Whole shaft visibility and mechanical performance for active MR catheters using copper-nitinol braided polymer tubes

Abstract

Background: Catheter visualization and tracking remains a challenge in interventional MR.Active guidewires can be made conspicuous in "profile" along their whole shaft exploiting metallic core wire and hypotube components that are intrinsic to their mechanical performance. Polymer-based catheters, on the other hand, offer no conductive medium to carry radio frequency waves. We developed a new "active" catheter design for interventional MR with mechanical performance resembling braided X-ray devices. Our 75 cm long hybrid catheter shaft incorporates a wire lattice in a polymer matrix, and contains three distal loop coils in a flexible and torquable 7Fr device. We explored the impact of braid material designs on radiofrequency and mechanical performance.

Results: The incorporation of copper wire into in a superelastic nitinol braided loopless antenna allowed good visualization of the whole shaft (70 cm) in vitro and in vivo in swine during real-time MR with 1.5 T scanner. Additional distal tip coils enhanced tip visibility. Increasing the copper:nitinol ratio in braiding configurations improved flexibility at the expense of torquability. We found a 16-wire braid of 1:1 copper:nitinol to have the optimum balance of mechanical (trackability, flexibility, torquability) and antenna (signal attenuation) properties. With this configuration, the temperature increase remained less than 2 degrees C during real-time MR within 10 cm horizontal from the isocenter. The design was conspicuous in vitro and in vivo.

Conclusion: We have engineered a new loopless antenna configuration that imparts interventional MR catheters with satisfactory mechanical and imaging characteristics. This compact loopless antenna design can be generalized to visualize the whole shaft of any general-purpose polymer catheter to perform safe interventional procedures.

Figures

Figure 1

Braiding setup and catheter shaft structure. (a) Copper and nitinol braiding layer (arrow) exposed after outer Pebax layer is removed. (b) Four grooves (arrows) are created in the inner polymer shaft to house transmission lines. (c) Schematic of a finished 7 Fr guide catheter with a 0.035" guidewire compatible lumen. (1) Three solenoid coils incorporated in the distal shaft for tip profiling (2) Inner nitinol-copper braiding layer used as a core conductor of the loopless antenna (3) Outer braiding layer used as a shield for the loopless antenna. (4) micro-miniature connectors (MMCX).

Figure 2

(a) Flexibility test setup and result. The catheter distal tip was fixed perpendicular to force meter that was mounted on a motorized stage. (b) The schematic representation of tip flexibility measurement setup. (c) Distal tip flexibility for different braiding layer configurations. Resistance force at the catheter distal tip was measured while the shaft was bent from 5 cm away until the tip reaches 1.5 cm vertical displacement.

Figure 3

Torque transmission test fixture setup and test result. (a) The distal shaft is fixed to a freely-rotating collet (arrow) that indicates the angular response to torque. (b) A meter on the proximal shaft recorded transmitted torque after 90–270° tip rotation. (c) Comparative torquability values for different braiding layer configurations. Higher torque response values indicate greater torquability.

Figure 4

The active catheter prototype. (a) The atraumatic distal tip of the catheter (arrow). (b) Three channel active catheter with matching circuit boxes. (c) Schematic of the loop antenna matching/tuning and decoupling circuit a) MMCX (micro BNC) connector b) DC block section c) matching/tuning section d) decoupling section e) BNC connector. (d) Schematic of the loopless antenna matching/tuning and decoupling circuit a) MMCX connector b) DC block section c) matching/tuning section d) decoupling section e) BNC connector.

Figure 5

in vitro and in vivo visibility performance. (a) Phantom MR image acquired with three channel catheter. (b) The active catheter was inserted percutaneously from the femoral artery, through the aorta, and into the left subclavian artery. (c) Multi-slice volume-rendered real-time MR of procedure described in panel (b) depicting the anatomic context. Device-related signal is evident in all slices. Independent catheter receiver channels allow colorized reconstruction of the tip (first (arrow) and third coil, green; middle coil, red; catheter shaft, blue).

Figure 6

Signal-to-noise ratio profile of the active laser catheter is mapped in normalized SNR units.

Figure 7

The heating test results. (a) maximum temperature rise versus insertion length graph when the active catheter placed to the isocenter. (b) maximum temperature rise measurements for different horizontal offset position relative to isocenter.