Oral Abstract Session
SCMR 22nd Annual Scientific Sessions
Background: Cardiovascular catheters and guidewires, typically long and narrow structures manipulated from their proximal end, are preferably constructed of metallic alloys which deliver improved mechanical torquability and pushability. The MRI’s transmission-coil induces currents on the metal surface and common-mode currents on wires within, which may heat surrounding tissues . Non-contact resonant radio-frequency traps “Baluns” are used in MRI receiver coil cables . We prototyped a novel miniature Balun (MBalun) for metallic-braided catheters, to mitigate heating concerns.
Methods: The MBalun (Fig. 1A) was constructed from a pitch=3 (=[interwinding-distance/wire-diameter]) solenoid surrounding a metallic-braided-shaft with an insulator between them . The solenoid and braid were shorted at one end (Fig. 1A), and resonated with a capacitor at the other. Electromagnetic simulation evaluated the Specific-Absorption-Rate (SAR) around a 35-cm long current-driven wire passing within 3 successive MBaluns. Fig. 1B illustrates an 8-Fr MR-tracked&Impedance-tracked metallic-braided deflectable EP ablation catheter with 11 MBaluns. Heat mitigation was validated in a gel phantom using temperature measurements during 3.99Watt/kg SAR imaging (@1.5T). Imaging was performed in 2 swine using Steady-State-Free-Precession (SSFP) during catheter navigation from the femoral vein to the right atrium (RA).
Results: Figure 2A shows the result of SAR modeling for a current-carrying 35-cm wire passing through three successive MBaluns. The simulation was performed inside a soft-tissue-mimicking medium. MBaluns provided ~30.3 dB total SAR reduction at the receiving port (right side). Temperature-testing results from an MBalun-equipped EP catheter in an ASTM gel phantom during 15-min of 3.99 W/Kg SAR imaging are shown in figure 2B. A temperature increase of 0.7 °C was seen at the distal tip electrode. Temperature rises of ~0.3 °C and ~0.6 °C were observed at the distal MR-Tracking-coil and MBalun, respectively, less than recorded by a reference probe far from the catheter. Breath-held ECG-gated SSFP (128x256, 23x30cm FOV, TR/TE/flip=3.5ms/1.5ms/45°, 4views/seg, 5mm slice) image acquired with all receiver-channels (spine-array, body-array, 4 MR-Tracking coils). Yellow arrows denote location of MR-tracking coils. The lack of signal from catheter shaft (Red Arrows) demonstrates the MBalun’s effectiveness. Figure 2D shows a frame of real-time (2 frames-per-second) ungated single-shot SSFP (64x128, 30x30cm FOV, TR/TE/flip=3ms/1.5ms/25°) image, during catheter navigation. Only two distal MR-Tracking coils are seen due to a curved-catheter orientation.
MBaluns reduce safety risks for metallic-braided EP ablation catheters in MRI-guided cardiovascular interventions, allowing construction of MRI-compatible devices with improved torquability.