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OBJECTIVE. This article reports on the preliminary evaluation of a new technique for guiding intravascular interventional procedures with MR imaging. Active real-time position monitoring of catheters with MR imaging is made possible by incorporating a small RF coil into the tip of the catheter. The purpose of this study was to evaluate the practicability and localizing precision of this MR catheter tracking technique in vitro and in vivo in comparison with fluoroscopy. MATERIALS AND METHODS. Feed cables employing a 0.9-mm-diameter coaxial cable, a 0.5-mm-diameter partially shielded coaxial cable, and a twisted pair cable attaching RF coils at the catheter tip to a coaxial plug at the catheter base were assessed. Further, miniature copper loop RF coils of two, three, and four turns were tested. In vitro validation of MR tracking was achieved by using a phantom consisting of a water-filled harvested segment of human aorta and iliac arteries embedded in gel. Accuracy of catheter placement was compared with MR and fluoroscopy. Subsequently, the MR tracking technique was evaluated in a swine model using a prototype 5-French MR tracking catheter. RESULTS. A fully shielded coaxial cable was found to be crucial for localizing the attached RF coil by means of the tracking technique. The number of coil turns had a lesser impact. Positions of the catheter tip measured with the MR technique and with fluoroscopy correlated well (r > .98), with a 6-mm 95% confidence interval of positional differences. Active real-time tracking of the coil-tipped catheters was achieved both in vitro and in vivo. The 5-French tracking catheter was successfully placed in the splenic and renal arteries of the swine. CONCLUSION. Robust in vivo tracking and accurate placement of catheters equipped with miniature RF coils are possible with MR imaging.