A medical device includes an insertion tube, having a proximal end and a distal end, which is configured for insertion into a body cavity of a subject. An optical assembly is contained in the distal end and configured to form an image of a target region in the body cavity. A working channel passes through the insertion tube and is configured to convey a fluid from the proximal to the distal end. The working channel has a segment adjacent to the distal end that is narrowed so as to cause the fluid to exit the working channel into the target region in a diverging cone.
