Our purpose in this study was to evaluate the steerability of a combined catheter guidewire system with MR tracking in an open-configuration .5T MR system and compare it with fluoroscopic guidance. Experiments were performed with an aorta-shaped glass phantom with different-size branches connected to a roller flow pump to simulate pulsatile flow. A .035" guidewire was used in conjunction with a 5F Cobra 2-shaped catheter. For active MR guidance, a small RF coil was incorporated into the tips of all devices. In addition to fluoroscopic guidance, we used MR tracking devices to selectively catheterize all branches in the .5T open magnet (Signa SP, GEMS). Time requirements for the latter were compared with those needed under conventional angiographic conditions using standard catheters and guidewires. Active MR tracking permitted the simultaneous real time (4 updates/sec) guidance of both guidewire and catheter. Under MR guidance, all branch vessels were successfully catheterized. We found no significant differences in time requirements between guidance with the MR tracking device and fluoroscopic guidance. However, cannulation under fluoroscopic guidance with standard angiography devices required significantly less time (P < .05). Selective catheterization of small branch vessels is possible with active MR tracking of a combined catheter/guidewire system. Limitations of MR tracking device material and design result in a considerable lengthening of the cannulation procedures.