Navigated DENSE strain imaging for post-radiofrequency ablation lesion assessment in the swine left atria

Abstract

Aims: Prior work has demonstrated that magnetic resonance imaging (MRI) strain can separate necrotic/stunned myocardium from healthy myocardium in the left ventricle (LV). We surmised that high-resolution MRI strain, using navigator-echo-triggered DENSE, could differentiate radiofrequency ablated tissue around the pulmonary vein (PV) from tissue that had not been damaged by radiofrequency energy, similarly to navigated 3D myocardial delayed enhancement (3D-MDE).

Methods and results: A respiratory-navigated 2D-DENSE sequence was developed, providing strain encoding in two spatial directions with 1.2 × 1.0 × 4 mm(3) resolution. It was tested in the LV of infarcted sheep. In four swine, incomplete circumferential lesions were created around the right superior pulmonary vein (RSPV) using ablation catheters, recorded with electro-anatomic mapping, and imaged 1 h later using atrial-diastolic DENSE and 3D-MDE at the left atrium/RSPV junction. DENSE detected ablation gaps (regions with >12% strain) in similar positions to 3D-MDE (2D cross-correlation 0.89 ± 0.05). Low-strain (<8%) areas were, on average, 33% larger than equivalent MDE regions, so they include both injured and necrotic regions. Optimal DENSE orientation was perpendicular to the PV trunk, with high shear strain in adjacent viable tissue appearing as a sensitive marker of ablation lesions.

Conclusions: Magnetic resonance imaging strain may be a non-contrast alternative to 3D-MDE in intra-procedural monitoring of atrial ablation lesions.

Keywords: Atrial fibrillation; Magnetic resonance imaging ablation; Porcine model.

Figure 1

Respiratory-triggered META-DENSE sequence. (A) Diagram of respiratory cycle (upper), and waveform of cardiac-gated META-DENSE with surrounding respiratory navigator echoes (lower). Navigators are accepted when they fall in a pre-determined window in the respiratory cycle. (B) Enlarged view of one cardiac cycle, showing navigators preceding and following the META-DENSE sequence. When the navigator position criteria are met, the sequence waits for an ECG trigger, and then executed the DENSE pulse sequence. Navigators after DENSE are compared with those preceding the sequence, detecting the magnitude of motion occurring during image acquisition. If motion during acquisition is excessive, the data are rejected and re-acquired later on. (C) The META-DENSE pulse sequence, consisting of a DE (D and E) preparation, followed by a mixing time (TM) (∼300 ms) in which displacement is measured. During the TM, an inversion pulse is played to suppress an anti-echo artefact. Finally, a multi-echo Fast Spin Echo readout (∼140 ms) is played (only first echo is shown). META-DENSE is repeated three times; a reference scan without DE (no gradient between the two 90 s in the DE prep), followed by x encoding (dashed blue gradient) and y encoding (solid black). To further suppress artefacts, each x and y encode is repeated, with 90° RF phase modulation (from 90_+x to 90_+y) in the second repetition, which also increases the signal-to-noise.

Figure 2

High-resolution respiratory-triggered DENSE of an infarcted sheep, with an inflated-balloon patch on the inferior wall of the LV to prevent remodelling (white arrow). (A) Two long-axis 3D-MDE images show mid-ventricular non-transmural scar (red arrow). (B) Colour-coded short-axis DENSE strain, showing strain-magnitudes ranging from normal (20%, red) to lack of strain (0%, blue). Circumferential strain (left) and radial strain (right). Chronic infarct and patch (orange bracket) appear as low-strain regions. (C) Long-axis DENSE strain, with longitudinal (left) and radial (right) strains. White dashed line indicates where short-axis images (B) intersect long-axis slice. Non-transmural infarct appears as large strain (live tissue) along the endocardial surface (right image) with low strain (necrosis) from the mid-wall to the epicardial surface. Note that the sign of radial strain is positive and circumferential strain is negative.

Figure 3

Incomplete RF ablation of two swine RSPV. The first swine includes figures A–F, the second swine includes figures G–L. (A) EAM maps overlaid with RFA points (brown dots); Anterior (A1) and Superior (A2) views, showing gaps (green arrows). Actual size of gaps could not be determined from EAM. Activation maps were obtained prior to ablation. (B) Atrial luminal renderings (pink), segmented from 3D-MRA, are overlaid with a 3D-MDE scar map (red), showing the gaps in RSPV ablation (B2 and B3). (C) Images detailing the orientation of distal (C1) and proximal (C2) DENSE slices placed perpendicular to the RSPV ostium. Slice positions are also shown in 5B2. (D) Colour-coded DENSE myocardial shortening (negative sign) and myocardial-thickening (positive sign) strain magnitude for the distal (D1) and proximal (D2) slices. White lines outline regions with <8% strain. (E) Reformatted 3D-MDE slices at the position and orientation of D1 (E1) and D2 (E2). Dashed black line indicates position of RSPV wall, white lines outline enhanced ablated regions, and green arrows denote lack of enhancement (ablation gap). (F) Stained gross histology photograph of the LA wall at the RSPV ostium, overlaid with the areas (dashed red line) where transmural ablation lesions were found, with unstained ablation gaps denoted (green arrows). Incomplete RF ablation of the RSPV in another swine. (G) EAM maps overlaid with RFA points (brown dots); Anterior (G1) and Superior (G2) views, showing gaps (green arrows). Activation maps were obtained prior to ablation. (H) Luminal anatomy overlaid (H1–H4), with 3D-MDE scar, showing regions of incomplete ablation. Dashed white lines (H2 and H4) designate positions of 2D-DENSE slices placed perpendicular and parallel to RSPV ostium. Colour-coded DENSE strain for the distal (I1) and proximal (I2) perpendicular slices. White lines outline, 8% strain regions. (J) Position of ROIs placed on DENSE slice oriented along the RSPV to RIPV plane (see H4); (J1) ROI (red) encompassing the RSPV, and (J2) ROI encompassing the RIPV. (K) Strain results for ROIs covering RSPV (K1) and RIPV (K2). (L) Gross histology photograph of the LA wall at the RSPV ostium region, overlaid with ablation lesion regions (dashed red line).

Figure 3

(continued).