Intracardiac MR imaging (ICMRI) guiding-sheath with amplified expandable-tip imaging and MR-tracking for navigation and arrythmia ablation monitoring: Swine testing at 1.5 and 3T

Abstract

Purpose: Develop a deflectable intracardiac MR imaging (ICMRI) guiding-sheath to accelerate imaging during MR-guided electrophysiological (EP) interventions for radiofrequency (500 kHz) ablation (RFA) of arrythmia. Requirements include imaging at three to five times surface-coil SNR in cardiac chambers, vascular insertion, steerable-active-navigation into cardiac chambers, operation with ablation catheters, and safe levels of MR-induced heating.

Methods: ICMRI's 6 mm outer-diameter (OD) metallic-braided shaft had a 2.6 mm OD internal lumen for ablation-catheter insertion. Miniature-Baluns (MBaluns) on ICMRI's 1 m shaft reduced body-coil-induced heating. Distal section was a folded "star"-shaped imaging-coil mounted on an expandable frame, with an integrated miniature low-noise-amplifier overcoming cable losses. A handle-activated movable-shaft expanded imaging-coil to 35 mm OD for imaging within cardiac-chambers. Four MR-tracking micro-coils enabled navigation and motion-compensation, assuming a tetrahedron-shape when expanded. A second handle-lever enabled distal-tip deflection. ICMRI with a protruding deflectable EP catheter were used for MR-tracked navigation and RFA using a dedicated 3D-slicer user-interface. ICMRI was tested at 3T and 1.5T in swine to evaluate (a) heating, (b) cardiac-chamber access, (c) imaging field-of-view and SNR, and (d) intraprocedural RFA lesion monitoring.

Results: The 3T and 1.5T imaging SNR demonstrated >400% SNR boost over a 4 × 4 × 4 cm³ FOV in the heart, relative to body and spine arrays. ICMRI with MBaluns met ASTM/IEC heating limits during navigation. Tip-deflection allowed navigating ICMRI and EP catheter into atria and ventricles. Acute-lesion long-inversion-time-T1-weighted 3D-imaging (TWILITE) ablation-monitoring using ICMRI required 5:30 min, half the time needed with surface arrays alone.

Conclusion: ICMRI assisted EP-catheter navigation to difficult targets and accelerated RFA monitoring.

Keywords: acute RF lesion imaging; cardiac electrophysiology; interventional MRI.

FIGURE 1

1.5T ICMRI guiding sheath. (A) The complete metallic‐braided catheter showing locations of the heat‐ameliorating MBaluns along the shaft as well as the two levers, which activate the tip deflection (dark green) and imaging‐coil expansion or contraction (light green). Expanded view of an MBalun mounted on the braid shows the solenoidal wires and the FPC capacitor. (B) Distal end of ICMRI, showing the imaging‐coil expanded (35 mm ID) during imaging or (C) folded (6 mm OD) during vascular access. (D and E) Pull‐wire deflected distal ends of ICMRI, along with the protruding distal‐tip of a deflected EP catheter, demonstrating the anatomical‐access capabilities permitted by manipulating both devices. (F) On‐face view of the expanded imaging‐coil and supporting struts, showing the location of the four MRT coils, , , and the protected pre‐amplifier electronics of the imaging‐coil. (G) MR Image of expanded imaging‐coil shows the tetrahedron shape of the four MRT coils, which allow accurate tip localization. (H) Construct of the 1.5T three‐layer loop‐design MRT FPC board (length 14 mm, width 1.2 mm, thickness 0.25 mm). The MRT coil contains both the antenna and embedded parallel and series T&M capacitors. The 3T MRT coils were of similar construct but contained only two layers (to reduce inductance) and utilized lower‐value T&M embedded capacitors

FIGURE 2

ICMRI imaging‐coil. (A) 3T FPC pentagon‐shaped coil with thin‐film embedded series capacitors on its shaft (red arrows). The 2‐winding coil circuit also includes an integrated front‐end (Yellow arrow) placed on the connecting stem, which includes T&M embedded capacitors, anti‐parallel pin diodes for high‐signal amplifier protection and a low‐noise pre‐amplifier, all encased within the (black) epoxy waterproof box. The 1.5T coil is similarly built, except for different capacitors on the pentagon and on the stem. (B) Enlarged view of the antiparallel pin‐diode wafers and low‐noise amplifier wafer taken with a microscope during component wire‐bonding, and before encasement in the epoxy box. Circuit detail is shown in Supporting information Figure S1A. (C) GRE image (parameters: TR/TE/θ = 3.3 ms/1.2 ms/30°, 4.5 × 4.5 cm² FOV, 225 × 225, 2 mm slice‐width, 0.2 × 0.2 × 2 mm³, 3 s acquisition) of a sliced Kiwi fruit 20 mm away from the coil demonstrates the coil's effective FOV when it is open

FIGURE 3

System electronics box. (A) Electronics box connects to ICMRI and conditions the signals in the four tracking channels (Track1‐4) and in the imaging channel (Image). (B) Enlarged Imaging channel, showing the π circuit T&M which compensates for the phase shifts produced by the 1 m length microcoax, along with a passive decoupling (antiparallel pin diodes) circuit and 1 KV (0.1 µF) capacitive blocking circuits that protect the patient from DC leakage currents and High AC voltages. (C) The LNA at the ICMRI tip is powered by two parallel rechargeable MRI‐conditional Lithium‐Polymer batteries which provide 3.7‐4.0V DC (not shown), although only 3.0V reach the LNA due to losses in a resistor and in the micro‐coaxial cable. The DC current passes through an MRI‐conditional Bias‐T circuit, which prevents RF noise from entering from the battery, and also prevents DC current from going upstream to the MRI receiver. The B‐T was composed of a parallel 0.1 µF 1 KV capacitor to ground, followed by two series 2.2 µH air‐core inductors. At the point designated with a black ± sign, the DC signal is combined with the amplified raw MRI signals from the LNA’s output. (D) The imaging channel receives the imaging‐coil's amplified signal from (C) and also has a π circuit to T&M the amplified signal coming from the ICMRI tip, as well as a 1 kV blocking capacitor to protect the patient. Circuit diagrams are included in the Supporting information Figure S1.

FIGURE 4

1.5T (A, B) and 3T (C, D) heating tests performed on a disconnected ICMRI in a non‐convective ASTM gel phantom during 15‐min continuous 4 Watt/kg SAR imaging with the LNA not powered. (A, C) 1.5T and 3T folded (6 mm OD) imaging‐coil, as used during navigation. (B, D) 1.5T and 3T expanded (35 mm OD) imaging‐coil, as used during imaging. IC denotes the epoxy box which encloses the LNA and its protecting pin‐diodes, which is connected to a heat sink on the FPC, which is not effective in the gel phantom. During navigation with the coil folded, ICMRI slightly exceeded ASTM/IEC limits of 1.5°C maximum heating at 1.5 and 3T. With the imaging‐coil expanded, portions of the 1.5 and 3T ICMRI imaging‐coil, at points distal to the MBalun, exceeded 1.5°C after only approximately 80 s, rising to 12°C and 13.2°C, respectively, after 15 min

FIGURE 5

3T ICMRI imaging‐coil SNR results ex‐vivo and in‐vivo. (A, B) T2‐weighted FSE images from expanded imaging‐coil in an ex‐vivo pig left atrium, distanced from the surface arrays to emulate human conditions. (A) SNR of the IMCRI imaging‐coil alone, and (B) in an array together with elements of the 32‐coil 3T body and spine arrays. The ICMRI imaging‐coil provided an SNR ratio of 400 ± 30% versus the surface coils over a 4 × 4 × 4 cm³ FOV. (C, D) In‐vivo axial and coronal GRE images taken with ICMRI navigated to a swine right atrium and then expanded. (E–G) Profiles along the Red, Yellow and Blue lines in (C, D) show the SNR ratios of the imaging‐coil (center location indicated by green arrow) versus 32‐channel surface‐array elements. In the Anterior‐Posterior direction, the imaging‐coil gives a 460 ± 40% SNR ratio vs the surface arrays, in the Left‐Right direction a 440 ± 40% SNR ratio and in the Head‐Feet direction a 300 ± 70% SNR ratio, over a 4 cm length

FIGURE 6

ICMRI deployed in the right ventricle at 1.5T. Respiratory‐navigated & ECG‐gated (trigger delay 432 ms) 3D GRE scan. ICMRI gave a 510 ± 30% SNR boost over the 32‐channel spine & body arrays over a 5 × 5 × 5 cm³ FOV). (A, B) Images of the ICMRI imaging‐coil in the Right Ventricle, with only the ICMRI imaging‐coil and the surface arrays enabled, demonstrate its strong signal (C, D) View of the signals from the MRT coils alone on the expanded ICMRI tip, performed by not powering ON the imaging‐coil, demonstrates their ability to indicate the distal‐tip location. In this tetrahedron configuration, the tracking‐coils’ location within physiological cycles can be tracked and used for imaging motion compensation. (D) Is an enlarged view of (C). The artifacts in B‐D are Gibbs artifacts that result from the strong signal intensity of the local coils

FIGURE 7

1.5T 3D Slicer workstation visualization. Display at a point during navigation, showing insertion of ICMRI, with folded imaging‐coil and a protruding EP catheter, through a trans‐septal hole into the Left Atrium. (A) short axis slice; (B): 3D view of ICMRI (blue) navigated into LA through the hole in the septum. Highlighted (red) structures are cardiac‐chamber walls segmented from the SSFP data; (C): sagittal slice; (D): short‐axis left atrial image. Yellow and blue points (highlighted with arrows) show the locations of two of the MR‐tracking coils mounted on the ICMRI shaft. A video of navigation in a simulated human left atrium is shown in Supporting information Video S1.

FIGURE 8

RF Ablation at 1.5T with ICMRI monitoring. Experiment1 (A‐D). (A) The ICMRI guiding‐sheath and an EP catheter are brought to the left atrium together. (B) ICMRI’s imaging‐coil is expanded in the LA and turned ON, demonstrating its image intensity, together with the surface arrays, relative to the surface arrays alone (what is observed further away from ICMRI). (C) ICMRI and the EP catheter were then pulled back into the right atrium, and ablation lesions created in the right atrial roof with the EP catheter (setting: 20 Watts for 90 s each). TWILITE was then performed with ICMRI alone in a 5:30 min 1 × 1 × 1 mm³ resolution 14 cm FOV acquisition covering only the right atrium, showing the acute lesions created (white arrows), with ablation gaps visualized between some. (D) Histological photograph of the lesions. Experiment2 (E‐H). An ICMRI catheter was advanced to the RA, and the imaging‐coil expanded below the LA. The EP catheter was then advanced alone into the LA. Ablation lesions were created on the roof of the LA. TWILITE was then performed in a 7:0 min 0.8 × 0.8 × 1.0 mm³ resolution 14 cm FOV acquisition, showing the acute lesions (white arrows). (E) and (F) are short axis views, (G) is a coronal view, and (H) Histological photograph of the lesions.