A catheter system for use during biliary procedures, including a first elongate member carrying a guidewire lumen, and a channel extending longitudinally along the member which provides access to the guidewire lumen and defines two edge surfaces. A second elongate member is also included which is disposed within the guidewire lumen and mates with the cross-sectional profile of the guidewire lumen of the first elongate member, thereby converting a guidewire lumen having a C-shaped design into one having an O-shaped design. The catheter may be used in rapid exchange catheter procedures requiring the use of small diameter guidewires which would otherwise slip out of the channel during use.