Real-time magnetic resonance imaging-guided radiofrequency atrial ablation and visualization of lesion formation at 3 Tesla

Abstract

Background: Magnetic resonance imaging (MRI) allows visualization of location and extent of radiofrequency (RF) ablation lesion, myocardial scar formation, and real-time (RT) assessment of lesion formation. In this study, we report a novel 3-Tesla RT -RI based porcine RF ablation model and visualization of lesion formation in the atrium during RF energy delivery.

Objective: The purpose of this study was to develop a 3-Tesla RT MRI-based catheter ablation and lesion visualization system.

Methods: RF energy was delivered to six pigs under RT MRI guidance. A novel MRI-compatible mapping and ablation catheter was used. Under RT MRI, this catheter was safely guided and positioned within either the left or right atrium. Unipolar and bipolar electrograms were recorded. The catheter tip-tissue interface was visualized with a T1-weighted gradient echo sequence. RF energy was then delivered in a power-controlled fashion. Myocardial changes and lesion formation were visualized with a T2-weighted (T2W) half Fourier acquisition with single-shot turbo spin echo (HASTE) sequence during ablation.

Results: RT visualization of lesion formation was achieved in 30% of the ablations performed. In the other cases, either the lesion was formed outside the imaged region (25%) or the lesion was not created (45%) presumably due to poor tissue-catheter tip contact. The presence of lesions was confirmed by late gadolinium enhancement MRI and macroscopic tissue examination.

Conclusion: MRI-compatible catheters can be navigated and RF energy safely delivered under 3-Tesla RT MRI guidance. Recording electrograms during RT imaging also is feasible. RT visualization of lesion as it forms during RF energy delivery is possible and was demonstrated using T2W HASTE imaging.

Copyright © 2011 Heart Rhythm Society. Published by Elsevier Inc. All rights reserved.

Figures

Figure 1. Prototype of the MRI compatible ablation catheter and schematic set up of the ablation delivery system and components inside and outside the EP-MRI suite

(A) A 7F, 110 cm, MRI compatible radiofrequency ablation catheter. The catheter uses a plunger mechanism for deflection, the four tracking micro-coils are shown by the red arrows. (B) Setup for ablation and radiofrequency delivery system. The ablation catheter, MRI scanner interface, MRI scanner, patch electrode and one of the filter boxes are in the EP-MRI suite; and a second filter box and a standard ablation current generator are housed outside.

Figure 2. Assessment of catheter tip position and catheter tip-tissue interface by RTMRI and FLASH sequences

An MRI compatible catheter is guided under RT-MRI (A–C) from the inferior vena cava to the lateral wall of the RA, this is seen in orthogonal planes (coronal, sagital, and axial views). The tip of the ablation catheter touching the atrial wall (red arrow) is seen in high-resolution T1w-FLASH images (D–E). RA: right atrium, LA: left atrium, RV: right ventricle, and LV: left ventricle.

Figure 3. Bipolar atrial and ventricular intra-cardiac recordings obtained during RT-MRI scanning

The tracing on top (red) represents an atrial bipolar intra-cardiac electrogram (1) and a far-field ventricular signal (2); the presence of the QRS (3) on the surface EKG (green) confirms the timing and location of these recordings (panel A). The tracing on top (red), demonstrates a ventricular bipolar intra-cardiac electrogram (1) superimposed (2) with a surface QRS (panel B).

Figure 4. Catheter tip-tissue interface imaging and lesion formation visualization on the atrial septum

(A,G) Catheter tip localization from T1w-FLASH sequence. Green arrow indicates the position of catheter tip. (B–F and H–L) Temporal progression of lesion formation in the RA visualized by T2w-HASTE during RF energy delivery (20W, 120sec and 20W, 90sec respectively). Time after start of the ablation is shown in each image: (B) at the beginning of ablation and (C–F) at 10, 15, 30 and 240sec after the start of the ablation, respectively; (H) at the beginning of ablation and (I–L) at 18, 30, 42 and 78sec from the start of ablation respectively. RA: right atrium, and LA: left atrium.

Figure 5. Correlation between RT-MRI findings during lesion formation (T2w-HASTE) and macroscopic tissue samples

(A) T2w-HASTE image acquired 20 seconds from the start of a 20W ablation for 90sec (impedance 100 Ohms) shows a lesion in the posterior wall of the RA (small green arrow). This lesion measured approximately 5 mm in diameter by 2 mm in depth on macroscopic examination, large green arrow on (B). On (C) a T2w-HASTE image sequence acquired obtained during the first 20 seconds from the start of a 20W ablation for 120sec (impedance 95 Ohms) shows a lesion in the cavo-tricuspid isthmus. This lesion measured approximately 10 mm in diameter by 4 mm in depth on macroscopic examination, large green arrows on (D) and (E). LA: left atrium, RA: right atrium, FO: fosa ovalis, CS: coronary sinus os, and TVA: tricuspid valve annulus.

Figure 6. Correlation between T2w-HASTE images of lesion formation and 3D reconstruction based on LGE-MRI and macroscopic tissue examination in the left atrium

(A,B) T2w-HASTE images of RF ablation in left arial appendage (LAA). The green arrow points to the catheter tip (A) and the lesion (B) being formed in the LAA after power-controlled ablation (20W, 30sec). (C) Segmented, 3D rendered image of an ex-vivo LGE MRI scan demonstrating lesions in LAA, blue circle corresponds to the lesion depicted in (B). In LGE imaging a dark area is visualized consistent with hematoma/hemorrhage within the atrial wall. (D) Macroscopic specimen demonstrating the lesion.

Figure 7. Correlation between enhancement size in T2w-HASTE MRI (vertical axis) and ex-vivo measurements (horizontal axis) of lesion for four 20W power-controlled lesions

A correlation coeficient of 0.92 was found for the T2w-HASTE images acquired approximately 20 seconds after beginning of ablation (top panel); and a correlation coeficient of 0.68 for the T2w-HASTE images acquired approximately 45 seconds after beginning of ablation (bottom panel). Table 1 shows the actual MRI and ex-vivo lesion size measurements.