The fabrication of low profile “active” intravascular devices using conventional techniques is not feasible and repeatable for clinical interventional MRI procedures [1,2]. We aimed to introduce a reliable method to fabricate micro inductively coupled RF resonators that maximize the coupled signal strength for intravascular device visualization under MRI. First, we performed Finite Element Method (FEM) simulations of both helical and tilted solenoid designs to perform the computational analysis of their electrical and magnetic characteristics Then, we designed a unique CNC based electro-mechanic system (figure 1) for this fabrication process. Based on FEM simulation results, we fabricated optimized complex resonator geometry on non-planar catheter surfaces using CNC based custom made thin film coating system via conductive ink (Conductive Compounds, NH, USA) to eliminate the need for using rigid analog circuit components such as insulated solenoid wire and capacitor and also lower the overall catheter profile.The received signal signature of both straight and tilted helical coil geometries were studied in details in order to bring out visibility performance of different resonator designs for interventional MRI applications. The mathematical expressions of these helixes were pointed out and their magnetic flux densities were stated via dipole design methodology. The prototypes of both straight and tilted helix coils were fabricated via conductive ink using exact dimensions used in FEM simulations on the catheter shaft surface with ±1 µm tolerance (Figure 2). Inductance values and coil characteristics of fabricated coils were compared with simulation results using Network Analyzer (4395A Network Analyzer, Agilent Technologies, CA). Fabricated coils were tested as matched with convenient capacitors under 1.5 T MR system (Siemens, Erlangen, Germany) in ASTM 2182 gel phantom using GRE sequence (TE/TR =1.8/8.2 ms, flip = 5°, FOV = 300 mm, matrix = 192 x 192, slice thickness = 5 mm) to observe their coupled signal strengths. Additionally simulations of 1.5 Tesla MRI catheter device with double helical coils were performed using the same methodology via HFSS program, and the resonance frequency is found approximately as 63 MHz.

Figure 2 shows fabricated straight and tilted helix coils adhering to simulated dimension. After fabrication, error margin of custom made thin film coating system was measured as ±1 µm.

As it is indicated in Figure 3, the results of simulations of both straight and tilted helical coils are consistent with laboratory measurements. Using the same dimensions and structures both in simulations and fabrications has provided congruent data. For 13 turns tilted helix coil design while simulation result is 914 nH, measured inductance value is 859 nH and for 13 turns straight helix coil design while the simulation result is 583 nH, measured inductance value is 539 nH.

Figure 4 shows the simulation results for magnetic field of straight and helical coils presented. Parallel to directionality measurements, straight helical coils has strong magnetic fields through the normal vector of their curvature, however the reduction of magnetic field due to change of direction is less severe in tilted helical coils than straight helical coils. It is clear that the FEM simulations are reliable for coil simulations in order to express their magnetic and electrical properties.

Additionally simulations of tilted double helix resonator via HFSS program were performed. The resonance frequency was observed around 62.74 MHz, which was 1.06 MHz away from the design frequency that was 63.8 MHz. The imperfect conductive coating thickness may cause such a small variation.

Finally, as can be seen in figure 5, under MRI, coupled signal strength of tilted helix coil is seen brighter than straight helix coil.

We fabricated micro RF resonators successfully over 6 Fr catheter surface using custom made CNC thin film coating system. The proposed solenoid geometries were determined through FEM simulations. The bench top measurements over the resonator prototypes through network analyzer were consistent with FEM simulation results. Also it was shown that the visibility of tilted resonator has less orientation dependency relative to main magmetic field direction compare to straight helix coils under MRI.