A catheter comprising a first lumen for receiving a guidewire and having an exit opening at a distal tip of the catheter. A second lumen, adjacent the first lumen and separated from the first lumen by a common wall, the second lumen configured for receiving contrast fluid, wherein perforations are formed in the common wall; a guidewire in the first lumen the guidewire being configured to be capable of advancement distally to extend out of the exit opening, the guidewire having two conditions: a first condition in which the guidewire is advanced distally to block the perforations whereby fluid may not flow from the second lumen to the first lumen; a second condition in which the guidewire is withdrawn proximally to unblock the perforations whereby fluid may flow from the second lumen into the first lumen and thence out of the exit opening.