Apparatus (10) is provided for use with a biologically-compatible-fluid pressure source. The apparatus includes an inflatable guide member (30), configured to be mounted in a vicinity of a small intestinal site, and to form a pressure seal upon inflation. An elongate carrier (24) is configured to be slidably advanced through the guide member into a small intestine (20) of the subject. An imaging capsule (26) is coupled to a distal portion of the carrier. The imaging capsule includes an imaging element (42) and a piston head (40). The piston head forms a pressure seal with a wall of the small intestine and is advanced distally through the small intestine in response to pressure from the fluid pressure source.