We developed a deflectable guiding catheter that uses Kevlar braiding for the catheter shaft and for the pull string in place of stainless steel or nitinol. The catheter is deflected using a 5 cm nitinol slotted tube and a nitinol spring that work together to provide up to 180° deflection in a single plane. These elements also provide kink resistance during deflection, a constant radius of curvature and restoring force upon release of deflection. The passive design eliminated all long metallic components from the deflectable guiding catheter while preserving the desired mechanical properties. As expected, we did not observe any RF heating greater than 1°C of the passive deflectable guiding catheter and inductive deflectable guiding catheter under any conditions in vitro or in vivo. Using the active receiver coil design, we observed RF heating up to 6°C at the maximum horizontal offset in vitro. The presence of a coaxial cable running along the catheter shaft most likely contributed to the formation of standing waves and eddy currents along the catheter, increasing the potential for heating at the tip coils. However, the heating was acceptable (less than 1.0°C at 15 minutes) under expected use position near the center of the scanner bore in vitro and during in vivo heating tests using all catheter iterations.
We demonstrated three different catheter designs having three different visualization strategies. The passive deflectable guiding catheter — the simplest design — could be used in tandem with an active guidewire to overcome its limited conspicuity. The active deflectable guiding catheter design offered the most versatility and provided the best visibility. Specifically, the active deflectable guiding catheter could be used in tandem with a commercial nitinol guidewire to allow catheter exchange while preserving guidewire access to an anatomic target. This allowed the operator to exchange the active deflectable guiding catheter over the commercial guidewire for another catheter or sheath once access was gained into the anatomical target. The inductive deflectable guiding catheter had enhanced tip visibility compared with the passive design, but was inferior to the active catheter design. The small level of heating observed at maximum horizontal offset in vitro shows that the active deflectable catheter heats more than the other catheter iterations. However, heating was not observed in vitro in the center of the bore or in vivo with the catheter advanced into the inferior vena cava from the femoral vein. Application-specific in vivo heating tests should be conducted on the active deflectable catheter for applications requiring use closes to the edge of the scanner bore.
Karmakar and colleagues created a deflectable catheter incorporating the metallic catheter shaft braiding and metal pull wire into a loopless antenna, connected to de-tuning circuitry at the proximal end to reduce heating during RF transmit (9
). Another group has designed a magnetically-assisted remote control steerable catheter, but having limited deflection ability at certain orientations with respect to the main magnetic field (10
). Other groups have developed catheters with ferromagnetic beads and use pulse sequences to apply MR gradient forces that navigate the catheter tip during scanning (12
); however, the ferromagnetic materials create large artifacts that obscure small vessel branches and make navigation difficult.
Delineating the shape and orientation of the catheter tip is crucial to navigating vessel branches and tortuous anatomy. Other groups have incorporated active tracking coils onto fixed curve catheters in order to test the ability to navigate difficult targets such as vessel branching points (14
). Such catheters are easily visualized under MR but lack control of deflection to actively navigate into anatomical targets.
The primary limitation of our prototype is the large outer diameter. Future iterations will employ lower profile Kevlar thread that will allow a thinner catheter wall, a reduced deflection force necessary, and a lower-profile pulling string. Our prototype pullstring breaks at twice the maximum required deflection force. This safety margin appears to suit our main application of target vessel selection, wherein pullstring failure would create little hazard apart from requiring device substitution. Alternative nitinol spring designs might enhance the restoring force of the single-thread Kevlar pull-string deflection mechanism.
In conclusion, we have developed a deflectable guiding catheter that is a significant addition to the interventional MRI catheter armamentarium. Kevlar braiding in the catheter shaft, Kevlar pulling string, and a short (less than 5 cm) nitinol deflection mechanism preserved important mechanical properties while improving the RF safety profile of the catheter. We have shown that this deflectable guiding catheter, using different visualization techniques, has appropriate mechanical characteristics, including range of deflection and torqueability, to access challenging anatomy using guidewires and therapeutic devices. Our deflectable guiding catheter is a versatile tool that can enable a wide range of catheter-based procedures under real-time MRI guidance.