A valve-plane-defining catheter includes a hollow flexible sheath having a distal end and defining a distal opening, a guidewire assembly, and three radiopaque valve-nadir markers. The guidewire assembly has a guidewire slidably disposed in the flexible sheath and having a distal end, three wire arms each having a terminating end and a proximal end offset from the terminating end and attached to the distal end of the guidewire to dispose the three wire arms approximately 120 degrees from one another about a circle defined by the terminating end of the three wire arms. The three radiopaque valve-nadir markers disposed respectively at the terminating end of each of the three wire arms, each of the valve-nadir markers being sized to fit within the distal opening.
