seal with a wall of the lumen after the carrier has been inserted into the lumen, and to be advanced distally through the body lumen in response to pressure from the
fluid pressure source. The apparatus is configured to facilitate distal advancement of the
piston head by facilitating passage of fluid out of the lumen from
a site within the lumen distal to the piston head. The apparatus additionally includes an optical
system, coupled to the carrier in a vicinity of the
distal portion, the optical
system having distal and proximal ends. The optical
system includes an
image sensor, positioned at the proximal end of the optical system; an optical member having distal and proximal ends, and shaped so as to define a lateral surface, at least a
distal portion of which is curved, configured to provide omnidirectional lateral viewing; and a convex mirror, coupled to the distal end of the optical member, wherein the optical member and the mirror have respective rotational shapes about a common rotation axis.