Synopsis

Metal-reinforced catheters have improved steerability and pushability. Currently, long (>wavelength/8) metallic-braided or metallic-tube-reinforced active MRI-compatible devices do not exist, due to risks of heating of surrounding tissue. Metallic-backbone devices may be possible if heating-sources are attenuated. Concentric-tube RF traps were miniaturized (“MBaluns)” by using leaky (loosely-wound) solenoids in-place of the external tube, providing transverse magnetic fields to attenuate (a) surface electric-fields on metals and (b) common-mode propagation on internal-cables. We validated (1) a 1.1mm outer-diameter (OD) active metallic-tube-based guidewire and (2) a 2.6mm OD metallic-braid-based electrophysiology catheter via; Electromagnetic modeling, Network-Analyzer electrical tests, phantom imaging and navigation into swine hearts.

Introduction

Invasive deflectable cardio-vascular devices (sheathes, catheters, guidewires) frequently incorporate metallic tubes/rods or are reinforced with metallic braids. Metals’ mechanical properties provide for reduced kinking, better steerability, improved pushability and torque transmission, allowing for remote manipulation of lengthy (L>1000mm) small external-outer-diameter (OD_E<3mm) structures. However, current MRI-compatible devices contain only short lengths (<λ/8, where λ is wavelength) of metal¹ , reducing body-coil-induced Radio-Frequency (RF) surface electrical (E) currents on the metal, which can heat surrounding tissues. Additionally, switched RF fields and gradients create transient magnetic (M) fields on lengthy metals^1,2 leading to undesirable B₀ inhomogeneity and B₁ hyperintensity. Without metallic support, MRI-compatible catheters are generally larger, or have inferior maneuverability. Passive guidewires can be constructed of bisected metal rods, but are hard to visualize². Active MRI catheters have sensors on their distal shaft (MR-tracking micro-coils, ECG/impedance-tracking electrodes), resulting in multiple electrical cables running up the catheter shaft. Without external (metallic) conductive shielding, the cables receive body-coil-induced common-mode currents, and require individual heat-amelioration solutions³, taking-up considerable space, and possibly attenuating desired (differential-mode) signals.

The goal was development and testing of MRI-compatible Electrophysiology devices with full-length metallic-braid or metallic-tube shafts. Periodic miniature resonant RF-traps (MBaluns) were placed on the metals, intended to suppress surface currents on the metal and common-mode currents on internal cables.

Methods

Methodology: MRI receiver-coil assemblies, enclosing multiple cables, are overlaid with a conductive-shield and periodic “floating” (non-contacting) resonant RF traps (Baluns) which attenuate common-mode currents in the cables and shield. Baluns are constructed of elongated (~10 cm) internal and external concentric copper tubes, separated by a dielectric layer (ε) and shorted at one end, producing a transverse magnetic field (M_t). The Balun’s inductance is resonated at the Larmor frequency with a parallel-capacitor.

Active-Catheter Design: If Baluns are reduced in diameter, reasonable-length concentric tubes cannot generate sufficient inductance. To compensate, we replaced the external tube with a leaky solenoid, where large inter-winding distances (∆), relative to the wire diameter (l), and the large (N>50) number of windings, generate a large M_t and inductance. The internal Balun tube was replaced by a non-ferrous conductive braid, since the braid’s woven structure does not support large surface E currents. The leaky solenoids were soldered to the braid, and resonated with miniature capacitors (MBaluns), attenuating common-mode currents on cables within the braid, and residual E fields on the braid surface. Figure 1 shows finite-difference Electromagnetic simulation of an MBalun placed off-center within and irradiated by a “Bird-Cage” body-coil, showing a large confined M_t field between the two surfaces.

Active-Guidewire Design: For larger form factors (L/OD_E) and larger stiffnesses, a solid internal tube was required. Periodic high-conductivity metal coatings were added to the tube underneath the MBaluns, with surface-wave impedances⁴ Z=(1+j)/δϬ, where δ is the skin depth and Ϭ is the conductivity, which differ from the tube’s, providing for strong surface reflections, and thereby only sustaining higher-frequency fields^4,5.

Devices Constructed: (1) Active guidewire with backbone of OD=0.5mm nitinol tube (Fig. 2) L=1170mm, OD_E=1.1mm, 2 multilayer MR-Tracking coils at the distal end⁶, 48AWG coax. MBalun-design1; 50mm-length/MBalun, N=150, ∆=0.3mm, l=0.1mm, ε thickness=0.2mm,8 MBaluns/guidewire (2) Irrigated Electrophysiology catheter with backbone of OD=1.9mm tinned-copper braid (Fig. 3) L=1000mm, OD_E=2.6mm, 4 MR-Tracking coils⁶ at distal end, 42AWG coax. MBalun-design2; 15mm-length/MBalun, N=50, ∆=0.3mm, l=0.1mm, ε thickness=0.2mm, 9 MBaluns/catheter. MBaluns were resonated at 63.8 MHz with 0.5mm thick capacitors.

Network-Analyzer (NA) RF tests: Signals on coaxial cables; (a) minimal desired-signal (differential-mode) attenuation; (b) large induced-signal (common-mode) attenuation. Currents on metallic braids and tubes; (c) large attenuation of induced signal. Common-mode signals were induced onto cables, braids, and tubes using large transmit-coils. Control tests performed on devices without MBaluns.

MRI large-phantom (150x30cm²) High-SAR tests: Single-shot scans; (a) minimal-TR 90⁰ flip-angle Steady-State-Free-Precession (SSFP); (b) 180⁰ flip-angle (ETL=256) Fast-Spin-Echo. Signal-hyper-intensity around shafts was investigated.

MRI Swine tests: Both devices were navigated into swine right atriums in a Siemens Avanto. Abdominal and spine-arrays were used, along with dedicated MR-tracking receivers. Imaging: Real-time SSFP and ECG-gated high-resolution SSFP.

Results

Network Analyzer: Cable differential-mode signals were not (+0.2dB) attenuated. Common-mode signals on cables were attenuated by (1) 15dB/Balun and (2) 8 dB/Balun, respectively. Surface-waves on metallic-tubes or metallic-braid were attenuated by 15dB/MBalun and 9dB/MBalun, respectively.

MRI large-phantom High-SAR tests: No (+5degree) flip-angle increase was observed around shafts. Appearance was of a dark shaft, with hyper-intense distal MR-Tracking coils.

MRI Swine navigation: MR-Tracking coils were well visualized inside heart (Fig. 4, 5). Device shafts were completely dark, validating MBalun effectiveness. Active-guidewire manipulation was easy.

Conclusions

Acknowledgements

References

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