Catheter assembly in which an imaging transducer is positioned proximally of a treatment device at the distal end of a catheter. Unrestricted flow between a fluid lumen in the catheter and the treatment device is provided by an annular passageway formed between the walls of a marker tube which extends through the transducer and a guide wire tube which passes through the marker tube, and by a plenum chamber which is formed between the distal end of the catheter and the proximal end of the marker tube in communication with the fluid lumen and the flow passageway. The catheter, the treatment device, and the transducer are all constructed as individual modular units which can be assembled together as desired to form the assembly.